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DEPARTMENT   OF   THE    INTERIOR 


MONOGRAPHS 


United  States  Geological  Survey 


VOLUME    XXVIII 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 

1897 


UNITED  STATES  GEOLOGICAL  SURVEY 

CHARLES  D.  WALCOTT,  DIRECTOR 


THE 


MARdUETTE  IllON-BEARING  Mim  OF  MICHIGAN 


J^TJ^Js^S 


CHARLES  RICHARD  VAN  HISE  aud  WILLIAM  SHIRLEY  BAYLEY 


INCLUDING 


A   CHAPTEE    ON    THE    REPUBLIC    TROUGH 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 
1897 


CONTENTS, 


Page. 

Letter  op  transmittai, xix 

Outline  of  this  monograph xxi 

Introduction 1 

Chapter  I. — Geological  explorations  and  literature;  by  W.  S.  Bay  ley 5 

Chapter  II. — The  Basement  Complex,  by  W.  S.  Bay  ley 149 

Section  I. — The  Northern  Complex 150 

The  Mona  schists 152 

Distribution  and  topography 152 

Relations  to  adjacent  formations 153 

Petrographical  character 154 

Basic  schists 154 

The  dense  varieties 154 

The  banded  varieties 156 

Other  varieties 158 

Acid  schists 159 

The  Kitcbi  schists 160 

Distribution  and  topography 161 

Relations  to  adjacent  rocks 162 

Petrographical  character 162 

Basic  schists 162 

Macroscopical 162 

Microscopical 164 

Acid  schists 167 

Origin  of  the  Kitchi  schists 168 

The  gneissoid  granites 169 

Distribution  and  topography 169 

Relations  to  adjacent  rocks 170 

Biotite-granites : 171 

Petrographical  character 171 

Macroscopical 171 

Microscopical 171 

Muscovite-granites 174 

Origin  of  the  granites 175 

The  hornblende-syenite 176 

Distribution  and  topography 176 

Relations  to  adjacent  rocks 176 

Petrographical  character 176 


VI  CONTENTS. 

Chapter  II. — The  Basement  Complex,  by  W.  S.  Bayley — Continued.  Page. 

Section  1. — The  Northern  Complex— Continued. 

The  intrusives  in  the  Northern  Complex 178 

The  basic  dikes 178 

The  acid  dikes 182 

The  peridotite 183 

The  Presq  ue  Isle  area 183 

The  Opin  area 184 

Ferruginous  veins  in  the  Northern  Complex 186 

Summary 188 

Section  II.— The  Southern  Complex.. 190 

Distribution  and  topography 191 

Comparison  with  Northern  Complex 192 

The  schists 192 

The  micaceous  schists 195 

Muscovite-schists 195 

Biotite-schists 196 

Feldsjiathic  biotite-schists 196 

Hornbleudic  biotite-schists 198 

Structure 198 

Composition  and  origin 200 

The  hornblendic  schists 203 

Greenstone-schists 204 

Amphibole-schists 206 

Micaceous  amphil>ole-schists 208 

Origin 208 

The  gneissoid  granites 209 

Petrographical  character 209 

Macroscopical 209 

Microscopical 210 

The  Palmer  gneisses 211 

Relations  to  adj.acent  formations 211 

Petrographical  character 213 

Composition  and  origin 216 

Intrusives  in  the  Southern  Complex 218 

Summary : 218 

Section  III. — Isolated  areas  within  the  Algonkian 220 

Chapter  III. — The  Lower  Marquette  series,  by  C.  R.  Van  Hise 221 

Section  I. — The  Mesnard  quartzite 221 

Distribution,  exposures,  and  topography 221 

Folding 222 

Petrographical  character 223 

Macroscopical 223 

Microscopical 224 

Relations  to  underlying  formation 230 

Thickness 231 

Interesting  localities 232 

MudL.'ike 232 


CONTENTS.  VII 

Chapter  III.— The  Lower  Marquette  series,  by  C.  R.  Van  Hise— Continued.  Page. 
Section  I.— The  Mesnard  quartzite— Continued. 
Interesting  localities— Continued. 

Mount  Omimi 234 

Mount  Mesnard 236 

Mount  Chocolay 237 

Migisi  Bluffs 238 

Lake  Mary 239 

Section  II.— The  Kona  dolomite 240 

Distribution,  exposures,  and  topography 240 

Folding 242 

Petrographieal  character 244 

Macroscopical 244 

Microscopical 244 

Relations  to  adjacent  formations 251 

Thickness 252 

Interesting  localities 253 

Eastern  area 253 

Ragged  Hills 253 

Kona  Hills 254 

Section  III.— The  Wewe  slate 256 

Distribution,  exposures,  and  topography 2.56 

Folding 257 

Petrographieal  character 258 

Macroscopical 258 

Microscopical 263 

Relations  to  adjacent  formations 269 

Thickness 271 

Interesting  localities 272 

Makwa  Hills 272 

Eastern  area 272 

Goose  Lake 273 

Wewe  Hills 275 

Section  IV.— The  Ajibik  quartzite 282 

Distribution,  exposures,  and  topography 282 

Folding 285 

Petrographieal  character 286 

Macroscopical 286 

Microscopical 289 

Relations  to  adjacent  formations 294 

Thickness 299 

Interesting  localities 300 

Michigamme  area 300 

Broken  Bluffs - 301 

Area  west  of  Teal  Lake 302 

Area  east  of  Teal  Lake 304 

Eastern  area 307 

Wewe  Hills 308 


VIII  CONTENTS. 

Chapter  III. — The  Lower  Marquette  series,  by  C.  R.  V.an  Hise— Continued.  Page. 
Section  IV. — The  Ajibik  quartzite — Continued. 
Interesting  localities — Continued. 

Ajibik  Hills 308 

Goose  Lake 310 

Cascade  area 310 

Sees.  27  and  28,  T.  47  N.,  R.  27  W 312 

Republic  and  Western  tongues 313 

Sees.  29  and  30,  T.  18  N.,  R.  27  W 313 

Section  V.— The  Siamo  slate 313 

Distribution,  exposures,  and  topography 314 

Folding 315 

Petrographical  character 316 

Macroscopical 316 

Microscopical 318 

Relations  to  adjacent  formations 321 

Thickness 322 

Interesting  localities 322 

Michigamme  area 322 

Nonpareil  mine 324 

Siamo  Hills 324 

Area  east  of  Teal  Lake 326 

Eastern  area 326 

West  half  of  T.  47  N.,  R.  26  W 327 

Section  VI. — The  Negaunee  formation 328 

Relations  to  eruptives 329 

Distribution,  exposures,  and  topography 330 

Folding 332 

Relations  to  underlying  and  overlying  formations 333 

Thickness 336 

Petrographical  character 336 

Macroscopical 336 

Microscopical 366 

Interesting  localities 375 

Michigamme  aud  Spurr 375 

Boston  and  Dexter  areas 377 

Excelsior  area 378 

Lake  Bancroft  area 378 

Teal  Lake  area i 378 

Negaunee-Ishpemiug  area 379 

Area  southeast  of  Ishpeming 380 

Cascade  r.ange 382 

Foster-Lowthian  area 383 

Saginaw-Goodrich  area 383 

Escanaba  River  area 384 

Humboldt  area 385 

Champion  .area 389 

Republic  area 389 

Magnetic  mine  area 390 


CONTENTS.  IX 

Chapter  III.— The  Lower  Jlarqiiette  series,  by  C.  R.  Van  Hise— Continued.  Page. 
Section  VI. — The  Negaunec;  formation— Continued. 

The  iron-ore  deposits •'^^ 


The  ore  horizons 


391 


in  of  the  ores 1*'0 


tnigi 

Prospecting 

Chapter  IV.— The  Upper  Marquette  series. 
Introduction,  by  C.  R.  Van  Hise 


40.-) 
40H 


408 


Section  I.— The  Ishpeming  formation,  by  C.  E.  Van  Hise 409 

The  Goodrich  quartzite *09 

Distribution,  exposures,  and  topography 409 

Folding *10 

Relations  to  adjacent  formations Ill 

Petrograpbical  character *11 

Macroscopical ^H 

Microscopical *1^ 


Thicliness 


416 


The  Bijiki  schist -11'' 

Distribution,  exposures,  and  topography *16 


Folding 


417 


Petrograpbical  character ^1^ 

Macroscopical ''l ' 

Microscopical *1° 

Relations  to  adjacent  formations "119 


Thickness . 


420 


Interesting  localities  of  the  Ishpeming  formation 420 

Michigamme  and  Spurr 420 

Lake  Michigamme  area - 41.3 

Boston  and  Dexter  area. 424 

Lake  Corning  area 4L5 

Ishpeming  area 42a 

Negaunee  area 427 

Cascade  area 429 


Goodrich-Saginaw  area . 


432 


Mount  Humboldt  area 

Champion  area 434 

Lake  Michigamme  area 436 

Republic  area 436 

Klomau  area 439 

Northern  Republic  and  Western  troughs 439 

Section  II.— The  Michigamme  formation,  by  C.  R.  Van  Hise 444 

Distribution,  exposures,  and  topography - 444 

Folding *1^ 

Petrograpbical  character 445 

Macroscopical *'*^ 

Microscopical **° 

Relations  to  the  underlying  formation 

Thickness 


452 


X  CONTENTS. 

Chapter  IV. — The  Upper  Marquette  series— Continued.  Page. 
Section  II.— The  Miehigamme  formation,  by  C.  R.  Van  Hise— Continued. 

Interesting  localities - 452 

Spurr,  Miehigamme,  and  Champion  area 452 

Eastern  area ■ 454 

Lake  Miehigamme  area 456 

Section  III.— The  Clarksburg  formation,  by  W.  S   Bayley 460 

Distribution,  exposures,  and  topography 460 

Relations  to  adjacent  formations ^61 

Thickness  and  folding 463 

Petrographical  character 463 

The  massive  greenstones 464 

The  lavas 467 

The  sediments  and  tutfs 468 

The  sediments 468 

Gradation  varieties  between  sediments  and  tufls 472 

Th  e  t  u  ff s 473 

The  hornblende-schists 475 

The  breccias  and  conglomerates 476 

Conclusions 480 

Interesting  localities 481 

Summary 484 

Chapter  V. — The  igneous  rocks,  by  W.  S.  Bayley 487 

Section  I. — The  pre-Clarksburg  greenstones 488 

The  bosses 189 

The  eastern  knobs 489 

Relations  to  Marriuette  sediments 489 

Petrographical  character 490 

The  western  knobs 499 

Relations  to  Marquette  sediments 499 

Petrographical  character 500 

The  dikes 506 

Petrographical  character 508 

Contact  effects - 513 

The  sheets  and  tuffs 514 

The  sheets 515 

The  tuffs 517 

Section  II. — The  post-Clarksburg  greenstones 518 

Petrographical  character 518 

Quartz-diabases 519 

Olivine-diabases 520 

Poriihyrites 521 

Basalts 522 

Summary 522 

Chapter  VI.— The  Republic  Trough,  by  Henry  Lloyd  Smyth 525 

Introduction 525 

SectionL— The  Archean 526 


CONTENTS.  XI 

Chapter  VI.— The  Reiniblic  Trough,  by  Henry  Lloyd  Smyth— Continued.  i-age. 

Section  II.— The  Lower  Marquette  series 528 

The  Ajibili  quartzito 528 

Tlxe  Negaunee  formation - 529 

Contacts  between  the  Lower  Marquette  series  and  the  Archean 532 

Section  III.— The  Upper  Marquette  series 535 

Contacts  of  the  Goodrich  quartzite  with  tlie   Lower  Marquette  series  and  with  the 

Archean 536 

Section  IV.— Later  igneous  iutrusives 538 

Section  V. — General  geology 538 

Faults 541 

The  ore  deposits 547 

Position  of  the  ore  deposits 547 

Relations  of  the  ore  deposits  to  the  geological  structure 549 

Origin  of  the  ore  deposits 551 

Chaptee  VII. — General  geology,  by  C.  R.  Van  Hise 554 

The  Basement  Complex 555 

The  Lower  Marquette  series 556 

The  transgression  horizon 556 

Unconformity  at  the  base  of  the  Lower  Marquette  series 557 

Deposition  of  the  Lower  Marquette  series 559 

Eruptives  of  Lower  Marquette  time 562 

Unconformity  at  the  top  of  the  Lower  Marquette  series 562 

The  Upper  Marquette  series 563 

Deposition  of  the  Upper  Marquette  series 563 

Folding  of  the  Basement  Complex,  Lower  Marquette  series,  and  Upper  Marquette  series 566 

Iutrusives 571 

Denudation 572 

Metamorjihism 573 

Correlation 575 

Index 580 


ILLUSTRATIONS. 


20 


Page 
Plate  I.  Portions  of  Burt's,   Hubbard's,  and  Ives's  niai)s  of  the  Upper  Peninsula  of 

Miehigau 

II.  Portion  of  Foster  and  Whitney's  map  of  tlie  Lake  Superior  laud  district 26 

III.  Fig.  1.  Portion  of  Brooks's  map  of  the  Upper  Peninsula  of  Michigan 58 

2.  Irving's  outline  map  of  the  Marquette  district 58 

IV.  Fig.  1.  Greenstone  schist  knob,  Dead  River 152 

2.  River  course  through  Mona  schist l'^- 

V.  Pebblesfrom  Kitchi  schist 164 

VI.  River  course  through  granite 1™ 

VII.  Fig.  1.  Weathered  surface  of  Kona  dolomite 246 

2.  Brecciated  chert  at  the  base  of  the  Kona  dolomite 246 

VIII.  Fig   1.  Brecciated  chert  in  Kona  dolomite 250 

2.  Brecciated  chert  in  Kona  dolomite 250 

IX.  Fig.  1.  .Shattered  Wewe  slate 262 

2.  Brecciated  Wewe  slate - 262 

X.  Fig.  1.  Recomposed  rock,  resembling  granite,  from  Wewe  slate 280 

2,  Ferruginous  Siamo  slate,  showing  overthrust  fault 280 

XI.  Dome  structure  of  griiuerite-magnetite-schist,  cau.sed  by  intrusive  greenstone.  328 

XII  Inclusions  of  griiuerite-magnetite-schist  in  intrusive  greenstone,  east  of  Spurr 

mine - ^30 

XIII.  General  view  of  Lake  Angeline  from  the  east,  showing  bluiils  of  greenstone, 

and  lowlands  underlain  by  the  Negaunee  formation 332 

XIV.  View  southwest  from  Lake  Bancroft,  showing  on  the  right  a  greenstone  bluff, 

and  in  the  distance  greenstone  hills  and  lowlands,  underlain  by  the  Negau- 
nee formation ^^*  , 

XV.  Open  pit  of  Cleveland  hard-ore  mine,  looking  west,  showing  minor  folds  in 

j  asper '^^ 

XVI.  View  of  westward-pitching  fold  in  No.  1  pit,  Lake  Superior  Iron  Company, 

looking  west.     The  rocks  in  the  center  of  the  figure  are  the  Goodrich 

quartzlte,  and  these  are  underlain  by  the  Negaunee  .jasper 338 

XVII.  Fig.  1.  Cherty  siderite,  from  the  Marquette  district 340 

2.  Cherty  siderite,  from  the  Peuokee  district 340 

XVIII.  Fig.  1.  Magnetite-griinerite-schist,  from  Republic  mine 342 

2.  Sideritic  magnetite-griinerite-schist,  from  sec.  13,  T.  47  N.,  R.  27  W 342 

XIX.  Fig.  1.  Griineritlc  magnetite-schist,  from  Republic  mine 344 

2.  Ferruginous  slate,  from  sec.  7,  T.  47  N.,R.  26  W 344 

3.  Ferruginous  slate  or  jasper,  from  sec.  7,  T.  47  N.,  R.  26  W 344 

XIII 


XIV 


ILLUSTRATIONS. 


Page. 

Plate  XX.  Fig.  1.  Ferruginous  chert,  from  Taylor  mine 346 

2.  Ferrugiuous  chert,  from  soiith  of  Jacksou  mine,  sec.  1,  T.  47  N.,  R.  27  W 346 

XXI.                 Hematitic  chert,  from  Negaunee 348 

XXII.                 Hematitic  cliert,  from  Neganuee 350 

XXIII.  Magnetitic  chert,  from  the  Michigamme  mine 352 

XXIV.  .Taspilite,  from  the  Grand  Rapids  mine,  Negauuee 354 

XXV.                 .laspilite,  from  Jasper  hluff,  Ishpeming 356 

XXVI.  Fig.  1.  Folded  jaspilite,  from  Jasper  bluff,  Ishpeming 358 

2.  Brecciated  jaspilite,  from  Jasper  hluff,  Ishpeming 358 

XXVII.  Fig.  1.  Jaspilite,  from  Jackson  mine,  Negauuee 360 

2.  Ore  and  jasper  conglomerate,  from  north  of  Lowthian  mine 360 

XXVIII.                 The  ore  deposits 394 

XXIX.                 The  ore  deposits 398 

XXX.                 Goodrich  quartzite  with  minor  fold  cut  by  dike,  Michigamme  mine 410 

XXXI.                 Conformable  exposure  of  Goodrich  quartzite  and  Bijiki  schist,  with  grada- 
tion zone  between,  near  Michigamme 412 

XXXII.  Fig.  1.  Thin  section  of  sedimentary  bed,  from  Clarksburg  formation,  showing  sec- 
ondary hornblende  crystals 470 

2.  Thin  section  of  fragmental  rock,  from  near  base  of  Clarksburg  formation..  470 

3.  Thin  section  of  banded  tuff,  from  Clarksburg  formation 470 

4.  Thin  section  of  greenstone,  from  Republic,  showing  secondary  hornblende 

crystals 470 

XXXIII.  Sketch  map  of  the  dikes  of  Mount  Humboldt 508 

XXXIV.  Geological  map  of  the  southeast  end  of  the  Republic  Horseshoe 546 

XXXV.  Fig.  1.  A  pitching  fold  in  Siamo  slate,  sec.  21,  T.  47  N.,  R.  27  W 570 

2.  Fan  fold  iu  ferrugiuous  schist 570 

FIGURES   IN  THE   TEXT. 

Fig.    1.  Generalized  cross-section  of  Marquette  synclinorium,  showing  the  Marquette  type  of 

fold 4 

2.  Horizontal  section  of  ore  bodies  at  the  surface  of  the  Champion  mine 95 

3.  Cross-section  through  ore  bodies  at  the  Edwards  mine 96 

4.  Cliff  of  Kitchi  schist,  in  sec.  33,  T.  48  N.,  R.  27  W^ 161 

5.  Outlines  of  plagioclase  grains,  in  uoneonglomeratic  band  of  Kitchi  schist 165 

6.  Magnetite  in  fine-grained  diabase  or  basalt 180 

7.  Mica-schist  intruded  by  granite,  south  of  Champion  mine 193 

8.  Thin  section  of  feldspathic  biotite-schist,  from  sec.  3,  T.  47  N.,  R.  30  W 197 

9.  Cleavage  in  slate  between  two  limestone  beds 243 

10.  Reibungsbreccia  iu  cherty  quartzite  truncating  limestone  strata 243 

11.  Basal  conglomerate  of  Wewe  slate,  from  near  center  of  sec.  22,  T.  47,  N.,  R.  26  W 259 

12.  Shattered  slate  cemented  by  vein  quartz,  from  northeast  quarter  sec.  21,  T.  47  N.,  R,  26  W  263 

13.  Brecciated  slate  oemen  ted  by  vein  quartz,  from  same  locality  as  fig.  12 263 

14.  Ajibik  quartzite  resting  unconformably  upon  Kitchi  schist 296 

15.  Minor  overturned  folds  in  Siamo  slate 315 

16.  Relations  of  schistosity  and  bedding  in  Siamo  slate 315 


ILLUSTRATIONS.  XV 

Page. 
I'iG.   17.   Intrusive  greenstone  in  griinerite-m;ignetito-scliist,  from  near  center  of  sec.  12,  T.  47  N., 

R.29  \V 330 

18.  Minor  plications  in  ferruginous  slate  iuterlaniinated  with  schistose  greenstones,  on 

Chicago  and  Northwestern  Railway,  east  of  Negaunee 332 

19.  Folded  ferruginous  chert  of  Starwest  mine 334 

20.  Horizontal  plan  of  contact  of  Goodrich  quartzite  on  plicated  Negaunee  jaspilite 335 

21.  Cross-section  of  contact  of  Goodrich  quartzite  on  plicated  Negaunee  jaspilite 335 

22.  Jaspilite  of  Republic  mine,  showing  white  areas  of  chert  in  the  red  jasper 362 

23.  Horizontal  plan  of  one  of  the  minor  pitching  isoclinal  folds  in  griinerite-magnetite- 

schist 384 

24.  Section  showing  relations  of  griinorite-magnetite-scbist  and  intrusive  diorite,  Mount 

Humboldt 386 

25.  Plat  showing  relations  between  griinerite-magnetite-schist  and  intrusive  diorite,  Mount 

Humboldt 386 

26.  Section  showing  relations  of  jasper,  ore,  conglomerate,  and  quartzite  at  Michigamme 

mine 420 

27.  Position  of  specimens  of  greenstone  from  south  half  of  sec.  12,  T.  47  N.,  R.  27  W 492 


ATLAS    SHEETS, 


Sheet. 

Title I 

Conteiits jj 

Legend  and  Key  Map Uj 

General  geological  map  of  the  Marquette  district j V 

The  southwest  quaiter  of  T.  48  N.,  R.  30  W V 

The  northwest  quarter  of  T.  47  N.,  R.  30  W VI 

The  southwest  quarter  of  T.  47  N. ,  R.  30  W VII 

The  southeast  quarter  of  T.  48  N.,  R.  30  W VIII 

The  northeast  quarter  of  T.  47  N.,  E.  30  W IX 

The  southeast  quarter  of  T.  47  N.,  R.  30  AV X 

The  southern  part  of  the  Republic  Trough XI 

The  southwest  quarter  of  T.  48  N.,  R.  29  W XII 

The  northwest  quarter  of  T.  47  N.,  R.  29  W XIII 

The  southwest  quarter  of  T.  47  N.,  R.  29  W XIV 

The  southeast  quarter  of  T.  48  N.,  R.  29  W XV 

The  northeast  quarter  of  T.  47  N.,  R.  29  W XVI 

The  southeast  quarter  of  T.  47  N.,  R.  29  W XVII 

The  southwest  quarter  of  T.  48  N.,  R.  28  W XVIII 

The  northwest  quarter  of  T.  47  N.,  R.  28  AV XIX 

The  southwest  quarter  of  T.  47  N.,  R.  28  W XX 

The  southeast  quarter  of  T.  48  N. ,  R.  28  W XXI 

The  northeast  quarter  of  T.  47  N.,  R.  28  W XXII 

The  southeast  quarter  of  T.  47  N.,  R.  28  W XXIII 

The  southwest  quarter  of  T.  48  N.,  R.  27  W XXIV 

The  northwest  quarter  of  T.  47  N.,  R.  27  W XXV 

The  southwest  quarter  of  T.  47  N.,  R.  27  W XXVI 

The  southeast  quarter  of  T.  48  N.,  R.  27  W XXVII 

The  northeast  quarter  of  T.  47  N.,  R.  27  W XXVIII 

The  southeast  quarter  of  T.  47  N.,  R.  27  W XXIX 

The  southwest  quarter  of  T.  48  N.,  R.  26  W XXX 

The  northwest  quarter  of  T.  47  N.,  R.  26  W XXXI 

The  southwest  quarter  of  T.  47  N.,  R.  26  W XXXII 

The  southeast  quarter  of  T.  48  N.,  R.  26  W XXXIII 

The  northeast  quarter  of  T.  47  N.,  R.  26  W XXXIV 

The  southeast  quarter  of  T.  47  N.,  R.  26  W XXXV 

The  southwest  quarter  of  T.  48  N.,  R.  25  W XXXVI 

The  northwest  quarter  of  T.  47  N.,  R.  25  W XXXVII 

The  southeast  quarter  of  T.  48  N.,  R.  25  W XXXVIII 

The  northeast  quarter  of  T.  47  N.,  R.  25  W XXXIX 

MON  XXVIII II  XVII 


LETTER   OE  TRANSMITTAL. 


Departmj:nt  of  thk  Interior, 

United  States  Geological  Survey, 

Washington,  D.  C,  January  31,  1896. 

SiE:  I  transmit  herewith  the  manuscript  and  iUustrations  of  a  memoir 
and  the  plates  for  an  accompanying  atlas  upon  the  Marquette  Iron-bearing 
District  of  Michigan,  by  W.  S.  Bayley  and  myself. 

The  field  work  ujwn  which  the  present  report  is  based  began  more 
than  five  years  ago.  This  work  Avas  by  W.  N.  Merriam,  W.  S.  Bayley, 
H.  L.  Smyth,  J.  Morgan  Clements,  James  R.  Thompson,  and  C.  R.  Van 
Hise,  although  Mr.  Bayley  mapped  a  larger  area  than  anyone  else. 
Several  others  have  rendered  subordinate  assistance.  The  mapping  of  the 
area  from  west  of  the  center  of  R.  28  W.  to  Michigamme  was  mainly 
the  work  of  Mr.  Merriam.  He  also  studied  the  Republic  tongue,  but  his 
work  in  this  part  of  the  area  was  supplemented  by  a  much  more  detailed 
study  by  Mr.  Smyth.  From  the  center  of  R.  28  W.  to  the  center  of 
R.  27  W.,  the  mapping  was  partly  the  work  of  Mr.  Merriam  and  partly 
that  of  Mr.  Bayley.  East  of  the  center  of  R.  27  W.  to  Lake  Superior 
the  mapping  was  mainly  by  Mr.  Bayley,  although  Mr.  Clements  assisted 
one  field  season.  Mr.  Smyth  mapped  the  Republic  tongue  and  the  area 
to  the  west.  All  the  underground  work  in  connection  with  the  mines 
was  done  by  Mr.  Thompson.  My  own  part  of  the  task  was  the  structural 
study  of  the  whole  district,  to  which  I  gave  one  entire  field  season  and 
large  parts  of  several  others. 

The  field  work  on  the  western  part  of  the  district  by  Mr.  Merriam 
and  Mr.  Smyth  was  for  private  parties.  The  original  specimens,  notes,  and 
maps  were  placed  at  our  disposal  for  the  preparation  of  this  report.     To 


XX  LETTER  OF  TRANSMITTAL. 

tlie  gentlemen  furnishing  this  material  we  are  very  greatly  indebted. 
To  Mr.  James  E.  Thompson  our  especial  thanks  are  due  for  a  large 
amount  of  gratuitous  work,  and  especially  for  plats  and  sections  of  the 
majority  of  the  mines  of  the  district,  showing  the  relations  of  the  iron-ore 
deposits  to  the  surrounding  I'ocks.  To  the  agents,  superintendents,  and 
engineers  of  the  mines  of  the  district  we  are  indebted  for  numberless 
courtesies. 

Of  the  manuscript,  Chapter  I,  upon  the  literature;  Chapter  II,  upon 
the  Basement  Complex;  Section  III  of  Chapter  IV,  upon  the  Clarksburg 
formation,  and  Chapter  V,  upon  the  igneous  rocks,  were  prepared  by  W.  S. 
Bayley.  Chapter  VI,  upon  the  Republic  trough,  was  prepared  by  H.  L. 
Smyth.  Chapters  III  and  IV,  with  the  exception  of  Section  III  of  the 
latter  chapter,  upon  the  Algonkian,  were  prepared  by  C.  R.  Van  Hise,  as 
was  also  Chapter  VII,  upon  the  general  geology. 

The  drawing  for  the  atlas  was  done  by  E.  R.  Maurer  and  F.  E.  Morrow. 
The  beautiful  originals  for  the  colored  plates  were  prepared  by  Mr.  J.  L. 
Ridgway,  from  polished  specimens  furnished  him.  They  are  photographic 
in  their  accuracy. 

Very  respectfully,  your  obedient  servant, 

C.  R.  Van  Hise, 

Geologist  in  Charge. 

Hon.  Charles  D.  Walcott, 

Director  United  States  Geological  Survey. 


OUTLINE  OF  THIS  MONOGRAPH. 


The  Marquette  district  occupies  an  area  extending  from  Marquette  on  Lake  Superior  west  to 
Michigamme,  a  distance  of  sometliing  less  than  40  miles.  The  breadth  of  the  area  of  the  Marquette 
series  proper  varies  from  about  1  mile  to  more  than  6  miles.  From  the  western  part  of  the  main  area 
two  arms  project  for  several  miles,  one  to  the  soutljeast,  the  Republic  trough,  and  one  to  the  south, 
the  Western  trough. 

The  rocks  in  the  district  comprise  three  series,  separated  by  unconformities.  These  are  the 
Basement  Complex  or  Archean,  the  Lower  Marquette,  and  the  Upper  Marquette,  the  two  latter 
constituting  the  Algonkian  for  this  district.  The  Basement  Complex  gives  no  evidence  of  water 
deposition.  The  Lower  Marquette  and  Upper  Marquette  series  are  mainly  sedimentary,  although 
large  masses  of  igneous  rocks  are  included.  Each  of  the  series  of  the  district  consists  of  several 
formations.  The  transgression  of  the  Lower  Marquette  sea  took  place  slowly,  so  that  in  parts  of  the 
district  the  Lower  Marquette  succession  is  incomplete.  In  other  parts  the  succession  is  incomplete 
because  of  inter-Marquette  erosion.  After  the  Upper  Marquette  series  was  deposited  the  district  was 
folded,  faulted,  and  fractured  in  a  complex  fashion,  with  resultant  profound  metamorphism. 

Chapter  I  gives  a  history  of  geological  explorations  in  the  Marquette  district,  and  a  full  summary 
of  previous  literature. 

Chapter  II  treats  of  the  Basement  Complex.  This  occurs  in  two  main  areas,  one  north  of  the 
Marquette  series,  called  the  Northern  Complex,  and  one  south  of  the  Marquette  series,  called 
the  Southern  Complex.  There  are  also  isolated  areas  within  the  Algonkian.  The  Basement  Complex 
is  composed  of  schistose  and  massive  phases  of  crystalline  and  pyroclastic  rocks,  so  different  from  tlie 
Algonkian  sediments  that  there  is  rarely  any  difficulty  in  distinguishing  between  them.  The  schistose 
phases  are  acid,  intermediate,  and  basic.  They  are  cut  by  a  variety  of  massive  igneous  rocks,  basic, 
acid,  and  intermediate,  in  the  forms  of  bosses  and  dikes.  The  rocks  are  undoubtedly  of  widely 
different  ages,  but  we  are  unable  to  separate  them  into  sharply  defined  series  upon  the  basis  of  age. 

The  Northern  Complex  is  treated  under  the  divisions  Mona  schists,  Kitchi  schists,  gneissoid 
granites,  hornblende-syenites,  basic  dikes,  acid  dikes,  peridotite,  and  ferruginous  veins.  The  Mona 
and  Kitchi  rocks  are  greenstone  schists,  which  are  believed  to  be  largely  recrystallized  volcanic 
materials.  Their  original  forms  iucluded  both  tuffs  and  lavas.  Basic  schists  are  predominant,  but 
acid  schists  are  found.  The  Mona  schists  are  uouconglomeratic  green  schists.  The  Kitchi  schists 
contain  numerous  pebble-like  bodies,  which  give  them  in  many  places  a  conglomeratic  appearance. 
The  gneissoid  granites  and  syenites  are  plutonio  intrusive  rocks  within  the  greenstone  schists.  The 
basic  dikes  are  mainly  diabase.  The  majority  of  these  are  schistose,  and  earlier  than  the  upper  beds 
of  the  Marquette  series.  A  few  are  fresh,  and  these  are  probably  of  Keweenawan  age.  The  peridotite 
is  older  than  the  Cambrian  sandstone,  and  younger  than  the  greenstone  schists  of  the  Basement 
Complex.  The  ferruginous  veins  are  believed  to  be  water-deposited,  and  were  formed  previous  to 
the  depositio*!  of  the  Lower  Mar(]uette  series. 

XXI 


XXII  OUTLINE  OF  THIS  MONOCxEAPH. 


The  Southern  Complex  is  treated  under  the  divisions  micaceous  schists,  amphibole-schists,  gneisB- 
oid  granites,  Palmer  gneiss,  and  intrusives.  The  micaceous  schists  include  muscovite-schists,  biotite- 
schists,  feldspathic  biotitc-schists,  and  hornbleudic  biotite-sehists.  The  amphibole-schists  include 
greenstone  schists,  hornblende-schists,  and  micaceous  hornblende-schists.  The  intrusives  are  mainly 
basic  and  acid  dilies.  The  greeenstone  schists,  the  granites,  and  the  dike  materials  are  similar  to  the 
corresponding  rocks  of  the  Northern  Complex.  The  granites  are  intrusive  in  the  schists.  The  isolated 
areas  within  the  Algonkian  are  gneissoid  granite  and  schistose  greenstones,  that  differ  in  no  essential 
respect  from  the  corresponding  rocks  of  the  Northern  Complex  and  Southern  Complex,  The  Base- 
ment Complex  was  deeply  denuded  before  Lower  Marquette  time,  as  is  shown  by  the  fact  that  the 
plutonic  rocks  yielded  their  detritus  to  the  basal  formation  of  the  sedimentary  series. 

Chapter  III  treats  of  the  Lower  Marquette  series.  The  Lower  Marquette  series  is  composed  of 
the  following  formations,  from  tlie  base  upward :  The  Mesnard  quartzite,  the  Kona  dolomite,  the  Wewe 
slate,  the  Ajibik  quartzite,  the  Siamo  slate,  and  the  Negaunee  iron  formation.  For  each  of  these 
formations  the  distribution,  exposures,  topography,  folding,  petrographical  character,  relations  to 
adjacent  formations,  thickness,  and  interesting  localities  are  discussed.  In  treating  the  Negaunee 
iron  formation,  the  iron-ore  deposits  and  prospecting  are  also  considered. 

The  Mesnard  quartzite  f*matiou,  from  110  to  670  feet  thick,  is,  as  the  name  indicates,  chiefly  a 
metamorphosed  sandstone.  However,  in  this  formation  are  other  varieties  of  rock.  At  the  bottom 
is  a  conglomerate,  which  at  most  places,  in  grading  into  the  quartzite,  passes  through  slate  and 
graywaeke.  The  conglomerate  is  basal,  in  any  particular  locality  being  composed  of  coarse  and  fine 
detritus  from  the  immediately  adjacent  rocks  of  the  Basement  Complex.  At  the  top  of  the  formation 
is  a  thin  belt  of  slate.  The  Mesnard  quartzite  ia  the  first  deposit  of  the  transgressing  Lower  Mar- 
quette sea.  By  the  time  the  sea  had  transgressed  a  short  distance  upon  the  Marquette  district  the 
Kona  dolomite  began  to  be  formed,  and  hence  the  Mesnard  formation  is  confined  to  the  eastern  part 
of  the  district.  In  a  large  way  the  Mesnard  formation  is  folded  into  an  east-west  syncliue  witli  a 
westward  pitch.  The  two  limbs  unite  south  of  Marquette  and  complete  a  U.  Superimposed  upon 
this  fold  of  the  first  order  are  close  folds  of  higlier  orders,  running  in  various  directions,  but  more 
continuously  east-west.  The  rocks  of  the  formation  vary  from  those  which  have  been  indurated 
mainly  by  siliceous  cementation  to  those  which  have  been  completely  metamorphosed.  At  various 
places  along  the  contact  horizon  of  the  Marquette  series  and  the  Basement  Complex  the  mashing  and 
shearing  have  been  so  profound  as  to  transform  both  into  crystalline  schists,  which  appear  to  grade 
into  each  other.  The  coarser-grained  kinds  of  the  dynamically  metamorphosed  rocks  are  extensively 
fractured,  while  the  finer-grained  kinds  are  mashed  without  macroscopical  fractures.  Microscopically, 
every  original  particle,  small  or  large,  shows  strain  effects. 

The  Kona  dolomite,  from  425  to  1,375  feet  thick,  is  largely  an  altered  limestone.  The  formation, 
however,  includes  interstratified  layers  of  slate,  graywaeke,  and  quartzite,  with  gradation  phases 
between  these  and  the  pure  dolomite.  The  Kona  dolomite,  like  the  Mesnard  quartzite,  is  confined  to 
the  eastern  part  of  the  district.  The  formation  has  been  folded  in  a  complex  manner,  the  folds  running 
east-west  and  north-south.  Consequent  upon  the  folding  and  the  different  resisting  powers  of  its 
layers,  the  topography  of  the  formation  is  exceedingly  rough.  When  deformed  the  dolomite  yielded 
in  most  places  without  prominent  fractures  or  cleavage,  but  in  the  interstratified  slates  cleavage 
was  developed  in  many  places,  and  the  graywackes  and  quartzites  were  fractured  or  brecciated. 
The  dolomite  varies  through  a  slate  into  the  Mesnard  quartzite  below,  and  by  a  lessening  of  the 
calcareous  constituent  gradually  passes  into  the  Wewe  slate  above. 

The  Wewe  slate,  550  to  1,050  feet  thick,  is  chiefly  a  metamorphosed  mudstone.  With  the  slates 
are,  however,  conglomerates,  quartzites,  graywackes,  mica-slates,  and  mica-schists.  The  Wewe  slate, 
like  the  two  previous  formations,  is  confined  to  the  eastern  part  of  the  district.     The  formation  in  a 


OT  TUNE  OF  THIS  MONOGRAPH.  XXIII 

large  way  is  folded  into  a  jjreat  westward-pluuging  syucliue,  and  upon  this  major  fold  are  superim- 
posed folds  of  a  higher  order.  The  western  boundary  of  the  Wewe  formation  was  the  western  limit 
of  the  seashore  at  the  time  of  the  deposition  of  this  formation,  and  thus  basal  conglomerates  are  here 
found,  which  in  all  respects  are  similar  to  those  of  the  Mesnard  formation,  being  composed  chiefly  in 
each  case  of  detritus  derived  from  the  immediately  subjacent  rocks  of  the  Archean.  As  the  result  of 
the  folding,  the  slates  show  very  generally  a  cleavage  or  lissility,  but  in  many  places  where  they  are 
coarse  or  brittle  they  are  fractured  through  and  through,  or  even  transformed  into  reibungsbreccias. 
The  formation  grades  into  the  Kona  dolomite  below  and  into  the  Ajibik  quartzite  above. 

The  Ajibik  quartzite,  from  700  to  900  feet  thick,  is  mainly  a  metamorphosed  sandstone.  The  time  of 
the  Ajibik  quartzite  marks  a  rapid  advance  of  the  Lower  Marquette  sea,  and  therefore  the  formation 
extends  to  the  western  end  of  the  district.  In  the  eastern  part  of  the  area  the  Ajibik  quartzite  grades 
down  into  the  Wewe  slate,  but  for  the  major  portion  of  the  district  it  rests  unconformably  upon  the 
Basement  Complex.  At  many  localities  contacts  and  basal  couglomerates  are  known.  In  some  places 
the  metamorphism  has  been  so  severe  as  to  transform  the  Basement  Comjilex  and  the  Ajibik  quartzite 
into  crystalline  schists,  with  parallel  structures.  The  quartzite  grades  above  either  into  the  Siamo 
slate  or  into  the  Negaunee  iron  formation.  In  dilferent  parts  of  the  district,  depending  iiijon  various 
conditions,  the  original  sandstone  has  been  transformed  into  quartzite,  cherty  quartzite,  ferruginous 
quartzite,  ferruginous  cherty  quartzite,  quartz-rock,  and  qu.artzite-breccia.  Some  of  the  dynamic 
breccias  so  closely  resemble  ordinary  conglomerate  as  to  deserve  the  name  pseudo-conglomerate. 

The  Siamo  slate,  from  600  to  1,200  feet  thick,  is  chiefly  an  altered  mudstone,  although  locally  it 
was  a  grit  or  sand-rock,  which  has  subsequently  been  changed  to  gray  wacke  or  quartzite.  The  larger 
area  of  the  formation  is  confined  to  the  eastern  part  of  the  district,  although  a  belt  runs  near  the 
north  side  of  the  Maniuette  series  to  the  west  cud  of  the  district.  The  major  folding  is  similar  to 
that  of  the  other  formations.  Superimposed  upon  the  larger  folds  are  secondary  folds,  and  these  at 
various  places  are  nionoclinal.  The  formation  is  very  generally  affected  by  a  cleavage  or  fissility,  and 
in  the  case  of  the  monoelinal  folds  the  cleavage  is  inclined  in  the  same  direction  as  the  axial  planes 
of  the  folds.  The  Siamo  slate  grades  into  the  Ajibik  quartzite  below  and  into  the  Negaunee  iron 
formation  above. 

The  Negaunee  formation,  from  1,000  to  1,500  feet  thick,  is  nonfragmental,  heavily  ferruginous 
throughout,  and  contains  the  greater  iron-ore  deposits  of  the  district.  It  is  therefore  called  the  iron- 
bearing  formation.  Large  quantities  of  intrusive  greenstones  are  associated  with  the  formation,  the 
masses  of  which  vary  in  magnitude  from  great  bosses  2  miles  or  more  long  and  a  half  mile  wide  to 
small  dikes.  The  largest  area  of  the  Negaunee  formation  is  in  the  east-central  part  of  the  district. 
From  this  area  two  lielts  extend  west  to  the  western  end  of  the  district.  Upon  the  whole,  the  forma- 
tion is  soft,  and  occupies  lowlands  between  the  more  resistant  greenstones  and  the  Ajibik  quartzite. 
The  formation  is  underlain  by  the  Siamo  slate  or  Ajibik  quartzite,  into  which  it  grades,  and  is  over- 
lain uncomformably  by  the  Upper  Marquette  series.  Petrographically  the  formation  comprises  sideritic 
slate,  which  may  be  griineritic,  magnetitic,  hematitic,  or  limonitic ;  griinerite-magnetite-schist ;  ferru- 
ginous slate;  ferruginous  chert;  jaspilite;  and  iron  ore.  The  ferruginous  chert  and  jaspilite  are  fre- 
quently brecciated ;  the  other  kinds  less  frequently.  The  sideritic  slate  is  the  original  form  from  which 
the  other  varieties  of  rock  have  developed.  The  griinerite-magnetite-schists  were  formed  hy  partial 
recrystallization  of  the  silica,  by  oxidation  of  the  iron  oxide  in  part  to  magnetite,  by  the  union  of  a 
part  of  the  silica  and  iron  protoxide,  producing  griinerite,  and  with  the  loss  of  carbon  dioxide.  The 
ferruginous  slates  are  the  direct  result  of  the  decomposition  of  the  iron  carbonate  and  the  peroxidation 
of  the  iron  with  partial  or  complete  recrystallization  of  the  silica.  The  ferruginous  cherts  difter  from 
the  ferruginous  slates  in  that  the  iron  oxide  and  the  chert  are  largely  concentrated  into  alternate 
bands.     The  jaspilites  difl'er  from  the  ferruginous  cherts  in  that  each  of  the  quartz  grains  of  the  chert 


XXIV  OUTLINE  OF  THIS  MONOGRAPH. 

bands  is  stained  red  by  included  liematite.  The  iron  ores  resulted  from  the  conceutration  of  the  iron 
oxides  through  the  agency  of  downward-percolating  waters.  These  concentration-bodies  usually  occur 
upon  impervious  basements  in  pitching  troughs.  The  pitching  troughs  are  formed  by  the  Siamo  slate, 
the  Ajibik  quartzite,  a  mass  or  dike  of  greenstone,  or  by  some  combination  of  these.  The  ore  deposits 
are  likely  to  be  of  large  size  where  as  a  result  of  the  folding  the  iron-bearing  formation  is  much  frac- 
tured, thus  permitting  the  ready  action  of  percolating  waters.  The  ore  deposits  occur  at  the  bottom 
of  the  Negaunee  formation,  within  the  Negaunee  formation,  and  at  the  junction  plane  between  the 
Negaunee  formation  and  the  overlying  Ishpeming  formation.  From  the  position  of  the  ore  deposits 
above  the  impervious  formations  it  is  concluded  that  their  concentration  occurred  during  or  subsequent 
to  the  folding  which  took  place  later  than  Upper  Marquette  time. 

Chaptek  IV  treats  of  the  Upper  Marquette  series.  This  series  is  composed  of  the  following 
formations,  from  the  base  upward:  The  Ishpeming  formation,  the  Michigamme  formation,  the  Clarks- 
burg formation.  As  in  the  case  of  the  Lower  Marquette  series,  for  each  fonuation  the  distribution, 
exposures,  topography,  folding,  relations  to  adjacent  formations,  petrographical  character,  thickness, 
and  interesting  localities  are  discussed. 

The  Ishpeming  formation  includes  two  classes  of  rock,  which  are  called  the  Goodrich  quartzite 
and  the  Bijiki  schist.  These  rocks  are  sufficiently  different  to  have  different  formation  names,  but 
the  Bijiki  schist  for  the  west  end  of  the  district  occupies  a  part  of  the  horizon  of  the  Goodrich 
quartzite  in  the  central  part.  The  quartzite,  from  600  to  1,550  feet  thick,  is  confined  to  the  central  and 
■western  parts  of  the  district.  The  main  area  of  the  formation  is  folded  into  a  synclinorium,  which,  as 
a  result  of  a  western  pitch,  terminates  to  the  east  at  Ishpeming.  On  account  of  the  resistant  character 
of  the  formation,  for  much  of  the  district  it  constitutes  a  ridge  separating  the  less  resistant  Negaunee 
formation  below  and  Michigamme  formation  above.  The  Goodrich  quartzite  rests  unconformably  upon 
the  Negaunee  formation.  For  the  greater  jiart  of  its  area  it  grades  up  into  the  Michigamme  or  Clarks- 
burg formations,  but  in  the  northwestern  part  of  the  district  it  passes  into  the  Bijiki  schist.  The 
least  metamorphosed  rocks  are  quartzites  and  quartzite-conglomerates,  the  grains  of  which  show  pres- 
sure effects.  The  more  metamorphosed  rocks  have  been  so  mashed  as  to  have  become  schist-conglom- 
erates and  micaceous  quartz-schists.  Where  the  formation  rests  upon  the  Archean  the  mica-schists  and 
mica-gneisses  also  occur.  Between  the  various  kinds  there  are  all  gradations.  At  the  base  of  th^ 
Goodrich  quartzite  is  a  basal  conglomerate.  For  the  major  part  of  the  district  this  conglomerate  rests 
upon  the  Negaunee  formation.  Its  detritus  is  therefore  derived  mainly  from  that  formation,  and  the 
rock  is  an  ore,  chert,  jasper,  and  quartz  conglomerate.  At  a  few  places  the  Archean  rocks  are  subjacent, 
and  their  materials  predominate  in  the  conglomerate.  The  Bijiki  schist,  from  0  to  520  feet  thick,  is  con- 
fined to  the  western  part  of  the  district.  The  rock  is  a  banded  grUuerite-magnetite-schist,  which  has 
been  derived  by  metasomatic  and  dynamic  processes  from  an  impure  siderite. 

The  Bijiki  schist  grades  into  the  Goodrich  ijuartzite  below  and  into  the  Michigamme  formation 
above. 

The  Michigamme  formation  occurs  in  a  single  large  belt,  stretching  from  the  center  to  the  western 
end  of  the  district.  It  is  folded  into  a  great  composite  syncliue  at  the  center  of  the  Marquette  syn- 
clinorium. The  rocks  were  originally  ferruginous  and  nonferrugiuous  muds  and  grits.  These  have 
been  altered  to  slates,  graywackes,  mica-schists,  and  mica-gneisses.  In  this  transformation  the  feld- 
spars have  decomposed  to  quartz  and  mica,  the  fragmental  quartz  has  been  granulated,  and  the  most 
metamorphosed  of  the  resultant  rocks  are  foliated,  completely  crystalline  schists.  The  formation 
grades  below  into  the  Goodrich  quartzite,  Bijiki  schist,  or  Clarksburg  formation.  Its  thickness  is  very 
considerable,  probably  2,000  feet  or  more,  but  no  accurate  estimate  can  be  given. 

The  Clarksburg  formation  differs  from  the  other  formations  of  the  Marquette  series  in  that  its  pre- 
dominant rocks  are  composed  of  volcanic  materials.     The  formation  embraces  basic  lava  flows,  tuffs. 


OUTLINE  OF  Tins  MONOGRAPH.  XXV 

ashes,  and  breccias,  which  locally  are  interleave.l  with  or  grade  into  gray  wacke,  slate,  or  conglomerate. 
Much  of  the  material  has  been  profoundly  metamorphosed,  and  schist-conglomerates,  mica-schists, 
and  hornblende-schists  have  resulted.  All  of  the  foregoing  rocks  are  cut  by  dikes  and  masses  of 
greenstone.  The  formation  is  confined  to  the  south-central  part  of  the  district.  It  grades  into  the 
Ishpeming  formation  or  the  Michigamme  formation  below  and  into  the  Mi.higamme  formation  above. 
Ko  accurate  estimate  of  the  thickness  of  the  formation  can  be  given.  The  volcanic  material  was 
poured  out  from  a  number  of  vents,  the  more  important  ones  which  have  been  recognized  being  located 
near  Clarksburg,  Greenwood,  and  Champion. 

Chapter  V  treats  of  the  igneous  rocks  of  the  Marquette  series  not  belonging  to  the  Clarksburg 
formation.  These  rooks  are  all  b.asic,  having  the  composition  of  altered  diabases.  As  to  age,  they  are 
divided  into  pre-Clarksburg  greenstones  and  post-Clarksburg  greenstones.  The  older  rocks  occur 
as  dikes,  bosses,  sheets,  and  tutf  beds,  although  the  latter  two  are  subordinate.  They  vary  from 
rather  fresh  diabases  to  schistose  rocks  which  are  micaceous  hornblende-schists,  chlorite-schists,  or 
talc-schists.  The  more  metamorphosed  forms  are  most  abundant  on  the  peripheries  of  the  masses, 
and  especially  close  to  the  rocks  of  the  Negaunee  formation,  and  such  rocks  are  often  heavily  ferrugi- 
nous. The  post-Clarksburg  greenstones  comprise  only  dikes  and  bosses.  They  are  much  fresher  than 
the  older  o-reenstones,  being  mainly  nonfoliated.  Their  alterations  are  chiefly  metasomatic.  They  com- 
prise olivine-diabases,  quartz-diabases,  porphyrites,  and  basalts.  It  is  conjectured  that  these  rocks 
are  correlative  with  the  eruptives  of  the  Keweenawan  series. 

Chapter  VI  treats  of  the  Republic  trough.  This  is  an  isoclinal  syncline,  extending  southeast 
from  the  western  end  of  the  district.  The  rocks  of  the  Republic  area  belong  to  the  Archean,  Lower 
Marquette,  and  Upper  Marquette  series.  The  Archean  rocks  comprise  granites,  gneisses,  and  crystal- 
line schists.  The  schistose  structure  is  especially  developed  adjacent  to  the  Algonkian  rocks.  The 
Lower  Marquette  series  includes  the  Ajibik  quartzite  and  the  Negaunee  formation.  The  Ajibik 
quartzite,  probably  not  exceeding  100  feet  in  thickness,  rests  unconformably  upon  the  Archean.  At 
one  place  a  coarse  basal  conglomerate  is  found  in  direct  contact  with  the  rocks  of  the  underlying 
series.  For  the  most  part  the  formation  has  been  transformed  to  a  micaceous  vitreous  (luartzite  or 
into  a  mica-schist.  The  Negaunee  formation  consists  of  two  horizons,  a  griinerite-magnetite-schist 
below,  and  a  specular  jasper  above.  The  Upper  Marquette  series  consists  of  the  Goodrich  quartzite 
and  the  Michigamme  mica-schist.  The  Goodrich  rock  has  been  transformed  to  a  quartz-schist  or  to 
a  micaceous  quartz-schist.  In  the  southeastern  part  of  the  trough,  at  the  bottom  of  the  Goodrich 
quartzite,  is  a  great  conglomerate,  the  detritus  of  which  is  derived  mainly  from  the  underlying  Negau- 
nee formation;  also  there  is  a  difference  in  dip  between  the  Goodrich  quartzite  and  the  Negaunee 
formation.  From  these  facts  it  is  certain  that  the  two  are  unconformable.  The  Michigamme  mica- 
schist  occupies  the  center  of  the  tongue.  This  grades  down  into  the  Goodrich  quartzite.  Basic 
intrnsives  occur  in  both  the  Upper  and  the  Lower  Marquette  series,  the  same  as  in  the  remainder  of 
the  district.  A  fault  with  hade  nearly  but  not  quite  parallel  to  the  bedding  occurs  on  the  eastern 
aide  of  the  trough  near  Republic.  The  iron-ore  deposits  are  at  the  contact  of  the  Ishpeming  and 
Negaunee  formations  or  within  the  Negaunee  formation.  The  important  deposits  are  at  the  end  of  the 
tro°  gh,  and  especially  at  the  bottom  of  subordinate  plunging  syuclmes.  They  are  secondary  concen- 
trations, produced  by  downward-percolating  waters. 

Chapter  VII  treats  of  the  general  geology,  and  involves  a  consideration  of  the  B.asement  Com- 
plex, the  Lower  Marquette,  and  the  Upper  Marquette  series.  The  Lower  Marquette  has  a  possible 
maximum  thickness  of  6,120  feet,  but  it  is  not  probable  that  any  single  section  will  give  as 
great  a  thickness  as  5,000  feet.  The  Upper  Marquette  series,  excluding  the  volcanics,  is  probably 
less  than  5,000  feet  thick;  including  the  volcanics  it  is  probably  more  than  5,000  feet  thick. 
Before  the  beginning  of  the  deposition  of  the  Lower  Marquette  series  the  Basement  Complex  had 
been  deeply  eroded.     The  transgression  h-.rizon  of  the  Lower  Marquette  series  is  a  conglomerate, 


XXVI  OUTLINE  OF  THIS  MONOGRAPH. 

which  quickly  passes  upward  into  a  quartzite.  As  the  sea  occupied  a  considerable  time  in  advancing 
over  the  land  area,  and  as  the  advance  was  from  the  east,  several  formations  were  deposited  in  the 
eastern  part  of  the  district  before  the  entire  area  was  submerged.  These  formations  are  the  Mesnard 
quartzite,  the  Kona  dolomite,  and  the  Wewe  slate.  Thus  we  have  these  formations  and  the  overlying 
Ajibik  quartzite  overlapping  one  another  to  the  west.  It  follows  that  the  transgression  horizon  is 
somewhat  arbitrarily  divided  between  four  formations. 

The  Lower  Marquette  series  rests  unconformably  upon  the  Archean.  This  is  shown  at  many 
localities  by  numerous  unconformable  contacts  and  great  basal  conglomerates,  the  main  part  of 
the  detritus  being  in  each  case  identical  with  the  rocks  of  the  Basement  Complex  at  that  locality. 
Within  the  sediments  of  the  Lower  Slaniuette  series  are  a  few  thin  lava  beds,  showing  volcanic 
activity  iu  Lower  Marquette  time.  After  the  deposition  of  the  Lower  Marquette  series  the  land  was 
raised  above  the  sea,  and  erosion  set  in  and  continued  for  a  long  time.  The  denudation  was  deep 
enough  in  some  places  to  remove  the  entire  Lower  Marquette  series.  The  Upper  Marquette  series  was 
therefore  deposited  unconformably  upon  the  Lower  Marquette  series  and  the  Archean.  For  the  major 
part  of  the  district  the  immediately  subjacent  rocks  belong  to  the  Negauneee  formation,  and  the  basal 
conglomerate  for  this  area  consists  mainly  of  detritus  derived  from  that  formation.  For  smaller 
areas  other  formations  of  the  Lower  Marquette  series  or  the  rocks  of  the  Basement  Complex  underlie 
the  Upper  Marquette  rocks,  in  which  cases  the  detritus  is  derived  mainly  from  them.  Above  the 
conglomerate,  the  iirst  deposit  of  the  advancing  Upper  Marquette  sea,  a  sandstone  was  piled  up, 
which  was  later  transformed  to  the  Goodrich  quartzite.  In  the  western  part  of  the  district,  above 
this  came  a  sideritic  slate,  which  has  been  changed  to  a  griinerite-magnetite-schist — the  Bijiki  schist. 
Above  the  Bijiki  schist  followed  the  muds  and  volcanics  which  have  been  transformed  respectively  to 
the  rocks  of  the  Michigamme  and  Clarksburg  formations.  Within  the  Lower  and  Upper  Marquette 
series  are  abundant  intrusives. 

After  the  deposition  of  the  Upper  Marquette  series  the  three  series  of  the  district  were  folded 
together.  The  folding  is  of  a  complex  character.  The  largest  but  least  conspicuous  folds  have  an 
east-west  direction.  The  major  east-west  fold  is  a  great  synclinorium,  which  in  the  central  part  of 
the  area  is  of  the  abnormal  type.  Upon  the  primary  folds  are  secondary  ones,  upon  these  tertiary  ones, 
and  BO  on  to  microscopic  jilications  in  the  case  of  the  finer-grained  rocks.  The  more  plastic  forma- 
tions yielded  mainly  by  flowage ;  the  less  plastic  formations  yielded  partly  by  fracturing,  although  in  a 
large  way  obeying  the  general  folding  of  the  district.  At  many  places  the  fracturing  was  so  complete 
as  to  produce  reibungsbreccias,  or  even  pseudo-conglomerates.  A  microscopical  study  shows  that  not 
a  cubic  inch  of  material  has  escaped  dynamic  action.  Every  original  grain  of  fair  size  gives  evidence 
of  interior  movement.  The  rocks  have  been  kneaded  throughout.  As  a  result  of  the  dynamic  action, 
there  has  also  been  faulting,  but  with  two  or  three  exceptions  the  faults  are  so  small  as  to  be  unim- 
portant. The  manner  in  which  tlie  rocks  have  responded  to  deformation  shows  that  when  folded  they 
were  iu  the  zone  of  combined  fracture  and  flowage.  It  is  believed  that  they  were  buried  under  a 
thickness  of  several  thousand  feet  of  sediments,  perhaps  as  much  as  10,000  feet. 

The  various  formations  of  the  Marquette  series,  as  a  result  of  the  dynamic  and  other  processes, 
were  metamorphosed  in  different  ways,  dependent  upon  their  composition  and  position.  In  the  softer 
rocks  cleavage  and  fissility  were  generally  developed,  and  the  rocks  were  extensively  transformed  to 
slates  or  schists.  In  the  harder  rocks  these  structures  are  less  prevalent,  although  at  many  places 
along  the  major  planes  of  accommodation,  and  especially  at  the  contacts  between  the  Basement  Com- 
plex or  Archean  and  the  Marquette  series  between  the  Lower  Marquette  and  Upper  Marquette  series, 
they  also  have  been  transformed  into  crystalline  schists.  At  the  former  place  the  Archean  rocks  have 
been  transformed  into  similar  crystalline  schists.  The  rooks  upon  opposite  sides  of  a  contact  have  par- 
allel schistosity,  and  therefore  there  are  here  apparent  grailations  between  the  unconformable' Lower 


OUTLINE  OF  THIS  MONOGRAPH.  XXVII 

Marquettr  and  Upper  Marquette  series,  aud  between  the  uuconformable  Lower  Maniuette  st-rics  and 
the  Archean.  After  the  last  period  of  dynamic  metamorphism  it  is  believed  that  the  crystals  of  horn- 
blende, garnet,  staurolite,  andalusite,  and  chloritoid,  which  are  now  unstrained,  developed,  under 
quiescent  conditions.  The  metamorphism  is  much  more  profound  in  the  -western  part  of  the  district 
than  in  the  eastern  aud  central  parts.  This  variation  in  metamorphism  corresponds  with  the  closeness 
of  folding. 

During  and  subsequeut  to  the  later  folding  of  the  district  the  area  has  undergone  vast  denudation. 
The  character  of  the  folding  shows  that  great  mountain  masses  must  have  been  produced,  which  have 
been  now  reduced  to  approximate  plains,  so  that  the  district  is  merely  bluffy. 

The  Lower  Marquette  and  Upper  Marquette  series  are  correlated  with  the  Lower  Uuronian  aud 
Upper  Huronian  series  of  the  north  shore  of  Lake  Superior.  They  are  also  correlated  with  the  Lower 
Menominee  and  Upper  Menominee  series.  The  correspondence  of  tlie  formations  of  the  Manjuette 
series  to  those  of  the  Menominee  series  is  only  approximate. 


THE  MARQUETTE  IRON-BEARING  DISTRICT  OF 
MICHIGAN. 


By  C.  R.  Van  ni«B  and  W.  S.  Bayley. 


INTRODUCTION. 

This  report  is  a  final  account  of  the  Marquette  district,  the  oldest 
important  iron-producing  area  of  the  Lake  Superior  region.  Already  two 
detailed  reports  have  been  issued  upon  it  by  the  Michigan  State  survey. 
The  lirst,  l)y  jMaj.  T.  B.  Brooks,  published  in  1873,  was  a  faithful  account 
of  the  structural  and  economic  geology  of  the  part  of  the  district  producing 
iron  ore  at  that  time,  no  attempt  being  made  to  completely  map  the  area. 
The  intrusive  character  of  most  of  the  greenstones  and  the  physical  break 
existing  between  the  Upper  Marquette  and  Lower  Marquette  series  were 
not  recognized.  While  for  closely  studied  localities  Major  Brooks's  map- 
ping is  remarkably  accurate,  it  was  not  possible  •under  the  circumstances 
to  fully  determine  the  general  succession.  The  second  report,  by  Dr. 
Carl  Rominger,  was  published  in  188L  This  report  is  accompanied  by  an 
areal  map  of  the  district  from  Lake  Superior  to  1  mile  into  R.  28  W.  The 
topography  is  carefully  indicated  by  hachures,  and  the  areal  distribution  of 
the  more  important  formations  is  delineated  with  a  fair  degree  of  accuracy, 
showing  that  the  district  had  been  traversed  with  g-reat  patience.  However, 
all  quartzites  are  placed  together,  without  reference  to  their  age,  and  the 
same  is  true  of  the  slates.  The  map  is  not  accompanied  by  any  sections. 
It  is  therefore  to  be  considered  as  a  lithological  rather  than  a  structural  map. 
Many  other  papers  upon  the  Marquette  district,  of  greater  or  less  impor- 
tance, have  been  published  by  Wadsworth,  Ir\'ing,  Pumpelly,  and  others. 
MON  xxviii 1  1 


2  THE  MARQUETTE   IRON-BEARTNG  DISTRICT. 

A  preliminary  report  has  been  published  l^y  us  in  the  Fifteenth  Annual 
Report  of  the  United  States  Geological  Survey. 

The  present  report  is  based  upon  a  detailed  examination  of  the  entire 
Alg-onkian  area  from  'Lake  Superior  to  Lake  Michiganmie.  Topographic 
maps  of  a  part  of  the  district,  made  by  the  United  States  Geological  Survey, 
have  been  supplemented  in  critical  areas  by  large-scale  plane-table  sheets. 
Practically  all  outcrops  have  been  accurately  mapped  on  a  large  scale. 
In  the  mining  part  of  the  area  advantage  has  been  taken  of  imderground 
workings  and  borings.  Owing  to  the  detailed  character  of  this  work, 
combined  with  the  advance  of  geological  knowledge  in  the  past  twenty 
years,  it  is  now  possible  to  present  a  much  more  satisfactory  account  of  the 
structure  of  the  district  than  has  yet  been  given. 

Notwithstanding  the  fact  that  mining  has  been  done  for  many  )-ears 
in  the  district,  it  is  little  traveled  away  from  the  roads.  The  timber  has 
been  cut  off  for  iron  smelting.  Where  the  cut  has  been  comparatively  recent 
the  fires  have  run,  and  there  is  now  a  tangle  of  fallen  timber  and  briars 
and  bushes.  "Where  the  cut  is  older  there  is  a  thick  second  growth,  20  to 
50  feet  high.  The  area  is  therefore  much  more  difficult  to  penetrate  than 
was  the  primeval  forest.  Moreover,  the  bushes  are  an  effectual  bar  to 
extended  vision,  except  from  high,  rocky  points.  While  the  district  is  not 
mountainous,  in  detail  much  of  it  is  exceedingly  rough,  so  that  in  travers- 
ing parts  of  it  one  is  nearly  always  ascending  or  descending  a  steep  slope. 
Other  parts  are  covered  by  a  mantle  of  glacial  deposits,  through  which  the 
rocks  rarely  protrude.  Because  of  the  ii-regularity  of  the  topography  and 
the  difficulty  of  seeing,  it  has  been  impossible  to  base  locations  on  the 
ordinary  topographic  maps.  For  the  larger  part  of  the  district  locating 
was  done  either  with  the  aid  of  the  plane-table  or  by  pacing  from  section 
corners  and  quarter  posts. 

The  rocks  of  the  district  comprise  three  series,  separated  by  uncon- 
formities. Each  of  these  series  consists  of  several  formations.  The 
transgression  of  the  sea  did  not  occur  over  the  entire  district  even 
approximately  at  the  same  time,  so  that  in  parts  of  it  the  succession  is  not 
complete,  and  in  other  parts  the  succession  is  incomplete  because  of  inter- 
vening erosion.  Finally,  the  district  has  been  folded  in  a  complicated 
fashion  in  two  directions,  with  resulting  profound  metamorphism. 


INTRODUCTION.  3 

It  is  plain  from  the  foregoing  that  the  district  is  one  of  exceeding 
ditttcuhy,  and  that  it  has  been  possible  to  unravel  its  intricacies  only  by 
patient  and  laborious  work. 

The  Algonldan  of  the  Marquette  district  is  divisil)lu  into  two  series, 
presumably  the  equivalents  of  the  Lower  Huronian  and  the  Upper  Huro- 
niau  of  other  disti-icts  of  the  Lake  Superior  region  and  of  the  Original 
Huronian  of  Canada.  These  two  divisions  are  separated  by  an  unconform- 
ity. In  this  paper  the  lower  clastic  series  of  the  Marquette  area  will  be 
called  the  Lower  ]\Iarquette  series,  and  the  upper  the  Upper  Marquette 
series.  The  Algonkian  rocks  are  bounded  on  the  north  and  on  the  south 
by  the  Basement  Complex,  or  Archean.  The  Archean  consists  of  an  intri- 
cate mixture  of  granite,  gneisses,  schists,  and  surface  volcanics.  All  are 
thoroughly  crystalline. 

The  Lower  Marquette  series  covers  the  larger  part  of  the  area  of 
Algonkian  rocks  east  of  Ishpeming,  and  forms  belts  on  the  north  and  south 
sides  of  the  Algonkian  area  west  of  Ishpeming.  The  easternmost  Upper 
Marquette  rocks  appear  at  Negaunee  and  at  Palmer.  Here,  however,  they 
are  in  patches,  the  east  end  of  the  main  area  appearing  at  Ishpeming. 
From  this  place  west  the  Upper  Marquette  rapidly  widens.  At  Lake  Michi- 
gamme  the  Algonkian  expands  into  a  broad  area,  from  which  several  arms 
extend.  In  each  of  these  arms  the  lower  series  occupies  the  outer  borders 
of  the  Algonkian  belts,  the  upper  series  appearing  in  the  centers. 

The  area  discussed  in  the  present  paper  is  limited  on  the  west  by  the 
east  mile  of  R.  31  W.,  and  on  the  south  by  T.  46  N.,  with  the  exception 
that  the  southern  extremity  of  the  Republic  tongue  extends  into  T.  45  N. 
The  Algonkian  rocks  of  this  area,  speaking  broadly,  are  in  a  great  syn- 
clinorium.  This  synclinorium  is  of  a  peculiar  and  complicated  character", 
which  will  be  fully  considered  later.  It  is  suflficient  here  to  say  that  in  tlie 
middle  of  the  district  the  rocks  in  the  outer  borders  of  the  Algonkian  belt 
are  in  a  series  of  sharply  overturned  folds.  The  Algonkian  rocks  on  either 
side  of  the  trough  have  moved  over  the  more  rigid  Archean  granite,  and,  as 
a  consequence,  on  each  side  of  the  Algonkian  trough  a  series  of  overfolds 
plunge  steeply  toward  its  center,  producing  a  structure  resembling  in  this 
respect  the  composed  fan  structure  of  the  Alps.  There  is,  however,  this 
great  difference  between  the  Marcjuette  structure  and  that  of  the  Alps,  that 


4  THE   MAEQUETTE    IRON-BEARING    DISTRICT. 

in  passing  from  tlie  sides  of  the  trough  toward  the  center  newer  rock  appear 
rather  than  older  ones,  so  that  in  the  center  of  the  synclinorium  the 
youngest  rocks  are  found.  It  is  as  if  the  composed  fan  folds  of  the  Alps 
were  sagged  downward,  so  that  the  structure  as  a  whole  is  a  synclinorium 
rather  than  an  anticlinorium.  The  structure  thus  differs  from  the  composed 
fan  structure  of  the  Alps  and  from  the  •  inverted  intermont  trough  of 
Lapworth,  Jind  may  be  called  an  abnormal  synclinorium^  (fig.  1).  This 
structure  prevails  in  the  central  part  of  the  area  from  Ishpeming  and 
Negaunee  westward  to  Clarksburg,  but  it  does  not  extend  to  Lake  Superior 
on  tlie  east  nor  to  Lake  Michigamme  on  the  west. 


While  the  more  conspicuous  folds  have  in  general  an  east-west  du-ection, 
the  rocks  have  also  been  under  strong  east-west  compression,  as  a  conse- 
quence of  which  the  folds  are  buckled  so  that  they  often  show  a  steep  pitch. 
In  places  the  north-south  folds  become  more  prominent  than  the  east-west 
folds,  and  control  the  prevalent  strikes  and  dips.  In  an  intermediate  area 
the  two  series  of  folds  are  about  equally  important,  thus  producing-  most 
irregular  strikes  and  dips.  These  north-south  folds  are  of  two  orders:  the 
first  of  great  magnitude,  but  small  dip;  the  second,  superimposed  on  the 
first,  of  less  length  of  wave,  but  with  steeper  dip. 

'Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Vau  Hise:  Sixteentb  Ann.  Rept. 
U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  612, 616-620. 


CHAPTER   I. 


By  W.  S.  Bayley. 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE. 

That  portion  of  the  Lake  Superior  region  known  as  the  Marquette  iron 
range  was  brought  to  the  notice  of  geologists  by  the  early  navigators  of 
the  lakes  during  the  first  quarter  of  the  present  century,  and  to  the  indus- 
trial connnunity  in  the  year  1850,  when  the  surveyors  of  the  Chippewa 
land  district  announced  the  discovery  of  great  masses  of  iron  ore  in  the 
valley  of  the  Carp  River.  The  literature  which  deals  with  the  geology  of 
the  district  far  exceeds  in  volume  that  devoted  to  all  the  other  iron-ore- 
producing  areas  in  Michigan,  Minnesota,  and  Wisconsin  taken  together. 
This  is  due  to  the  early  discovery  of  the  district,  to  its  importance  as  an 
ore  producer,  to  its  comparatively  easy  accessibility,  and  to  the  fact  that  its 
geolog}'  is  so  complicated  as  to  have  afforded  data  for  many  different  theories 
concerning  it.  These  theories  have  given  rise  to  frequent  and  sometimes 
violent  discussions,  and  though  many  have  been  proved  untenable,  they 
have  done  much  toward  the  development  of  correct  notions  of  the  origin 
and  the  geological  relationships  of  the  various  rocks  occurring  in  the  district. 

The  history  of  the  literature  on  the  Marquette  district  may  be  divided 
into  four  periods,  as  follows:  The  first  extending  from  the  year  1820  to  1850, 
the  date  of  the  appearance  of  Foster  and  Whitney's  joint  report;  the  second 
beginning  in  1850  and  ending  with  the  establishment  of  the  Michigan  survey 
in  1870;  the  third  beginning  in  1870  and  ending  with  the  publication  of 
Rominger's  report  in  1881;  and  the  fourth  embracing  the  time  that  has 
elapsed  since  1881. 


6  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

The  work  of  the  first  period  was  mainly  preUminary.  It  began  with 
the  general  notes  of  the  early  navigators  and  explorers,  and  ended  with  the 
statement  of  the  general  features  of  the  Marquette  geology  as  given  in 
Foster  and  Whitney's  report.  This  was  a  period  of  great  activity.  It 
included  the  Avi-itings  of  the  navigators  or  explorers,  Schoolcraft  and  Bay- 
field, of  the  first  State  geologist  of  Michigan,  Dr.  Houghton,  and  of  the 
United  States  surveyors  and  geologists,  Cunningham,  Gray,  Locke,  Chan- 
ning,  Burt,  Hubbard,  Jackson,  Foster,  and  Whitney.  The  various  reports 
made  were  devoted  to  a  general  discussion  of  the  geology  of  the  district 
and  to  the  classification  of  the  rocks  found. 

The  second  period  was  not  so  fruitful  of  results.  The  only  notable 
paper  published  during  this  time  was  that  of  Kimball,  which  appeared  in 
1865.  Whitney,  Rivot,  Whittlesey,  Hunt,  Bigsby,  and  Credner  also  co'u- 
tributed  to  the  discussion.  Very  little  that  was  new  was  added  to  the 
knowledge  of  the  region,  the  principal  articles,  with  the  exception  of  the  one 
bv  Kimball,  being  in  confirmation  or  in  contradiction  of  the  Aarious  points 
raised  by  Messrs.  Foster  and  Whitney — mainly  with  respect  to  the  di^-isi- 
bility  of  the  "Azoic"  series.  In  Kimball's  article  the  sequence  of  the  rock 
beds  in  several  portions  of  the  iron  range  is  noted,  and  a  theory  of  the 
origin  of  the  iron  ores,  in  opposition  to  that  advanced  b}-  Fo.ster  and 
Whitney,  is  advanced. 

The  third  period  was  again  a  time  of  activity,  made  so  through  the 
eff"orts  of  the  Michigan  and  Wisconsin  surveys.  Alexander  Winchell, 
Brooks,  Julien,  Wright,  Wichmann,  and  Rominger  contributed  to  the  State 
reports,  while  Wads  worth,  Crosby,  and  others  published  in  the  various 
journals.  In  this  period  the  publications  took  a  much  wider  range  than  in 
preceding  periods.  Brooks  and  Rominger  each  published  length)^  reports 
dealing  with  the  general  geology  of  the  entire  district.  Detailed  descrip- 
tions of  observations  are  the  rule  in  all  the  papers  written,  and  the  con- 
clusions are  based  on  these.  In  this  period  also  the  first  detailed  maps  of 
the  mining  region  were  made.  In  general  the  principal  work  done  was  the 
recording  of  accurate  observations.  This  period  is  also  noted  for  the  vigor- 
ous controversy  that  arose  between  the  advocates  of  the  theory  which 
ascribed  to  the  jaspers  and  ores  of  the  region  an  eruptive  origin  and  the 


GEOLOGICAL   EXPLORATIONS   AND    LITERATUEE.  ^ 

adherents  of  the  theory  which  regarded  them  as  of  sedimentary  origin. 
The  controversy  continued  to  be  waged  violently  through  the  greater  por- 
tion of  the  fourth  period,  mitil  finally,  near  its  close,  it  was  settled  to  the 
satisfaction  of  both  sides. 

The  fourth  period  is  noted  especially  for  the  writings  of  the  geologists 
of  the  United  States  Geological  Survey,  more  particularly  those  of  Irving, 
Pumpelly,  Van  Hise,  Williams,  and  Smyth,  although  the  work  of  the  newly 
reorganized  Michigan  survey  was  an  important  element  in  settling  several 
of  the  questions  that  had  been  raised  in  the  earlier  discussion.  Wadsworth, 
Patton,  Lane,  and  Rominger  contributed  to  the  Michigan  reports.  Among 
the  other  geologists  who  published  during  this  j^eriod  are  Alexander  Win- 
chell,  N.  H.  Winchell,  Hunt,  Reyer,  Birkinbine,  and  Putnam.  The  prin- 
cipal problems  attacked  were  the  classification  of  the  Marquette  formations 
into  series,  the  definition  of  the  series,  and  their  correlation  with  the  series 
existing  elsewhere  in  the  Lake  Superior  region.  During  this  period  also  a 
number  of  the  doubtful  questions  as  to  the  origin  of  certain  members  of 
the  various  formations  were  settled,  so  that  at  its  close  nearly  all  the  geolo- 
gists with  a  personal  knowledge  of  the  region  came  netirer  to  agreement 
than  at  any  earlier  time.    . 

With  this  brief  outline  of  the  general  direction  taken  by  the  geological 
literature  of  the  district,  we  pass  at  once  to  the  consideration  of  the  litera- 
ture itself,  discussing  the  several  articles  in  the  order  of  tlieir  publication. 
A  synopsis  of  each  article  is  given,  and  this  is  accompanied  by  a  commen- 
tary intended  to  explain  the  circumstances  under  which  the  different  papers 
were  Avritten.  Where  possible  the  authors'  own  words  have  l)een  quoted 
freely  in  the  abstracts,  since  it  is  thought  better  to  allow  them  to  explain 
their  own  views  than  to  make  the  explanations  for  them.  Now  and  then  a 
critical  comment  has  been  introduced,  but  the  comment  is  in  criticism  of 
some  statement  of  fact  by  an  author,  and  not  of  his  views. 


THE   MARQUETTE   IKON-BEAEING   DISTRICT. 


Schoolcraft,  Henry  R.  Narrative  journal  of  travels  from  Detroit  northwest 
throngli  the  great  chain  of  American  lakes  to  the  sources  of  the  Mississippi  River, 
in  the  year  1820.    Albany,  1821.     Pages  157-160.     With  map  and  plates. 

The  first  reference  to  the  geology  of  the  Marquette  district  is  found  in 
a  volume  by  Schoolcraft,  who,  in  the  course  of  his  travels  along  the  south 
shore  of  Lake  Superior,  as  a  member  of  the  exploring  party  under  Governor 
Cass,  of  Michigan,  observed  the  existence  of  granite  at  the  point  now  known 
as  Granite  Point,  some  9  or  10  miles  north  of  the  present  city  of  Marquette, 
and  the  unconformity  between  it  and  the  sandstone  immediately  overl3dng 
it.  The  granite,  "rising  out  of  the  lake  to  a  height  of  200  feet,  is  connected 
with  the  shore  by  a  neck  of  land  consisting  of  red  and  gray  sandstone  in 
horizontal  layers."  Greenstone  veins,  varying  in  width  from  2  to  30  feet, 
were  noticed  cutting  the  granite.  "The  sandstone  laps  upon  the  granite, 
and  fits  into  its  irregular  indentations  in  a  manner  that  shows  it  to  have 
assumed  that  position  subsequently  to  the  iqihea-sdng  of  the  granite.  Its 
horizontality  is  perfectly  preserved,  even  to  the  immediate  point  of  contact, 
which  is  laid  bare  to  the  view."  A  geological  section  is  published,  showing* 
the  unconformity  of  the  sandstone  on  the  granite.  The  author  does  not 
pretend  to  know  the  age  of  the  sandstone,  but  he  thinks  "its  position  would 
indicate  a  near  alliance  to  the  'Old  Red  sandstone.'"  The  country  back  of 
the  lake  appeared  to  Schoolcraft  to  consist  largely  of  granite. 

Thus  the  existence  of  granite  cut  by  trap  dikes  in  the  Marquette  region, 
and  of  sandstone  much  younger  than  the  granite,  was  first  made  known  by 
Schoolcraft,  who  also  pictured  and  made  classical  one  of  the  best-known 
unconformities  on  Lake  Superior. 

1829. 

Bayfield,  H.  "W.  Outlines  of  the  geology  of  Lake  Superior.  Trans,  of  the 
Lit.  and  Hist.  See.  of  Quebec,  Vol.  I,  1829,  pages  1-43. 

Commander  Bayfield,  in  1829,  on  his  tour  around  the  lake,  made  a 
number  of  observations  concerning  the  geology  of  its  coasts.     He  confirms  . 
Schoolcraft's  discovery  of  the  existence  of  granite  at  Granite  Point  and  of 
horizontal   sandstone    resting   immediately  upon  it.     He  finds  this   same 


GEOLOGICAL  EXPLORATIONS  AND   LITEEATUEE-1841.  <J 

sandstone  with  similar  relations  to  the  granite  all  along  the  south  coast 
of  the  lake  from  this  point  as  far  west  as  the  east  side  of  Keweenaw  Bay. 
Like  Schoolcraft,  he  regards  it  as  probably  the  "Old  Red."  The  granite 
and  certain  greenstones  associated  with  it  are  cut  by  "veins  of  hornblende," 
which,  however,  never  pass  upward  into  the  sandstone. 

1S41. 

Houghton,  Douglass.  Fourth  annual  report  of  tlie  State  Geologist, 
Douglass  Houghton.  State  of  Michigan,  House  of  Representatives,  No.  27. 
Reprinted  in  "Memoirs  of  Douglass  Houghton,  first  State  Geologist  of  Michigan," 
by  Alvah  Bradish.    Detroit,  1889.    302  pages. 

The  first  general  statements  made  with  respect  to  the  relations  of  the 
Lake  Superior  rocks  to  one  another  are  to  be  found  in  the  reports  of  Dr. 
Houghton,  first  State  geologist  of  Michigan.  The  fourth  report,  published 
in  1841,  deals  principally  with  the  geology  of  the  copper  region,  although 
the  general  geological  relations  of  the  difi"erent  portions  of  the  Lake  Superior 
region  to  one  another  are  discussed,  and  the  boundaries  of  the  present 
Marquette  district  are  outhned.  The  presence  of  "Primary"  and  "Meta- 
morphic"  rocks  in  the  vicinity  of  the  present  city  of  Marquette  was  known 
to  the  author,  though  he  refers  to  this  locality  only  in  a  general  way  as 
the  region  north  of  the  Chocolate  River.  At  Presque  Isle,  moreover,  he 
recognized  the  presence  of  a  serpentinous  trap  rock,  which  he  thought  had 
been  raised  up  through  the  sandstones  now  lying  upon  it,  for  the  original 
horizontal  stratification  of  the  sedimentary  rock  has  been  so  disturbed  that 
its  strata  now  dip  away  from  the  trap  in  all  directions.  At  the  junction  of 
the  two  rocks  the  sandstone  has  been  shattered  and  impregnated  with 
calcareous  matter,  and  both  rocks  have  lost  their  characteristic  features 
(j3p.  180-181).^ 

The  main  features  of  the  geology  of  the  Upper  Peninsula  of  Michigan 
are  here  for  the  first  time  approximately  outlined.  The  primary  rocks  are 
reported  as  extending  northwestward  from  Little  Presque  Isle,  a  small  point 
jutting  into  Lake  Superior,  a  little  southeast  from  River  des  Morts,  now 
known  as  Dead  River.     Along  the  lake  shore  they  are  known  as  far  west 

'An  references  by  page  number  in  this  review  are  to  the  report  as  reprinted  in  the  Memoir  of 
Douglass  Houghton. 


10  THE  MARQUETTE   lEON-BEARING  DISTRICT. 

as  the  Hm-ou  Islands,  while  inland  the}"  stretch  westward  as  far  as  the 
source  of  the  Wisconsin  River  (p.  166).  The  Ig-rger  portion  of  the  Primary 
series  consists  of  granite  or  syenite  (hornblende-granite).  In  its  southeast- 
ern portions  granite  is  the  predominant  rock.  Toward  the  northwest  the 
character  of  the  series  changes  almost  imperceptibly.  Quartz  becomes  less 
and  less  abundant  in  the  granite,  and  hornblende  more  abundant,  until 
finally  the  rock  passes  into  a  granular  compound  of  feldspar  and  horn- 
blende, or  into  a  greenstone.  The  granitic  members  of  the  series  (which 
are  those  included  within  the  present  Marquette  district)  are  traversed  in 
all  directions  by  greenstone  dikes  of  various  magnitudes  that  have  produced 
contact  effects  in  the  granite  on  both  sides  of  them.  These  dikes  are  con- 
nected with  the  greaf  greenstone  masses  to  the  northwest.  They  are  not 
only  of  the  same  composition  as  the  greenstone,  but  as  the  large  areas  of 
the  greenstone  are  approached  the  dikes  become  more  and  more  abundant 
in  the  granite,  "until  at  length  it  becomes  difficult  to  determine  which  of 
the  rocks  predominates  in  quantity."  Occasionally  veins  of  other  rocks  than 
greenstone  are  to  be  found  cutting  the  gi-anite,  and  "in  a  single  instance 
what  was  regarded  as  a  true  vein  of  porphyry,  having  a  width  of  nearly 
3  feet,  was  noticed,  which  vein  is  crossed  at  angles  of  53°  and  107°  by  a 
vein  of  greenstone,  having  a. width  somewhat  less  than  that  of  the  porphyry. 
In  this  instance  the  greenstone  is  clearly  the  most  recent  vein"  (p.  176). 

It  is  evident  that  Houghton  did  not  discriminate  between  the  younger 
greenstone  dikes  in  the  granite  and  the  greenstone-schist  comprising  the 
large  areas  of  greenstone  to  which  he  makes  reference.  The  latter  do 
not  send  apophyses  into  the  granite,  but,  on  the  contrary,  the  granite 
sends  dikes  into  the  gi-eenstone-schists.  There  is  a  gradation  such  as 
Houghton  describes,  l)ut  the  granite,  and  not  the  greenstone-schist,  is  the 
invading  rock.  The  small  dikes  in  the  granite  are  as  described  in  the  report, 
but  they  are  not  genetically  connected  with  the  greenstone-schists. 

"Flanking  the  primary  rocks  on  the  south,"  writes  Houghton,  "is  a 
series  of  stratified  rocks  consisting  of  talcose,  mica,  and  clay  slates,  slaty 
hornblende  rock,  and  quartz  rock,  the  latter  rock  constituting  by  far  the 
largest  proportion  of  the  whole  group."  Passing  from  the  granite  southward 
near  the  lake  shore,  the   series  consists  of  a  serpentine  rock  into  which 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE-1841.  11 

the  o-vanite  insensibly  grades,  hornblende-slates,  talcose  and  micaceous 
slates"  and  elav-slates,  and  finally  quartzites.  The  series  dips  irregularly  to 
the  s..uth  and  southeast,  while  the  cleavage  of  the  slates  is  very  uniform 
to  till'  north  (p.  168).  These  "metamorphic"  rocks  are  confined  exclu- 
sively to  the  range  of  hills  lying  upon  the  southeast  side  of  the  granitic 
rocks.  They  occupy  a  belt  of  country  having  an  average  width  not 
exceeding  l^'to  8  ndles.  The  precise  limit  of  the  series  to  the  southwest 
is  unknown.  The  southeastern  boundary  is  the  Chocolate  River.  The 
alternations  of  the  different  members  of  the  series  are  so  complicated  that 
the  author  does  not  attempt  to  describe  them.  He  contents  himself  with  a 
description  of  the  quartzite,  which  he  finds  to  be  granular  and  compact, 
and  a  statement  with  regard  to  the  serpentinous  rock  lying  innnediately 
soutli  of  the  granite.  This  rock  has  a  regular-jointed  structure  i-esemlding 
stratification.''  In  composition  it  is  very  close  to  greenstone,  since  it  consists 
essentially  "of  granular  feldspar  and  hornblende,  with  which  serpentine 
is  intimately  blended.  This  rock  only  occurs  in  the  talcose  slate  as  ^ve 
approach  the  granitic  region,  and  possibly  a  more  close  examination  may 
.show  it  to  lie  a  simple  series  of  dikes  lying  parallel  to  the  line  of  cleavage 
of  the  slate  rocks"  (pp.  182-183).  Like  the  primary  rocks,  the  metamorphic 
ones  are  traversed  by  trap  dikes. 

The  serpentinous  rocks  t..  which  the  reference  is  made  are  the  green- 
stone-schists which  were  so  long  regarded  as  part  of  the  sedimentary  series, 
and  whose  origin  was  a  matter  of  doubt  tmtil  CI.  H.  Williams  proved  them 
to  be  altered  basic  tuffs.  The  mica-slates  and  clay-slates  and  the  (piartzites 
are  members  of  the  Marquette  series. 

In  the  economic  portion  of  the  report  the  State  geologist  refers  to  the 
value  of  the  granite  and  the  greenstones  as  building  materials,  and  gi^'es  a 
list  of  the  minerals  found  in  the  primary  and  metamorphic  series.  Among 
those  in  the  metamorphic  series  novaculite  and  hematite  are  mentioned,  but 
the  latter  is  not  thought  to  be  of  much  value.  "Although  the  hematite  is 
abundantly  disseminated  through  all  the  rocks  of  the  metamorphic  group, 
it  does  not  appear  in  sufficient  quantity  at  any  one  point  that  has  been 
examined  to  l)e  of  practical  importance"  (i)}).  19()-1!»7). 


12  THE   MAKQUETTE   IRON-BEAEING  DISTRICT. 

1845. 

Cunningham,  Walter.  A  copy  of  a  report  of  "Walter  Cunningham,  late 
mineral  agent  on  Lake  Superior.  Dated  January  8,  1845.  Communicated  by  Hon. 
William  Wilkius,  Secretary  of  War,  February  11,  1845.  28tli  Congress,  2d  session, 
1844-45.     Senate  Documents,  Vol.  VII,  No.  98.     5  pages. 

After  Dr.  Houghton's  uufortunate  death,  the  first  informatiou  that 
comes  to  us  concerning  the  geology  of  the  Marquette  district  is  through 
the  reports  of  the  commissioners  appointed  to  examine  the  country  ceded 
to  the  United  States  Government  by  the  Chippewa  Indians,  a  large  tract, 
including  all  of  the  area  embraced  in  the  iron  and  copper  districts  of  the 
south  shore  of  Lake  Superior,  in  addition  to  much  other  land.  Gren.  W. 
Cunuino-hanr  had  been  sent  1)V  the  War  Department  into  the  "Chippewa 
land  district"  to  examine  the  countr}'  and  to  gather  information  with  respect 
to  the  extent  of  the  mineral  lands  on  the  south  shore  of  the  lake,  that  these 
lands  might  be  designated  "mineral  lands"  by  the  proper  authorities,  and 
so  treated. 

Greneral  Cunningham  submitted  his  report  in  184.5.  In  it  the  Chocolate 
River  was  fixed  upon  as  defining  the  southern  boundar)'  of  the  mineral 
tract,  not  because  iron  ores  had  been  discovered  near  it,  but  presumably 
because  "in  the  vicinity  of  Death  River  important  discoveries  of  lead  ore 
have  been  made."  Very  little  was  afterward  heard  of  these  discoveries, 
but,  at  the  time  he  wrote,  Cunningham  believed  these  occurrences  of 
galena  marked  the  northeastern  end  of  the  lead  range  whose  southwestern 
extension  formed  the  great  lead  region  of  Iowa,  Illinois,  and  Wisconsin. 

Stoc;kton,  John.  A  report  of  John  Stockton,  superintendent  of  the  mineral 
lands  on  Lake  Superior,  with  maps,  etc.  Dated  February  24,  1845.  Transmitted  to 
the  Senate  March  17,  1845.  28th  Congress,  2d  session,  1844-45.  Senate  Documents, 
Vol.  XI,  ^o.  175,  pages  2-22. 

Upon  the  designation  of  the  "mineral  lands"  by  General  Cunningham 
they  were  placed  under  the  charge  of  General  Stockton  as  superintendent, 
with  headquarters  at  Copper  Harbor,  on  Keweenaw  Point. 

In  his  own  report  Stockton  deals  almost  exclusively  with  that  ]>ortion 
of  the  mineral  lauds  in  which  copper  was  known  to  occur. 


GEOLOGICAL   EXPLORATIONS   AND   LITEKATUKE— ISiG.  13 

Gray,  A.  B,     Report  by  A.  B.  Ciray.     Ibid.,  pages  15-22.     With  map. 

Accompanying-  Stockton's  report  are  the  reports  of  his  several  assist- 
ants, among  whom  was  A.  B.  Gray,  who  refers  incidentally  to  the  rocks 
between  Granite  Point  and  the  Chocolate  River  as  "trap  and  granite, 
beai'ing  the  strongest  indication  of  a  metalliferous  nature"  (p.  18).  They 
contain  veins  of  galena,  chalcopyrite,  and  pyrite.  The  traps  and  granites 
appear  as  knobs,  some  of  which  "are  based  on  the  south  by  metamorphic 
and  sandstone  rocks."  "With  Gray's  report  is  published  a  map  of  the  west 
half  of  Lake  Superior,  showing  very  well  the  main  features  of  the  topog- 
raphy of  the  district  lying  between  the  Montreal  and  Chocolate  rivers,  but 
containing  no  geology. 

1S4G. 

Gray,  A.  B.  Report  of  A.  B.  Gray  relative  to  the  mineral  lands  on  Lake 
Superior.  Dated  March  10,  1846.  Transmitted  to  the  House  of  Representatives 
June  16, 1846.  29th  Congress,  1st  session,  1845-46.  Executive  Docnments,  Vol.  VII, 
No.  211.    23  pages.     With  map. 

In  a  second  report,  Avritten  about  a  year  after  that  last  referred  to,  Gray, 
who  in  the  meanwhile  had  been  appointed  assistant  superintendejat,  reports 
the  progress  of  his  surveys  of  the  Lake  Superior  mineral  district,  and  gives 
on  a  large-scale  map  the  location  of  claims  filed  by  mineral  prospectors. 
It  is  interesting  to  note  that  a  large  number  of  claims  had  been  leased  in 
the  district  with  which  we  are  concerned  at  present,  especially  in  T.  47  N., 
R.  26  W.,  and  T.  47  N.,  R.  27  W.,  the  tAvo  most  important  iron-producing 
townships  in  Michigan,  and  that  they  had  been  taken  up,  not  with  a  view  to 
prospect  for  iron  ore,  but,  in  all  probability,  in  the  hope  that  galena,  copper, 
or  an  ore  of  copper  might  be  discovered  on  them.  It  is  true  that  Samuel 
Peck  reported  the  existence  of  large  exposures  of  specular  iron  ore  in  the 
range  of  hills  south  of  Dead  River  and  about  10  or  12  miles  inland,  but  it 
is  plain  that  a  great  deal  less  interest  was  taken  in  this  discovery  than  in  that 
of  galena  on  Dead  River  and  near  the  Carp,  of  gold  in  the  "iron  range," 
and  of  black  oxide  of  copper  at  Presque  Isle  and  near  the  mouth  of  the 
Carp. 


14  THE  MAEQUETTE   TEON  BEAEIN(}   DISTEICT. 

EoGERS,  H.  D.  Verbal  communication  to  the  Boston  Society  of  Natural 
History,  April  1,  1S46.     Proc.  Boston  Soc.  Nat.  Hist.,  Vol.  II,  pages  124-125. 

In  the  same  year  in  which  Gray  wrote.  Professor  Rogers,  in  a  verbal 
communication  to  the  Boston  Society  of  Natural  History,  gave  an  account 
of  the  mode  of  occurrence  of  copper  at  Keweenaw  Point,  and  discussed  the 
age  of  the  sandstones  and  conglomerates  of  the  Lake  Superior  region.  In 
the  course  of  his  remarks  he  announced  the  discovery  of  a  contact  between 
a  red  sandstone  and  an  underlying  series  of  sandstones  which  he  thought 
to  be  the  equivalent  of  the  Primal  sandstone  and  slate  of  the  Appalachian 
series,  known  in  the  reports  of  the  'Kew  York  survey  as  the  Potsdam  sand- 
stone. The  location  of  the  contact  is  in  the  neighborhood  of  the  Choco- 
late and  Carp  rivers.  The  underlying  sandstone  is  highly  inclined  and  is 
"traversed  by  parallel  east  and  west  axes."  On  the  uplifted  edges  of  this 
sandstone  rest  unconformably  beds  of  the  conglomerates  and  sandstones  of 
the  red  sandstone  series,  with  very  gentle  northern  dips.  If  the  underlying 
sandstone  is  Potsdam,  then  the  red  sandstones  and  conglomerates  of  Lake 
Superior  are  post-Paleozoic.  The  author  concludes,  "from  various  points 
of  analogy  between  the  red  sandstone  itself,  its  trappean  dikes,  and  their 
mineral  associations  with  the  similar  components  of  the  Mesozoic  or  New 
Red  sandstone  of  the  Atlantic  States,  that  the  formation  in  question  is  of 
equivalent  age  and  origin  with  this  last-named  interesting  group  of  rocks." 

This  contact  was  later  described  and  pictured  by  Irving  and  other 
geologists.  Professor  Rogers  described  its  essential  features  well,  but  his 
conclusion  with  regard  to  the  age  of  the  overlying  rock  is  of  course 
valueless,  since  the  underlying  sandstones  are  Algonkian,  and  not  Cambrian. 
Moreover,  the  overlying  sandstones  are  not  cut  by  trappean  dikes. 

1848. 
Locke,  John.    Eeport  of  John   Locke  to  Dr.  C.  T.  Jackson,  describing  the 
observations  made  on  the  geology  of  the  mineral  lands  in  Michigan.    Dated  October 
27,  1847.    30th  Congress,  lat  session,  1847-48.     Senate  Documents,  Vol.  II,  No.  2, 
pages  183-199. 

By  acts  of  March  1  and  March  3,  1846,  the  mineral  lands  of  Lake 
Superior  were  taken  from  the  jurisdiction  of  the  War  Department  and 
placed  under  control  of  the  General  Land  Office,  at  that  time  a  branch 


GEOLOGICAL  EXPLORATIONS  AND   LITERATURE— 1848.  15 

of  the  Treasury  Department.  Thereupon  Dr.  C.  T.  Jackson  was  appohited 
by  the  Acting  Secretary  of  the  Treasury  "to  make  a  geological  survey  of 
the  Lake  Superior  district,  in  Michigan."  Dr.  Locke  was  selected  by  Dr. 
Jackson  to  be  assistant  geologist  to  take  charge  of  one  of  the  exploring 
parties,  whose  duty  was  to  examine  that  poi'tion  of  the  south  shore  of  the 
lake  east  of  Keweenaw  Point.  His  report  was  published  in  1848,  although 
the  reports  of  Jackson's  other  assistants  did  not  appear  until  the  records 
of  the  following  Congress  were  printed,  and  not  until  after  the  appearance  of 
the  reports  of  the  linear  surveyors,  whose  work  was  concluded  and  whose 
results  were  probably  known  to  Dr.  Locke  before  he  took  the  field.  These 
latter  reports  take  precedence  in  point  of  time,  althougli  they  were  not 
given  to  the  public  imtil  after  Locke's  report. 

In  his  report  Locke  mentioned  pure  iron  ore  as  occumng  on  Presque 
Isle.      lie  further  remarks  (p.  187): 

The  region  drained  by  Dead  and  Carp  rivers  is  full  of  Interest,  and,  geologically, 
is  a  "  compact  country,"  jireseuting  a  great  variety  in  small  distances.  *  *  *  [It 
consists  of  J  knobs  of  greenstone  and  augitic  trap,  surrounded  by  altered  sandstone  and 
slates.  Perhaps  not  so  much  "knobs"  as  ridges.  The  sandstone  is  changed  almost  into 
quartz,  and  the  slates  occur  so  much  transformed  that  at  some  points  it  is  difBcult  to 
distinguish  them  from  trap.  *  *  *  This  region  has  been  called  the  "  Cornwall  of 
America,"  in  reference  to  its  general  geology.  It  has  been  said  that  there  are  rocks 
there  suitable  to  contain  gold,  silver,  copper,  lead,  &c.  So  far  as  I  have  observed,  it 
is  a  gold  region  "  all  but  the  gold,"  a  silver  region  without  silver,  and  a  copper  region 
with  veins  and  bunches  of  ore  so  thin  and  scattered  as  to  be  unprofitable  for  working. 

The  author  of  the  report  refers  to  the  ore  of  the  Jackson  Iron  Com- 
l)any  as  similar  to  that  of  Missouri,  and  announces  the  discovery  of  some 
very  pure  iron  ore  along  the  western  edges  of  Ts.  47  and  48  N.,  R.  26  W., 
and  in  the  contiguous  towns.  He  also  identifies  the  "granite  "(if  the 
Michigan  map  (probably  referring  to  the  map  afterward  published  by 
the  linear  surveyors)  as  a  porphyritic  syenite. 

Channing,  Wm.  F.  Dr.  Chaiining's  synopsis  of  the  survey  in  charge  of  Dr.  John 
Locke.  (Second  section.)  3()th  Congress,  1st  session,  1847-48.  Senate  Documents, 
Vol.  II,  No.  2,  pages  207-208. 

Locke's  report  is  so  badly  written  tliat  it  is  fortimate  for  its  author's 
reputation   as  a  geologist   that  lie  had  witli   him  as  "first  subageut"   Dr. 


16  THE   MAEQUETTE   IRON-BEARING   DISTRICT. 

Channing,  who  has  g-iven  us  a  synopsis  of  the  geological  results  reached 
by  Locke's  party. 

After  referring  to  the  point  of  Presque  Isle  as  consisting  of  "trap  much 
modified  by  the  neighboring  sandstone,  with  which  it  has  a  junction  a 
short  distance  inland,"  and  to  the  character  of  the  ores  ("sulphuret  of 
copper  and  iron")  mined  there,  Channing  briefly  outlines  the  results  of  a 
trip  made  inland  from  the  Carp  River  "through  the  metamorphic  region 
and  the  iron  region  to  the  syenite  on  the  Escanawby  River,"  as  follows: 

On  Carp  River  the  principal  rocks  are  the  metamorphic  slates  and  sandstone 
quartz.  In  the  former,  in  T.  48,  R.  26,  two  veins  of  quartz  were  observed  contain- 
ing copper  pjrrites.  *  *  *  In  sec.  30,  T.  48,  R.  26,  a  quarry  of  hornstone  occurs, 
adjoining  the  quartz,  which  furnishes  oilstones  of  the  finest  quality. 

The  location  of  the  Jackson  Iron  Company  is  in  R.  27,  T.  47,  sec.  1.  The  iron 
region,  which  has  its  northern  limit  here,  was  observed  for  many  miles.  It  consists  of 
magnetic  and  unmagnetic  oxides  of  iron,  occasionally  associated  with  metamorphic 
slate  or  chlorite  slate.    *     *     * 

South  of  the  metamorphic  region  the  slates  and  sandstone  quartz  gradually 
pass  into  quartz  and  feldspar  rock  and  fine-grained  syenite. 

Still  farther  south  the  coarse  and  porphyritic  syenite  on  the  Escanawby  River 
comes  in.    These  rocks  were  rarely  found  to  contain  mica. 

The  published  documents  of  the  first  session  of  the  Thirty-first  Con- 
gress (1849—50)  contain  the  reports  of  the  linear  surveyors,  those  of  Dr. 
Jackson  and  his  assistants,  and  of  Messrs.  Foster  and  Whitney,  who  suc- 
ceeded Jackson  as  United  States  geologists.  Although  all  these  bear  the 
same  date  of  publication,  the  reports  of  the  linear  surveyors  had  been 
submitted  to  the  authorities  sevei'al  years  before  their  publication,  even 
before  the  date  of  publication  of  Locke's  first  report  already  referred  to, 
and  copies  of  them  had  evidently  been  furnished  to  Jackson  and  to  Foster 
and  Whitney  before  they  entered  upon  their  surveys;  so  that  the  reports 
bearing  date  of  1849-50  must  not  be  looked  upon  as  contemporaneous. 
The  work  of  the  linear  surveyors  was  prior  to  that  of  Jackson  and  his 
assistants,  while  the  work  of  Foster  and  Whitney  was  undertaken,  at  least 
in  part,  after  Jackson's  work.  The  value  and  importance  of  the  different 
reports  published  in  this  year  must  be  judged  in  the  light  of  this  knowledge. 

The  Chippewa  land  district  was  subdivided  into  townships  and  sec- 
tions by  William  A.  Burt,  Bela  Hubbard,  and  others,  under  the  direction 


GEOLOGICAL   EXPLORATIONS   AND    LITEKATUEE— 1830.  17 

of  Douglass  Houghton,  during  the  years  1845  and  184G.  Houghton 
mstructed  his  assistants  to  observe  the  ledges  on  their  lines  of  travei'se, 
1  mile  apart,  running  east  and  west  and  north  and  south,  to  i-ofjort  their 
character,  and  to  collect  specimens  from  them.  The  reports  of  Messrs. 
Burt  and  Hubbard  are  based  on  these  observations.  Geology,  of  course, 
was  of  secondary  interest  to  these  surveyors,  and  yet  their  geological 
work  has  been  of  great  value  to  all  later  geologists  who  have  made 
excursions  into  the  district  surveyed.  Foster  and  Whitney  used  the  linear 
surveys  to  great  advantage  in  prosecuting  their  explorations,  and  all 
geologists  who  have  followed  these  two  have  been  guided  to  important 
outcrops  by  the  notes  of  the  surveyors.  These  notes,  as  has  already  been 
mentioned,  are  printed  in  the  same  volume  of  the  Government  records  as 
the  reports  of  Jackson  and  of  Foster  and  Whitney,  but  since  the  latter 
gentlemen  made  use  of  them,  it  is  but  right  that  Messrs.  Burt  and  Hubbard 
should  be  given  priority  in  a  discussion  of  the  literature  of  the  region. 
Consequently,  in  this  chapter  their  reports  are  given  precedence  of  the 
reports  of  Jackson  and  Whitney,  although  appearing  later  in  the  same 
volume  that  contains  the  reports  of  these  latter  gentlemen. 

1S50. 

Burt,  Wm.  A.  Topography  and  geology  of  the  survey,  with  reference  to  mines 
and  minerals,  of  a  district  of  township  lines  south  of  Lake  Superior.  31st  Congress, 
1st  session,  1819-50.  Senate  Documents,  Vol.  Ill,  No.  1,  pages  811-832.  With  map, 
opposite  page  880. 

Burt,  as  the  result  of  his  first  year's  survey,  divided  the  rocks  seen  by 
him  during  his  journeys  along  the  section  lines  into  five  principal  groups: 
the  primary  rocks,  the  traps,  the  conglomerates,  the  sandstones,  and  the 
slates.  The  primary  rocks,  including  the  metamorphic  ones,  he  found 
generally  a  little  inland,  with  the  metamorphic  rocks  flanking  the  other 
members  of  the  primary  series  on  the  south.  The  rock  to  the  north  is 
"sienite,"  or  "  sienite  granite."  Hornblende  is  a  more  frequent  constituent 
than  mica,  hence  "sienite"  is  the  predominant  rock  type.  Greenstone 
intrusives  and  veins  of  quartz  and  feldspar  cut  the  ]!)rimary  rocks  in  all 
directions.     In  the  metamorphic  series  south  of  the  primary  granite  and 

MON  XXVIII 2 


18  THE  MARQUETTE   IKON-BEAIIING  DISTRICT. 

syenite,  "quartz,  compact  and  granular;  imperfect  talcose  slates,  which  are 
in  some  instances  slightly  argillaceous,  and  slaty  hornblende  "  are  the  pre- 
dominant types.  The  rocks  are  more  or  less  stratified,  and  are  imperfectly 
jointed.  Within  the  area  of  the  metamorphic  rocks  the  surveyors  saw 
several  knobs  of  syenitie  granite  and  some  dikes  of  greenstone.  On  the 
sketch-maji  accompanying  the  report  (ojjposite  p.  880)  are  given  the  outlines 
of  the  areas  occupied  by  granite,  metamorphic  rocks,  and  clay-slates  in 
T.  48  N.,  R.  26  W. ;  T.  49  N.,  R.  25  W.,  and  T.  47  N.,  R.  26  W.     (See  PI.  I.) 

Hubbard,  Bela.  General  observations  upon  the  geology  and  topograpliy  of 
the  district  south  of  Lake  Superior,  subdivided  in  1845  under  direction  of  Douglass 
Houghton,  deputy  surveyor.  33  st  Congress,  1st  session,  1849-50.  Senate  Docu- 
ments, Vol.  Ill,  No.  1,  pages  833-842. 

Hubbard's  report  is  upon  the  same  district  as  is  that  of  Burt,  referred 
to  above.  The  author  divides  the  townships  sin-veyed  in  1845  into  two 
classes,  in  one  of  which  he  places  Ts.  46  N.,  47  N.,  and  48  N.,  Rs.  24  W., 
25  W.,  and  26  W.,  all  in  the  Marquette  district.  He  then  proceeds  to 
describe  the  geology  of  these  towns.  He  adds  little  new  to  Burt's  descrip- 
tion, but  his  treatment  of  the  subject  is  more  satisfactory,  as  it  is  more 
comprehensive.  T.  46  N.,  Rs.  24  W.,  25  W.,  and  26  W.,  and  the  lower 
tier  of  sections  in  the  towns  north,  are  occupied  by  granites.  These  rocks 
appear  in  a  succession  of  rounded  knobs,  having  a  general  direction  a  little 
south  of  west.  They  "vary  much  in  character  and  composition,  being 
sometimes  hornblendic  and  approaching  a  perfect  sienite,  but  more  com- 
monly feldspathic,  or  composed  of  quartz  and  feldspar."  In  the  more 
southerly  portion  of  the  district  the  feldspar  is  red,  and  the  granite  of  a 
corresponding  tint.  Some  portions  of  the  rock  are  massively  stratified. 
A  second  area  of  granite  begins  on  the  coast  a  little  south  of  Presque  Isle 
and  runs  westerly  inland.  It  occupies  the  portion  of  T.  48  N.,  R.  25  W., 
lying  north  of  "Rio  des  Morts"  (Dead  River).  "The  granite  of  this 
portion  of  the  country  is  traversed  by  large  and  irregular  dikes  of  green- 
stone trap,  and  the  granite  itself  puts  on  a  trappean  character,  the  two 
rocks  being  sometimes  with  difficulty  distinguished  from  each  other.  This 
is  the  commencement  of  an  apparently  very  large  extent  of  granitic  coun- 
try extending  westerly  into  the  region  not  yet  surveyed  by  section  lines" 


GEOLOGICAL    EXPLORATIONS   AND   LITERATUIIE— 1850.  19 

(p.  834).  Between  the  uortlieru  and  southern  granite  areas  lies  the 
region  of  metamorpliic  rocks.  This  region  is  divided  into  two  parts — 
the  southern  or  quartzite  portion,  and  the  northern  or  trappean  portion, 
corresponding  to  our  present  division  into  the  fragmentals  and  greenstone- 
schists.  In  the  southern  area  are  found  white  and  brown  quartz  rocks, 
talcose  and  augitic  slates  and  clay-slates,  slaty  hornblende,  and  specular 
and  micaceous  oxides  of  iron.  These  rocks  cover  a  tract  of  country  lying 
"between  the  granites  on  the  south  and  a  line  bearing  north  of  west  from 
the  mouth  of  Carp  River  to  the  center  of  the  west  line  of  township  48, 
range  26."  This  tract  is  described  as  rolling,  with  numerous  ridges  trend- 
ing nearly  east  and  west.  The  central  portions  of  many  of  the  ridg-es 
seem  to  be  trap,  which  is  capped  and  flanked  by  the  metamorphosed 
rocks.  No  outcrops  of  this  trap  ay  ere  seen,  however.  Its  presence  was 
inferred  from  the  character  of  the  metamorpliic  rocks  and  their  dips,  which 
were  thought  to  be  in  all  directions  away  from  a  central  axis.  All  the 
metamorpliic  rocks  are  pervaded  by  "the  oxides  of  iron,  sometimes  inti- 
mately disseminated,  and  sometimes  in  beds  or  veins,"  which  are  frequently 
of  such  "great  extent  as  almost  to  entitle  them  to  be  considered  as  rocks." 
The  ores  are  described,  and  the  positions  of  some  of  these  outcrops  are 
noted.  The  northern  portion  of  the  metamorpliic  area  embraces  all  the 
country  l^etween  the  quartzite  group  just  described  and  the  granites  to 
the  north,  with  the  exception  of  about  5  square  miles  in  the  northeast  part 
of  T.  48  X.,  E.  2G  W.,  where  clay-slates  occur.  "This  division  of  the  meta- 
morjihic  region  is  characterized  by  the  frequent  occurrence  of  knobs  or 
uplifts  of  greenstone  and  augitic  trap,  making  their  appearance  rather 
irregularly  over  the  country,  and  surrounded  by  altered  sandstones  and 
slates."     The  greenstone  is  igneous  in  origin. 

This  report  of  Hubbard's  is  by  far  the  most  satisfactory  one  jiublished 
up  to  this  time.  It  supplements  Houghton's  general  report  of  1841,  and 
for  the  first  time  gives  a  fair  idea  of  the  character  of  the  country  now 
included  in  the  Marquette  area,  the  distribution  of  the  rocks  occupying  the 
district,  and  their  relations  to  one  another.  Of  course  the  report  is  frag- 
mentary. The  author  regarded  it  as  such,  and  yet  it  contains  in  shadowy 
outline  many  of  the  conclusions  of  later  geologists. 


20  THE   MARQUETTE   IRONBEAEING  DISTRICT. 

UuRT,  Wm.  a.  Geological  report  of  the  survey,  "  with  reference  to  iniues  and 
miuerals,"  of  a  district  of  township  lines  in  the  State  of  Michigan,  in  the  year  184G, 
and  tabular  statement  of  specimens  collected.  Dated  March  20, 1847.  31st  Congress, 
1st  session,  1849-50.  Senate  Documents  Vol.  Ill,  No.  1,  pages  842-875.  With  maps, 
opposite  ])ageS80. 

The  experiences  of  a  second  season  in  the  Upper  Peninsula  of  Michigan 
resulted  in  Burt's  second  report.  In  this  the  author  generalized  to  a,  greater 
extent  than  was  possible  in  his  earlier  report.  The  territory  surveyed  dunng 
this  second  season  embraced  all  of  the  "Marquette  district,"  in  addition  to  the 
country  north  and  south  of  it  for  a  number  of  miles.  The  general  geology 
of  all  this  area  is  briefly  outlined,  and  the  results  of  the  3^ear's  work  are 
indicated  on  a  map. 

The  granites  of  the  region  are  said  by  the  author  to  pass  in  some 
instances  into  syenitic  greenstones.  They  are  cut  by  trap  and  are  very 
often  gneissic  hi  structure.  In  Ts.  47  N.  and  48  N.,  Rs.  27  W.,  28  W.,  and 
29  AV.,  are  argillaceous  slates  that  are  the  extension  westward  of  the  slates 
observed  in  1845.  These  slates  dip  "  at  a  high  angle,  generally  conforming 
to  the  surrounding  granites,  or  flanking  the  numerous  protrusions  of  green- 
stone within  their  boundary.  They  appear  like  the  remnants  of  overlying 
rocks  among  the  greenstones,  which  have  escaped  the  denuding  effects  of 
causes  that  partially  stripped  this  region  of  similar  rocks  jDrevious  to  the 
completion  of  its  present  elevation"  (p.  546).  "The  talcose  slates  are  of 
many  varieties,  such  as  would  result  from  their  passing  into  argillitic  and 
into  horniolende  slates. "  The  gTcenstone  and  hornblende-slates  occupy  only 
a  small  portion  of  the  area  mapped  in  PI.  I,  though  they  formed  the  largest 
portion  of  the  area  sm-veyed.  The  greenstones  are  described  as  more  or 
less  granular  and  syenitic  rocks,  with  a  dark  color  when  moist.  Their  com- 
position is  hornblende,  feldspar,  and  quartz,  with  the  hornblende  largely 
predominating,  sometimes  almost  to  the  exclusion  of  the  other  constituents. 
The  hornblende-slates  are  fine-grained  and  compact  varieties  of  the  green- 
. stone,  possessing  a  "laminated  or  slaty  structure."  The  slates  are  cut  by 
quartz  veins  and  by  later  trap  dikes  which  are  supposed  to  unite  with  the 
.trap  range  of  Keweenaw  Point  to  the  west. 


GEOLOGICAL   EXPLORATIONS   AND    LITERATURE— 1850.  21 

The  author  concludes  his  remarks  on  the  general  geology  of  the  region 
by  mentioning  the  locations  of  fourteen  exposures  of  iron  ore  met  with  along 
the  traversed  lines.  He  infers  from  his  observations  that  the  region  is  an 
exceedingly  rich  one,  far  excelling  any  other  portion  of  the  United  States 
in  the  abundance  and  good,  quality  of  its  ores  (p.  852).  It  is  to  Burt's 
energy  and  to  his  discovery  of  ore  that  later  developments  of  the  iron  dis- 
trict are  due,  although,  as  we  have  seen,  Locke's  report  referred  to  the  ores 
of  the  region  two  years  before  that  of  their  discoverer  was  given  to  the 
public. 

Lists  of  specimens  collected  along  the  township  lines  are  appended 
to  the  reports,  and  it  is  these  that  have  afforded  such  great  aid  to  later 
geologists. 

Jackson,  Charles  T.  Report  on  the  geological  and  mineraloglcal  survey  of 
the  mineral  laud-s  of  the  United  States  iu  the  State  of  Michigan,  etc.  Dated 
November  10,  1849.  .31st  Congress,  1st  session,  1849-50.  Senate  Documents,  Vol. 
Ill,  No.  1,  Images  371-502.  With  maps.  Also  House  Documents,  Vol.  Ill,  No.  5, 
pages  371-502. 

Reference  has  already  been  made  to  the  appointment  of  Dr.  Jackson, 
in  1847,  as  geologist  for  the  survey  of  the  mineral  lands  of  the  State  of 
Michigan.  The  reports  of  Locke  and  Channing,  which  appeared  in  1848, 
were  the  first  fruits  of  the  survey.  In  1 850  the  reports  of  Jackson  himself 
and  of  several  of  his  assistants  were  submitted  to  the  Secretary  of  the 
Interior,  concluding  the  survey  luider  Jackson's  charge. 

In  his  own  report  Jackson  gives  a  minute  and  detaileil  account  of  his 
explorations  and  their  results.  Most  of  Jackson's  personal  observations 
were  made  m  the  copper  region.  In  the  report,  however,  he  notes  that  in 
1845  Mr.  Joseph  Stacy  explored  the  region  "between  the  mouth  of  Dead 
River  and  Lake  Michigan,  and  established  the  fact  that  there  was  an 
inexhaustible  amount  of  compact  and  micaceous  specular  iron  ore  in  that 
district."  Its  analysis  gave:  Silica,  3.88;  lime,  0.17;  peroxide  of  iron, 
96.11.  A  small  portion  of  the  ore  is  in  the  state  of  magnetic  oxide.  The 
localities  were  examined  more  carefully  in  1847  by  Assistant  Geologist 
Locke  and  Subagent  Channing,  whose  report  is  referred  to  below.     In  1848 


22  THE    MARQUETTE    IRON  BEARING    DISTRICT 

Jackson  received  a  specimen  of  pure  hematite  from  near  the  Carp  River 
(pp.  478-479).  The  maps  that  accompany  tlie  report  are  mainly  of  the 
copper  regions. 

Locke,  J.  United  States  geological  survey  of  public  lands  in  Michigan.  Field 
notes  of  184:7.  Accompanying  report  of  Dr.  Jackson.  .31st  Congress,  1st  session, 
1849-50.     Senate  Documents,  Vol.  Ill,  No.  1,  pages  572-586. 

Akhongh  Locke's  report,  which  accompanies  that  of  Dr.  Jackson,  is 
but  a  copy  of  the  author's  field  notes,  it  contains  a  few  points  of  interest 
concerning  the  iron  region.  Locke  and  his  assistants  coasted  along  the 
shore  of  Lake  Superior  from  L'Anse  to  beyond  the  Chocolate  River.  At 
Presque  Isle  the  junction  of  the  trap  and  sandstone  on  the  east  side  of  the 
point  Avas  observed,  and  fresher  trap  dikes  in  the  main  "trap  mass"  were 
noted.  Li  the  vein  mined  by  the  New  York  and  Lake  Superior  Mining  Com- 
pany is  a  mixtm-e  of  galena,  asbestos,  pyrite,  and  arsenopyrite.  Inland  from 
the  mouth  of  the  Carp  River  a  large  number  of  veins  of  quartz  containing 
copper  pyrites  were  seen  cutting  metamorjDhic  slates.  At  the  Jackson  loca- 
tion (sec.  1,  T.  47  N.,  R.  27  W.)  the  direction  of  the  iron  range  is  about  east 
and  west.  Much  ore  lies  in  loose  pieces  on  the  surface  of  the  ledges.  The 
best  of  it  is  of  a  loose  crystalline  structure,  but  about  one-half  consists  of 
"ribbon"  ore,  striated  with  red  veins,  which  deserve  examination  to  ascer- 
tain their  character,  while  a  third  variety  is  a  slaty  ore,  compact  and  pure. 
Along  the*  west  lines  of  sees.  6  and  7,  T.  47  N.,  R.  26  W.,  exposures  of  ore, 
metamorphic  sandstone,  and  feiTUginous  quartz  were  seen.  Continuing 
farther  south,  the  explorers  found  ores  of  various  qualities,  and  at  the  south- 
west comer  of  section  18  an  "augitic  rock."  On  the  east  branch  of  the 
Escanaba  River  they  ran  into  "red  sienite."  About  a  mile  north  of  the  Carp 
River,  near  the  coast,  there  occiirs  a  clay-slate,  which  farther  north  is  "highly 
metamorphic,  and  a  trappean  rock  occurs,  apparently  blending  with  the 
slate."  Trap  rocks  "seem  to  be  frequently  interfused  with  the  metamorphic 
rocks  in  this  region,  and  sometimes  to  receive  even  a  stratified  structure, 
when  slightly  changed  from  their  original  type."  "With  regard  to  the 
metamorphosed  character  of  the  regioji  to  which  these  notes      *      *     * 


GEOLOGICAL   EXPLORATIONS    AND    LITEEATUllE— IS.JO.  23 

refer,  there  can  be  no  doubt  in  placing  the  coarsely  crystalline  sienite  of 
the  Escauawby  and  southern  part  of  the  primitive  district  apart  from  the 
trap  and  copper-bearing  rocks  of  Lake  Superior." 

At  the  mouth  of  Dead  River  syenite  was  observed,  and  at  the  first  falls, 
1  mile  upstream,  a  talcose  slate.  Syenites  and  various  slates  were  noted 
at  other  places  in  the  vicinity  of  Dead  River,  and  the  syenite  was  seen  to  be 
cut  very  frequently  by  trap  dikes.  At  "Point  No.  2,  west  of  Presque  Isle," 
a  junction  of  red  sandstone  Avith  syenite  is  said  to  occur,  but  it  seems  to 
have  had  verv  little  significance  to  the  writer,  as  he  does  not  describe  it,  but 
mereh'  asserts  its  existence.  The  contact  is  probably  the  unconformity  at 
Granite  Point. 

Foster,  J,  W.  Notes  on  the  geology  and  topography  of  portions  of  tbe  country 
adjacent  to  Lakes  Superior  and  Michigan,  in  the  Chippewa  land  district.  Dated  May 
26, 1849.  31st  Congress,  1st  session,  1810-50.  Senate  Documents,  Vol.  Ill,  No.  1, 
pages  773-801. 

J.  W.  Foster,  another  of  Jackson's  assistants,  reports  the  results  of 
his  explorations  along  the  Michigamme  and  Menominee  rivers  to  Green 
Ba}-.  Only  one  or  two  observations  are  of  interest  to  us  in  the  pres- 
ent discussion.  Indications  of  the  presence  of  iron  ore  on  the  north 
side  of  Lake  Michiganune  are  plentiful.  Hornblendic  and  argillaceous 
slates  form "  a  range  bounding  the  lake  on  the  north,  and  within  the 
hornblende  rocks  are  beds  of  quartz.  In  sec.  1,  T.  46  N.,  R.  30  Vi.,  where 
Republic  was  afterward  located,  Foster  and  his  associates  crossed  an 
almost  perpendicular  cliff,  composed  of  such  pure  specular  oxide  of  u-on 
that  its  mineral  associates  were  difficult  to  determine.  This  pure  ore  forms 
the  brow  of  the  cliff.  Beyond  it  succeeds  a  bed  of  quartzite,  containing 
small  specks  of  ore  and  large  rounded  masses  of  the  same  material, 
forming  with  the  quartzite  a  conglomerate.  This  is  the  first  mention  of  a 
conglomerate  associated  with  the  iron  rocks,  and,  strange  to  relate,  this 
first  conglomerate  observed  was  the  last  one  whose  significance  in  the 
geological  history  of  the  region  was  realized. 

The  ore  was  regarded  by  Foster  as  continuous  with  that  of  Carp 
River,  because  the  mineralos'ical  and  g-eolog-ical  associations  of  the  ore  in 


24  THE   MARQUETTE   lUOX-BEARING    DISTRICT. 

both  localities  are  the  same.     Other  beds  of  the  same  ore  were  observed 
farther  south  along  the  Menominee  River.     Foster  writes  (p.  776): 

These  beds,  so  far  as  I  have  observed,  present  a  marked  similarity  in  mineralogical 
characters,  and  derive  their  origin  from  common  causes,  and  these  were  aqueous. 
The  jointed  structure  and  waved  stratification  of  some  of  the  beds  prove  that  igneous 
causes  have  operated,  since  their  deposit,  to  modify  and  change  their  character. 

Whitney,  J.  D.  Verbal  communication  to  the  Boston  Society  of  Natural 
History.     December  19,  1849.    Proc.  Boston  Soc.  Nat.  Hist.,  Vol.  Ill,  pages  210-213. 

J.  D.  Whitney,  who  was  also  associated  with  Jackson,  gave  the  main 
results  of  his  ^vork  in  an  address  to  the  Boston  Society  of  Natural  History, 
delivered  in  December,  1849,  anticipating,  to  some  extent,  his  official 
reports.  As  in  the  case  of  most  other  accounts  of  Lake  Superior  geology, 
this  one  is  devoted  principally  to  the  copper  region.  Reference,  however, 
is  made  to  immense  deposits  of  iron  ore,  existing  mainly  as  a  fine-grained, 
almost  chemically  pure  peroxide  of  iron.  "It  occupies  about  eighty  quarter 
sections  of  the  mineral  country,  and,  at  the  nearest  point,  is  about  12  miles 
from  the  lake."  The  quantity  of  the  ore  is  reported  to  be  beyond  calcu- 
lation. "It  appears  in  the  form  of  solid  ridges  and  knobs,  evidently  of 
igneous  origin,  the  highest  being  about  1,100  feet  above  the  level  of  the 
lake,  and  some  of  them  being  half  a  mile  long."  The  speaker  exhibited 
to  the  society  a  specimen  of  banded  jasper  and  ore.  In  reply  to  questions, 
Whitney  declared  that  no  New  Red  sandstone  occurs  in  any  of  the  region 
examined  by  him.  Since  he  had  examined  the  coast  of  the  lake  near  the 
Carp  and  Chocolate  rivers,  it  is  evident  that  he  disagreed  with  Rogers  as  to 
the  age  of  "the  Lake  Superior  sandstones  and  conglomerates. 

Foster,  J.  W.,  and  Whitney,  J.  D.  Synopsis  of  the  explorations  of  the 
geological  corps  in  the  Lake  Superior  land  district  in  the  Northern  Peninsula  of 
Michigan,  under  the  direction  of  J.  W.  Foster  and  J.  D.  Whitney,  U.  S.  Geologists. 
Dated  November  5,  1849.  31st  Congress,  1st  session,  1849-50.  Senate  Documents, 
Vol.  Ill,  No.  1,  pages  605-625.    With  maps. 

After  the  resignation  of  Dr.  Jackson  from  the  position  he  held  on  the 
Lake  Superior  Survey,  J.  W.  Foster  and  J.  D.  Whitney,  his  principal  assist- 
ants, were  appointed  to  succeed  him.     They  were  furnished  with  copies  of 


GEOLOGICAL   EXPLORATIONS    AND    LITERATURE— 1850.  25 

the  field  notes  of  the  Huear  surveyors,  in  whicli  many  ledges  of  iron-bearing 
rocks  had  been  located.  With  the  aid  of  these  and  their  own  observations 
they  presented  in  their  first  joint  report  a  fairly  good  general  view  of  the 
iron  district.  They  also  constinicted  a  map  which  gave  the  first  information 
we  possess  of  the  distribution  of  the  ore-bearing  rocks.  The  report  is 
simply  a  synopsis;  so  it  is  limited  to  general  statements.  With  reference 
to  the  iron  region,  we  (juote  (pp.  609-610): 

On  let'erring  to  the  map  wliich  accompanies  this  synopsis  it  will  be  seeu  that 
the  iron  occtu-s  in  a  metamorphic  formation,  bounded  by  two  granite  belts — one  ou 
the  north  and  the  other  on  the  south — and  that  it  is  prolonged  westerly  beyond  the 
Machigiimig  River.  This  formation  consists  of  hornblende,  talcose,  and  chlorite 
slates,  with  associated  beds  of  hornblende  and  felspar  rocks,  evidently  trappean  in 
their  origin.  In  that  portion  of  the  region  drained  by  Carp  and  Dead  rivers,  and 
even  in  the  head  waters  of  the  Escanaba,  the  trappean  rocks  rise  in  irregular  knobs 
and  ridges  from  100  to  200  feet  above  the  general  level  of  the  country,  and  from  800 
to  1,000  feet  above  the  lake  level.  To  the  west  and  south  of  Machi-gummi  (or  Big 
Lake)  the  ridges  are  less  abrupt,  and  there  are  some  townships  where  there  is  scarcely 
a  single  exposure  of  the  rock  in  place. 

A  description  of  the  ores  is  given,  their  banded  character  is  noted,  and 
the  great  abundance  of  good  ore  in  the  region  is  emphasized.  Nothing  else 
of  interest  concerning  the  iron  region  is  given  in  the  synopsis. 

On  the  map  the  color  for  the  metamorphic  rocks  of  the  Azoic  system  is 
made  to  cover  a  large  area  of  country  which  we  now  know  to  be  underlain 
by  older  rocks.  The  metamorphic  series  included  the  iron-bearing  rocks, 
but  besides  these  it  embraced  also  the  "green-schists"  north  of  the  iron  belt 
proper,  and  many  hornblende-schists  and  mica-schists  southeast  of  Lake 
Michigamme.  In  the  metamorphic  area  four  colors  are  used  to  distinguish 
the  four  rocks,  quartz,  saccliaroidal  limestone,  trappean  rocks,  and  the  undif- 
ferentiated schistose  series.  At  the  mouth  of  the  C'ar})  River  there  is  mapped 
a  small  area  of  sandstone,  belonging  at  the  base  of  the  Silurian  system. 
The  quartz  is  in  ranges,  beginning  as  two  ridges  at  the  lake  shore  on  both 
sides  of  the  Carp  River,  uniting  into  one  about  5  miles  inland,  and  continu- 
ing as  a  single  ridge  to  Teal  Lake  and  2  miles  beyond.  The  saccliaroidal 
limestone  is  represented  as  several  narrow  bands  occurring  along  the  north 
sides  of  the  eastern  quartz  ridges.     Presque  Isle  is  colored  for  basalt,  which, 


26  THE    MARQUETTE    IRON  BEARIXG   DISTRICT. 

like  the  granite  north  and  south  of  the  metamorphic  beh,  is  regarded  as 
younger  than  the  metamorphic  rocks. 

This  map,  which  from  the  necessities  of  the  case  was  ^'ery  general, 
served  as  a  good  basis  for  the  more  detailed  maps  published  later. 

1851. 

FosTEK,  J.  W.,  AND  WuiTNEY,  J.  D.  On  tlie  dift'erent  systems  of  elevation 
which  have  given  configuration  to  North  America,  with  an  attempt  to  identify  them 
with  those  of  Europe.     Proc.  Am.  Ass.  Adv.  Sci.,  Vol.  V,  1851,  pages  136-138. 

In  the  following  year  the  same  authors  published  a  general  paper,  in 
which  three  "grand  systems  of  elevation"  are  described  as  having"  deter- 
mined the  outlines  of  North  America.  The  first  of  these  is  the  "Lake 
Superior  system,"  which  ended  immediately  before  the  deposition  of  the 
Potsdam  sandstone.  The  culminating  points  of  this  pre-Potsdam  continent 
were  in  the  Lake  Superior  district.  It  "  stretched  out  in  a  long  and  narrow 
belt  of  land,  with  here  and  there  a  detached  island,  like  that  of  the  iron 
region  of  Missouri  or  that  of  Carp  Eiver."  Its  longest  direction  was  east 
and  west. 

Foster,  J.  W.,  and  Whitney,  J.  D.  On  the  Azoic  system  as  developed  in  the 
Lake  Superior  land  district.  (Abstract.)  Proc.  Am,  Ass.  Adv.  Sci.,  Vol.  V,  1851, 
pages  1-7. 

This  pajjer  is  an  abstract  of  the  well-known  report  referred  to  below. 
In  it  the  authors  refer  to  the  existence  in  the  Lake  Superior  region  of  a 
series  of  gneisses,  schists,  quartzites,  marbles,  and  iron  ores,  lying  uncon- 
formably  below  the  Potsdam  sandstone.  Most  of  the  rocks  are  regarded  as 
metamorphosed  sediments  that  have  been  changed  from  the  original  sand- 
stone into  subcrystalline  masses  that  have  lost  nearly  all  traces  of  their 
stratification.  They  have  been  subjected  to  the  most  violent  dislocations, 
appearing  now  as  vertical  beds  or  in  the  form  of  folds,  compressed  and  in 
some  cases  overturned.  With  these  are  associated  flows,  dikes,  and  bosses 
of  eniptive  rocks,  to  whose  existence  the  metamorphism  of  the  sediments 
is  ascribed.  Between  this  sj^stem  of  rocks  and  the  overlying  Potsdam 
sandstone  there  is  a  clear  and  well-defined  line  of  demarcation.     In  the 


GEOLOGIOAL   EXPLORATIONS    AND   LlTERATURE-lSni.  27 

rocks  below  this  line  are  evidences  of  intense  and  long-continued  igneous 
agency,  and  in  those  above  it  proofs  of  comparative  tranquillity  and  repose. 
These  jire-Potsdam  rocks  occupy  an  almost  continuous  belt  along  the  north 
shore  of  Lake  Superior,  and  are  extensively  developed  in  the  southern  shore, 
forming  the  watershed  between  the  respective  river  systems  of  Lake 
Superior,  Lake  Michigan,  and  the  Mississippi.  The  unconformable  super- 
position of  the  Potsdam  sandstone  of  the  Silurian  system  upon  the  quartzites 
of  the  Azoic  system  was  seen  near  Carp  River,  where  the  last-named  rocks 
occur  in  ripple-marked  beds  standing  nearly  vertical,  while  the  sandstone 
lies  around  it  in  nearly  horizontal  beds. 

The  Azoic  series  was  characterized  by  immense  deposits  of  iron  ore, 
and  the  Silurian  strata  by  deposits  of  copper.  Near  Teal  Lake  is  a  high 
hill  composed  of  alternating  layers  of  jasper  and  iron  ore  that  are  curiously 
contorted  and  plicated. 

It  is  impossible  to  form  a  correct  notion  of  the  thickness  of  the  Azoic 
series.  If  measured  across  the  edges  of  the  strata,  we  should  have  a 
thickness  greater  than  that  of  the  whole  fossiliferous  series.  The  strata, 
however,  are  plicated  and  folded,  so  that  in  measuring  across  their  edges 
the  observer  is  passing  over  a  repetition  rather  than  a  succession  of  beds. 

Foster,  J.  W.,  and  Whitney,  J.  D.  Report  on  the  geology  and  topography 
of  the  Lake  Superior  hind  district.  Part  II.  The  iron  region,  together  with  the 
general  geology.  Dated  November  12,  1851.  32d  Congress,  special  session,  1851. 
Senate  Documents,  Vol.  Ill,  No.  4.  400  pages.  With  plates  and  maps.  Abstract  in 
Bull.  Soc.  Geol.  Prance,  1850,  pages  89-100. 

A  little  later  in  the  same  year  Foster  and  Whitney  published  the  report 
which  sums  up  all  the  information  concerning  the  geology  of  the  Lake 
Superior  region  gathered  by  the  authors  during  their  four  years'  connection 
with  the  survey  of  the  Chippewa  land  district,  first  as  assistants  of  Dr. 
Jackson,  and  during  the  last  two  years  as  the  geologists  in  charge  of  the 
survey.  In  this  report  the  authors  present  an  account  of  the  geology  of 
the  entire  Upper  Peninsula  of  Michigan.  For  the  first  time  we  here  learn 
of  the  general  relations  to  one  another  of  the  various  rock  systems  in  this 
region,  and  obtain  the  first  definite  information  with  respect  to  the  detailed 


28 


THE   MARQUETTE   IRON  BEARING   DISTRICT. 


geology  of  the  iron-producing  district.  Although  imprinted  with  "The 
Iron  Region"  on  its  title-page,  the  description  of  the  iron  region  proper 
occupies  only  about  95  pages  of  the  report. 

The  map  accompanying  the  report  is  in  general  like  that  published 
with  the  authors'  earlier  report  (PI.  II),  though  the  colors  have  been 
changed.  Presque  Isle  has  for  some  reason  been  colored  as  granite,  and 
"basalt"  has  been  left  out  of  the  scheme  of  colors.  There  are  a  few  other 
differences  in  the  two  maps,  but  in  the  main  they  are  identical. 

The  report  is  introduced  by  a  tabular  statement  of  the  order  of  suc- 
cession of  the  rocks  existing  within  the  limits  of  the  district  studied.  This 
table  as  given  by  the  authors  (p.  2)  is  as  follows: 

Classification  of  the  rocks. 


Granite. 

Plutonic  rocks 

Syenite. 
[Feld.^par  and  quartz  rock. 

ISO'^ous 

Of  Tarious  ages 

[Greenstone,   or   dolerite, 
porphyry. 

Basalt,  amygdaloid. 
iTrappeanor  volcanic  rocks...  j  Hornblende  and  serpen- 
[      tine  rocks. 

Masses  of  specular  and 
1    magnetic  oxide  of  iron. 

Gneiss,  mica,  and  hornblende 
slate. 

Metamorpliic... 

A  ( ic  a   ste  1 

Chlorite,  talcose,  and  argilla- 

Formations.. 

ceous  slate. 
Beds  of  quartz  and  sacclia- 
roidal  marble. 

Potsdam  sandstone. 

ClUciferous  sandstone. 

Cbazy  limestone. 

Lower 

Bird's-eye  limestone. 
Black  Eiver  limestone." 

Trenton  limestone. 

Siluriiir  system 

Upper 

Galena  limestone. 

Hudson  Eiver  group. 
(Clinton  group. 
J  Niagara  group. 
[Onoudaga  salt  group. 

Aqueous 

Devonian  system 

Upper  Helderberg  series. 

Bedsof  sand,  clay,  and  gravel, 
rudely  stratified. 

Transported  blocks  of  gran- 
ite, greenstone,  etc. 

Alluvial  deposits 

Sand    and    pebble    beaches, 
marshes,  flats,  hooks,  spits, 

Of  these  various  divisions  of  rocks  there  are  present  in  the  region  with 
which  we   are  now  concerned  the  following':   the  Potsdam  sandstone  of 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 1S5L  29 

Silurian  age,  all  the  members  of  the  Azoic  system,  all  the  members  of  the 
Volcanic  series  except  the  basalts  and  amygdaloids,  and  representatives 
of  all  the  Plutonic  rocks. 

The  oldest  class  of  igneous  products  cousists  of  lioiublende  and  feldspar  rocks 
and  serpentine  rocks,  and  may  be  regarded  as  contemporary  with  the  Azoic  system. 
Next  in  order  are  the  granites  and  syenites,  which  are  intermediate  in  age  between 
tlie  Azoic  and  Siluriau  systems.  These  are  traversed  by  at  least  two  systems  of 
greenstone  dikes,  which  are  anterior  to  the  purely  sedimentary  deposits.    *     *     * 

Below  all  the  fossiliferous  groups  of  this  region  there  is  a  class  of  rocks  consist- 
ing of  various  crystalline  schists,  beds  of  quartz,  and  saccharoidal  marble,  more  or 
less  metamorphosed,  which  we  denominate  the  Azoic  system.  This  term  was  first 
applied  by  Murchisou  and  De  Verneuil  to  designate  those  crystalline  masses  which 
preceded  the  Paleozoic  strata.  In  it  they  include  not  only  gneiss  but  the  granitic 
and  plutonic  rocks  by  which  it  has  been  invaded.  We  adopt  the  term,  but  limit  its 
significance  to  tho.se  rocks  which  were  detrital  in  their  origin,  and  which  were  sup- 
posed to  have  been  formed  before  the  dawn  of  organized  existence.     (P.  .'?.) 

The  rocks  described  by  the  authors  as  comprising  the  Azoic  sy.stem 
include  "gneiss,  hornblende,  chlorite,  talcose,  and  argillaceous  slates,  inter- 
stratified  with  beds  of  quartz,  saccharoidal  marble,  and  immense  dejjosits 
of  specular  and  magnetic  oxide  of  iron."  Most  of  these  rocks  are  regarded 
as  metamorphosed  sediments  that  have  been  altered  by  intrusions  of  trajj, 
basalt,  and  serpentine,  whicli  occur  cutting  through  the  sediments  as  dikes, 
interleaved  with  them  as  sheets,  or  pi-otruding  through  them  as  bosses.  The 
rocks  are  contorted.  They  rarely  exhibit  the  characteristics  of  sediments, 
but  the  evidences  of  their  metamorphic  origin  are  plain,  since  they  become 
more  and  more  crystalline  as  the  great  "lines  of  igneous  outburst"  are 
approached.  "Gneiss  generally  flanks  the  granite,  succeeded  by  dark 
masses  of  hornblende  with  numerous  joints,  but  obscure  lines  of  bedding, 
which  often  graduates  into  hornblende  slate  or  chlorite  slate  as  we  recede 
from  the  purely  igneous  products"  (p.  14).  In  the  vicinity  of  the  settlement 
of  Marquette  "an  epitome  of  nearly  the  whole  geology  of  the  district"  may 
be  observed.  Here  the  authors  noted  the  existence  of  two  quartzite  ridges, 
one  on  each  side  of  the  Carp  River.  These  unite  farther  westward  and 
form  a  single  ridge  that  extends  beyond  Teal  Lake.  The  horizontal  Pots- 
dam sandstone  is  described  as  abutting  against  the  quartzite,  and  the  latter 


30  THE    MARQUETTE   lEON-BEARING   DISTRICT. 

rook  is  mentioned  as  containing  fragments  of  slate  and  jasper,  and  hence  as 
beino-  younger  than  these  rocks.  Grranite  is  said  to  have  intruded  the 
quartzite  of  the  southern  ridge,  causing  great  dislocations  in  its  beds,  and 
metaraorjjhosing  it  to  such  an  extent  as  to  destroy  its  bedding  planes. 
Siliceous  slates  and  marbles  are  interstratified  with  the  quartzites  near 
the  lake.  North  of  the  quartzite  range  the  country  as  far  north  as  the 
Dead  River  is  underlain  liy  chlorite-slates  and  talcose  slates,  intersected 
by  three  east-and-west  belts  of  igneous  rocks,  many  of  which  are  thought 
to  occur  as  sheets.  "Many  of  the  slates  appear  to  be  composed  of  pul- 
verulent greenstone,  as  though  they  might  originally  have  been  ejected 
as  an  ash  and  subsequently  deposited  as  a  sediment,  and  pass  by  imper- 
ceptible gradations  from  a  highly  fissile  to  a  highly  compact  state"  (p.  16). 
This  is  the  first  expression  of  the  view  that  some  of  the  greenstone-schists 
of  the  region  were  originally  volcanic  ashes,  although  the  illustrations 
oftered  are  not  always  of  the  rocks  which  were  later  shown  to  be  tuff's, 
and  the  processes  by  which  the  ashes  were  made  were  not  conceived  as 
the  same  in  nature  as  we  now  regard  them. 

Where  the  Azoic  slates  and  the  overlying  Potsdam  sandstone  are  in 
.  contact,  the  latter  may  plainly  be  seen-  to  be  the  younger  rock.  At  L'Anse 
(which  is  at  the  head  of  Keweenaw  Bay  and  outside  of  our  district)  there 
is  an  uuconformable  superposition  of  the  sandstones  and  the  slates.  The 
authors  picture  this  and  describe  it  in  some  detail  as  of  great  importance. 

This  .section  is  exceedingly  iustrnctive,  inasmuch  as  it  enables  us  to  draw  a  line 
of  demarcation  between  two  formations,  different  in  age  and  external  characters. 
While  the  newer  formation — the  Potsdam  sandstone — is  but  sUghtly  if  at  all  disturbed, 
and  little  changed  by  metamorphism,  the  older  or  Azoic  slates  are  contorted  and  folded 
into  numerous  arches,  and  in  several  places  invaded  by  igneous  rocks.  Their  structure 
has  been  changed  from  granular  to  subcrystalline,  and  the  whole  mass  is  intersected 
by  numerous  planes  of  lamination.     (P.  19.) 

The  granite  of  the  Azoic  was  fouml  to  form  many  of  the  points  of  the 
lake  north  of  Little  Presque  Isle,  and  to  occur  inland  in  rounded,  dome-like 
hills.  Everywhere  this  granite  is  cut  by  "powerful"  dikes  of  greenstone 
and  veins  of  quartz. 

In  describing  a  section  from  the  shore  of  Lake  Superior  across  Teal 
Lake  to  the  mouth  of  the  Escanaba  River,  the  authors  give  many  details  as 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 1851.  31 

to  the  occurrence  of  beds  of  conglomerate,  slates,  etc.  On  the  line  between 
sees.  29  and  32,  T.  47  N.,  R.  26  W.,  near  where  the  village  of  Palmer  is 
now  situated,  a  large  exposure  of  conglomerate  was  found  which  "is  made 
up  of  coai'se  blocks  of  various  sorts  which  belong  to  the  neighboring 
trappean  and  slaty  beds,  and  are  of  very  considerable  dimensions.  Among 
them  we  recognized  not  only  fragments  of  the  rock  associated  with  the 
iron,  but  masses  of  the  iron  itself,  and  of  the  banded  and  jaspery  varieties." 
This  is  evidently  the  description  of  an  exposure  of  Upper  Marquette  basal 
conglomerate.  The  authors  regard  it,  however,  as  a  friction  conglomerate, 
whose  origin  is  connected  with  the  "eruption"  of  the  adjacent  granite,  i.  e., 
the  granite  to  the  south.  The  ore  deposits  are  older  than  the  conglomerate, 
because  they  yielded  fragments  to  the  latter.  But,  since  these  and  other 
fragments  Avere  cemented  by  ferruginous  material,  it  '^^■as  presuijied  that 
"emanations  of  metallic  matter  must  still  have  been  issuing  from  beneath" 
when  the  conglomerate  was  formed  (p.  41).  The  relations  existing  between 
the  granite  and  the  Azoic  slates  are  thought  to  be  those  of  an  intrusive 
rock  to  an  intruded  series.  The  hornblende-slates  near  the  southeast  corner 
of  sec.  25,  T.  47  N.,  R.  27  W.,  for  instance,  are  penetrated  by  a  wedge  of 
granite,  "shooting  out  in  ramifying  branches."  Such  are,  indeed,  the 
relations  of  the  granite  to  these  hornblende-slates.  The  authors  did  not 
realize,  however,  that  these  hornblende-slates  are  older  than  man}^  of  the 
other  members  of  their  Azoic  series. 

From  the  quotations  that  have  been  given  it  is  seen  that  Foster  and 
Whitney  regarded  as  present  in  this  region  two  series  of  sedimentary  rocks, 
the  Potsdam  representing  the  Silurian,  and  the  Azoic  a  pre-Silurian  series. 
The  former  were  shown  to  be  much  younger  than  the  Azoic  rocks,  which 
are  much  metamorphosed.  The  Azoic  series  alone  is  cut  by  dikes  of  green- 
stone and.  by  granite ;  hence  both  these  rocks  are  younger  than  the  Azoic, 
but  not  so  young  as  the  Potsdam.     In  the  Azoic  occur  the  ore  beds. 

These  ore  beds  were  found  principally  in  a  belt  of  crystalline  schists 
and  intercalated  trappean  rocks,  bounded  on  each  side  by  a  belt  of  o-ranite. 
Many  occurrences  of  the  ore  were  located  in  the  present  Marquette  district, 
and  a  large  number  of  the  occurrences  are  described  at  some  length.  In 
some  of  these  exposures  dark  quartzites  are  associated  with  the  ores,  and  the 


32  THE   MARQUETTE   IRON  BEARING   DISTRICT. 

fragmental  rock  is  impreg-iiated  with  the  oxide.  This  observation  leads 
the  authors  to  conclude  that  the  diffusion  of  ore  through  the  rocks  must  be 
ascribed  to  some  general  cause  quite  independent  of  the  nature  of  the  rock 
itself  Man)'  descriptions  of  now  famous  ore  bodies  are  given.  It  is  fre- 
quently asserted  that  the  ore  beds  are  associated  with  the  greenstone  dikes, 
and  that  the  other  rocks  associated  with  the  ores  are  saturated  with  emana- 
tions of  iron  oxide.  In  sees.  10  and  11,  T.  47  N.,  R.  27  W.,  in  the  vicinity 
of  the  present  city  of  Ishpeming,  large  deposits  of  almost  pure  ore  were 
discovered.  Here  the  ore  "exhibits  many  of  the  characters  of  an  igneous 
erujDtive  rock,  and  can  not  be  regarded  in  any  other  light  than  as  a  huge 
lenticular  mass,  which  has  been  elevated  to  its  present  positioh  from  beneath 
while  in  a  semifluid  state,  exactly  in  the  same  way  as  the  trappean  ridges 
which  accompany  it  and  which  it  so  strikingly  resembles  in  general  outline 
and  jDosition."  After  discussing  the  various  theories  that  might  be  offered 
to  explain  the  existence  of  ore  and  jasper  in  the  forms  noted  in  the  region, 
the  authors  conclude  with  a  rtisumd  of  their  oj)inions  with  respect  to  the 
origin  of  the  Azoic  rocks  in  general  and  the  ore  beds  in  particalar.  AVe 
quote  the  resume  entire  (p.  G9): 

Wo  may  couceive  that  the  various  rocks  of  the  Azoic  series  were  originally 
deposited  iu  a  nearly  horizontal  ijosition,  at  a  period  prior  to  the  appearance  of  organic 
life  upon  the  earth;  that  these  stratified  deposits  were  comijosed,  for  the  most  part, 
of  finely  comminuted  materials,  principally  siliceous  and  argillaceous,  in  some  cases 
consisting  of  almost  pure  silex,  like  the  purest  portion  of  the  Potsdam  sandstone 
which  was  afterwards  deposited  upon  these  strata. 

During  the  deposition  of  these  strata,  at  various  intervals,  sheets  of  plastic 
mineral  matter  were  poured  forth  from  below  and  spread  out  upon  the  surface  of  the 
preexisting  strata.  These  igneous  rocks  are  exceedingly  compact  and  uniform  iu 
their  texture,  which  would  seem  to  indicate  that  they  were  under  heavy  pressure, 
probably  at  the  bottom  of  a  deep  ocean.  The  same  depth  of  water  is  also  inferred 
from  the  comparative  absence  of  ripple-marked  surfaces  throughout  the  whole  series. 

Daring  this  period  the  interior  of  the  earth  was  the  source  of  constant  emana- 
tions of  iron  which  appeared  at  the  surface  in  the  form  of  a  plastic  mass,  in  combina- 
tion with  oxygen,  or  rose  in  metallic  vapors,  or  as  a  sublimate,  perhaps  as  a  chloride; 
in  the  one  case  it  covered  over  the  surface  like  a  lava  sheet;  iu  the  other  it  was 
absorbed  into  the  adjacent  rocks  or  diffused  through  the  strata  in  process  of  formatton. 
Besides,  a  large  amount  of  iron  entered  into  the  composition  of  the  igneous  rocks  of 
this  period,  cliiefly  iu  combination  with  silica,  as  a  silicate  of  the  protoxide.     Portions 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 185L  33 

of  the  eruptive  masses  were  occasioually  subjected  to  denudation,  aud  the  ferruginous 
particles  were,  under  tbe  action  of  violent  currents,  spread  out  in  thin  beds,  or  swept 
into  some  depression  of  the  surface,  forming  a  lenticular  mass,  upon  which  the  strata 
were  afterwards  accumulated. 

When  the  siliceous  materials  had  become  impregnated  with  metallic  matter, 
which  may  have  been  scattered  more  or  less  uniformly  through  it,  a  rearrangement 
of  the  siliceous  and  ferruginous  particles  in  some  instances  took  place,  under  the 
action  of  segregating  forces,  by  which  the  whole  mass  assumed  a  banded  structure. 

Subsequently,  the  whole  series  of  beds,  slaty,  quartzose,  ferruginous,  and  trap- 
pean,  were  elevated  and  in  all  probability  folded,  perhaps  at  the  epoch  of  the  eleva- 
tion of  the  granite  ranges  on  the  north  and  south  of  the  ferriferous  belt  of  the  Azoic 
system. 

Fnim  this  quotation  we  see  that  while  the  authors  regarded  the  ore 
material  as  igneous,  they  nevertheless,  in  order  to  explain  the  banded  nature 
of  some  of  the  jasper  ores,  found  it  necessary  to  admit  the  action  of  segre- 
gating forces,  and  in  order  to  account  for  the  lenticular  forms  of  some  of 
the  deposits  they  made  use  of  the  ideas  of  denudation  and  deposition.  The 
reason  for  refusing  to  accept  the  theory  of  a  sedimentary  origin  for  the 
banded  ores  and  jaspers  in  general  is  apparent  from  the  following-  quotation 
(jDp.  67-G8): 

At  first  glance  this  banded  structure  might  be  regarded  by  some  as  the  result 
of  aqueous  deposition,  by  which  alternate  seams  of  quartzose  and  fen-uginous  matter 
were  spread  over  each  other,  and  the  whole  subsequently  solidified  and  welded 
together  by  hestt;  but  if  we  examine  the  circumstances  more  closely  it  will  be  found 
more  difficult  to  account  for  all  of  the  facts  under  this  hypothesis  than  might  at  first 
appear.  The  extreme  tenuity  of  these  bands,  which  are  often  no  thicker  than  a 
sheet  of  i)aper,  renders  the  supposition  of  their  analogy  to  strata  highly  improbable. 
In  fact,  this  banded  structure  in  many  of  the  Lake  Superior  ores — for  example,  at 
the  Cleveland  iron  knob — will  be  hardly  apparent  to  the  eye  on  fresh  fracture  of  a 
specimen,  the  weathered  surface  of  which  may  present  a  beautiful  series  of  intricate 
convolutions  of  alternate  bands  of  bright- red  and  steel-grey.  Besides,  on  examiuing 
this  mountain  mass  we  find  every  portion  exhibiting  equally  fine  and  equally  con- 
torted series  of  convolutions.  If  these  were  really  the  result  of  aqueous  deposition 
we  should  expect  from  analogy  with  other  deposits  of  a  similar  character  that  some 
of  the  layers  would  be  of  more  considerable  thickness  than  others,  and  that,  sui)pos- 
ing  the  contortions  to  have  been  caused  by  lateral  pressure  of  the  plastic  mass,  in 
some  cases  at  least  the  foldings  would  exhibit  a  considerable  radius  of  curvature, 
which  is  not  the  case  here. 
MON   XXVIII 3 


34  THE   MAEQUETTE   lEON-BEAEING   DISTEICT. 

Reference  has  already  been  made  to  the  discovery  by  the  authors 
of  great  numbers  of  greenstone  dikes  in  the  Azoic  schists,  and  of  knobs  of 
greenstone  scattered  among  those  of  the  ores.  They  find  great  difficulty 
in  distinguishing  between  the  greenstones  of  igneous  and  those  of  meta- 
morphic  origm.  "The  passage  of  one  into  the  other,  especially  in  the 
southern  portion  of  the  district,  seems  in  many  cases  to  be  gradual,  and 
their  general  appearance  and  structure  is  so  much  alike  that  it  is  often 
difficult  to  say  where  one  begins  and  the  other  ends."  Evidently  the 
authors  regard  many  of  these  greenstones  as  forming  portions  of  volcanic 
flows.     They  class  them  with  the  diorites. 

The  unconformity  at  Granite  Point  between  the  granite  and  the 
sandstone,  and  that  at  the  Car])  River  between  the  latter  rock  and  vertical 
quartzite,  were  observed  and  correctly  interpreted. 

From  1850  to  the  ajjpearance  of  Brooks's  report  in  the  Geology  of 
Michigan  in  1873  A'ery  little  additional  information  was  published  concern- 
ing the  relation  of  ore  deposits  to  their  associated  rocks  in  the  Marquette 
district.  A  number  of  short  papers  appeared  in  this  interval,  but  they 
treated  only  of  small  points  in  the  geology  of  the  region,  and  none  dealt 
with  the  relations  of  the  rocks  to  one  another. 

1852. 

BiGSBY,  John  J.  On  the  physical  geography,  geology,  and  commercial 
resources  of  Lake  Superior.  Edinburgh  l^ew  Philos.  Journ.,  No.  105,  July,  1852, 
pages  55-62. 

Bigsby  gives  a  general  account  of  the  geology  of  the  Lake  Superior 
region,  which  is  based  largely  on  the  reports  of  the  earlier  investigators. 
It  contains  nothing  that  had  not  already  been  commented  ujjon  by  others. 

1854. 

Whitney,  J.  D.  The  metallic  wealth  of  the  United  States.  Philadelphia, 
1854.     H.  E.  Schoolcraft.    510  pages. 

Whitney,  in  1854,  published  a  volume  containing  a  few  references  to 
the  iron  ores  of  Michigan,  repeating  the  statements  made  in  Foster  and 
"Whitney's  report  on  the  Iron  Region.     The  ores  are  described  as  occurring 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 1855.  35 

"  at  intervals  iii  a  belt  of  slates  from  6  to  25  miles  wide,  extending  for  a 
distance  of  150  miles  or  more  westward  into  the  State  of  Wisconsin." 


SCHOOLCEAPT,  Henrt  R.  Observations  on  the  mineralogy  and  geology  of 
the  country  embracing  the  sources  of  the  Mississippi  River  and  the  Great  Lake 
basins.  Summary  narrative  of  an  exploring  expedition  to  the  sources  of  the 
Mississippi  River  in  1820.     Philadelphia,  1855.     Pages  303-362. 

Henry  R.  Schoolcraft,  as  geologist  to  the  expedition  of  1820  to  the 
sources  of  the  Mississippi,  made  a  report  to  the  Hon.  John  C.  Calhoun, 
then  Secretary  of  War,  giving  a  brief  account  of  the  geology  of  the  region 
traversed  by  the  exploring  party  thirty-five  years  before.  In  this  we  find 
several  references  to  the  country  with  which  we  are  now  concerned.  The 
sandstone  near  Presque  Isle  and  at  Granite  Point  is  described  as  horizontal. 
The  first-named  point  was  recognized  as  "a  headland  of  serpentine,  resting 
against  which  is  a  curious  formation  of  magnesian  breccia"  (pp.  321-322). 
Magnetic  oxide  of  iron  is  reported  as  occurring  in  mountain  masses  in  the 
valley  of  the  Carp  River. 


Whitney,  J.  D.  On  the  occurrence  of  the  ores  of  iron  in  the  Azoic  system. 
Proc.  Am.  Ass.  Adv.  Sci.,  Vol.  IX,  1856,  pages  209-216. 

After  the  publication  of  the  "Report,"  Whitney  spent  portions  of  two 
seasons  in  the  Lake  Superior  region,  and  visited  the  iron  districts  of 
Missouri  and  northern  New  York.  The  author  finds  a  strong  analogy 
existing  between  the  relations  of  the  ores  with  the  rocks  associated  with 
them  in  these  two  districts  and  in  Scandinavia  and  the  relations  of  the 
Marquette  ores  with  their  associated  rocks.  All  the  facts  observed  con- 
firm him  in  the  belief  that  the  Lake  Superior  ores  are  Azoic.  They 
occur  in  large  quantities,  and  consist  almost  uniformly  of  specular  and 
magnetic  oxides.  Hydrous  ores,  carbonates,  etc.,  were  not  seen  in  the 
district,  though  it  is  thought  possible  that  they  may  exist  on  the  borders  of 
the  ore  deposits,  where  they  may  have  been  affected  by  the  adjacent  rocks. 
The  larger  deposits  are  described  as  lacking  the  characteristics   of  veins. 


36  THE  MARQUETTE   lEOif-BEAEING  DISTKICT. 

Some  of  the  smaller  ones  "approach  much  nearer  to  segregated  veins,  and 
might  be  classed  with  them,  were  they  not  developed  on  so  large  a  scale 
as  to  render  it  difficult  to  conceive  of  segregation  as  a  sufficient  cause  for 
their  production "  (p.  212).  Hence  the  author  declares  that  there  is  only 
one  hypothesis  that  will  explain  their  occurrence.  AVe  quote  as  follows 
(iDp.  212,  213-215): 

They  are  simply  parts  of  the  rocky  crust  of  the  earth,  and,  like  other  igneous 
rocks,  have  been  poured  forth  from  the  interior  in  the  molten  or  plastic  state.  No 
other  oi'igin  can  be  assigned  to  the  dome-shaped  and  conical  masses  of  Lake  Superior 
and  Missouri,  or  to  the  elongated  ridges  of  the  first-named  region. 

The  eruptive  origin  of  the  great  Lake  Superior  ore-masses  seems  also  well  sus- 
tained by  the  phenomena  which  they  exhibit.  They  alternate  with  trappean  ridges 
whose  eruptive  origin  can  not  be  doubted,  and  which  themselves  contain  so  much 
magnetic  oxide  disseminated  through  their  mass,  as  one  of  their  essential  ingredients, 
that  they  might  almost  be  called  ores.  These  eruptive  masses  include  the  largest 
and  purest  deposits  of  ore  which  are  known  in  the  Lake  Superior  or  the  Missouri 
iron  regions:  but  there  are  other  localities  in  both  these  districts  where  the  mode  of 
occurrence  of  the  ore  is  somewhat  dift'ereut  and  where  the  evidences  of  a  direct 
igneous  origin  are  less  marked.  This  class  comprehends  those  lenticular  masses 
of  ore  which  ai'e  usually  included  within  gueissoidal  rocks,  and  whose  dip  and  strike 
coincide  with  that  of  the  gneiss  itself,  but  whose  dimensions  are  limited.  *  *  * 
Such  beds  of  ore  as  these  may  in  some  cases  be  the  result  of  segregating  action;  but 
the  facts  seem  rather  to  indicate  that  they  are  made  up  of  the  ruins  of  preexisting 
igneous  masses  which  have  been  broken  and  worn  down  during  the  turbulent  action 
which  we  may  suppose  to  have  been  preeminently  manifested  during  the  Azoic  epoch, 
and  then  swept  away  by  currents  and  deposited  in  the  depressions  of  the  sedimentary 
strata  then  in  process  of  formation.  In  confirmation  of  this  hypothesis  *  *  *  it 
may  be  noticed  that  the  oi-es  occurring  in  this  form  and  position  are  less  pure  than 
those  of  decidedly  igneous  origin,  as  if  they  had  become  more  or  less  mixed  with 
sand  during  the  process  of  reconstruction,  so  that  they  not  unfrequeutly  require  to  be 
separated  from  their  earthy  impurities  by  washing  before  they  can  be  advantageously 
used.  Again,  it  may  be  observed  in  the  case  of  some  of  the  ore  beds  of  this  class 
that  the  bed  rock  or  foot  wall  is  considerably  rougher  or  more  irregular  in  its  outline 
than  the  hanging  wall  or  roof,  as  if  depositions  had  taken  place  upon  a  surface 
originally  rough  and  uneven,  the  upper  surface  of  the  ore  being  considerably  smoother 
and  more  regular  than  the  lower  one,  and  sometimes  separated  from  the  rock  by  a 
thin  seam  of  calcareous  matter. 

There  is  still  another  form  of  deposit  which  is  not  unfrequeutly  met  with  in  the 
Lake  Superior  region.    *     »     »     This  consists  of  a  series  of  quartzose  beds  of  great 


GEOLOGICAL    EXPLORATIONS   AND    LITERATURE— 185G.  37 

thickness,  and  passing  gradually  into  specular  iron,  which  freriuently  forms  bands 
of  nearly  pure  ore,  alternating  with  bands  of  quartz  more  or  less  mixed  with  the 
same  substance.  *  *  *  Deposits  of  this  character  are  usually  very  distinctly 
bedded.  »  *  *  These  deposits  seem  to  have  been  of  sedimentary  origin,  having 
been  originally  strata  of  siliceous  sand,  which  has  since  been  metamorphosed. 
The  iron  ore  may  have  been  introduced  either  by  the  sublimation  of  metalliferous 
vapors  from  below  during  the  deposition  of  the  siliceous  particles  or  by  precipi- 
tation from  a  ferriferous  solution,  in  which  the  stratified  rocks  were  in  process  of 
formation. 

RivOT,  L.  E.  Voyage  au  Lac  Superieur.  Ann.  des  Mines,  Ser.  V,  Tome  VII, 
1855,  pages  173-328;  Tome  X,  1850,  pages  365-474. 

Rivot,  of  the  Ecole  des  Mines,  Paris,  made  two  visits  to  the  Lake 
Superior  region  in  the  years  1854  and  1855,  respectively,  with  the  purpose 
of  studying  the  geology  of  the  copper  rocks,  more  particularly  on  Kewee- 
naw Point.  In  connection  with  his  study  he  made  a  general  survey  of  the 
granitic  and  the  iron-bearing  rocks.  According  to  this  author,  granites 
associated  with  gneiss  and  various  schists,  quartzite,  limestones,  slates,  and 
trap  form  a  belt  stretching  from  a  point  on  the  lake  north  of  Marquette  as 
far  west  as  the  State  of  Wisconsin.  They  constitute  a  metamorphic  series, 
and  in  them  are  great  beds  of  iron  ore,  associated  with  amphibole-schists. 
At  the  Jackson  mine  tlie  ores  are  accompanied  by  trap  and  by  talc-schists 
and  amphibole-schists,  in  which  the  "  primitive  stratification "  can  still  be 
detected.  To  the  north  and  south  the  iron-bearing  beds  are  limited  by 
conglomerates  composed  of  portions  of  all  the  rocks  noticed  in  the  region, 
cemented  by  a  ferruginous  matrix.  The  traps  associated  with  the  ores 
are  said  to  be  similar  to  those  in  the  Keweenawan  region.  They  pass  into 
the  amphibole-schists,  and  like  these  latter  are  believed  to  be  metamorphic 
and  "not  igneous  rocks"  (\).  413). 

All  the  "metamorphic"  rocks  are  believed  to  have  been  sediments 
which  have  since  their  deposition  been  metamorphosed  into  crystalline 
schists  and  granites,  the  latter  of  which  in  their  present  position  are  younger 
than  the  traps  and  sandstone  lying  upon  them.  In  other  words,  the  granite 
was  apparently  regarded  as  the  fused  basal  portion  of  a  sedimentary  series, 
which,  after  fusion,  intruded  the  upper  beds  of  the  series  (p.  231).  The 
granites,  schists,  traps,  and  sandstones  are,  seemingly,  all  believed  to  be  of 


38  THE   MARQUETTE   lEOX-BEARING   DISTRICT. 

Cambrian  age,  tlie  discordance  between  them  being  explained  as  due  to 
dislocations  caused  by  the  granite.  Even  the  sandstone  near  Marquette 
is  supposed  to  have  suffered  fracturing,  etc.,  through  the  action  of  the 
granite.  The  unconformity  at  Granite  Point  is  not  referred  to  specifically. 
Later  observations  have  shown  that  nearly  all  of  Rivot's  generaliza- 
tions regarding  the  origin  of  the  metamorphic  rocks  and  their  relations 
to  one  another  are  erroneous. 


Whitney,  J.  D.  Remarks  on  the  Huroniau  and  Laureutian  systems  of  the 
Canada  Geological  Survey.    Am.  Jour.  Sci.  (2),  Vol.  XXIII,  1857,  pages  305-314. 

Whitney,  in  1857,  published  an  article  which,  while  it  does  not  treat 
directly  of  the  Marquette  rocks,  does  so  indirectly.  The  Canadian  geolo- 
gists, as  a  result  of  their  studies,  had  come  to  the  conclusion  that  under 
the  Potsdam  of  the  western  Great  Lakes  there  are  two  great  unconform- 
able series  of  rocks,  called  by  them  the  Laurentian  and  the  Huronian  series. 
The  purpose  of  Whitney's  article  is  the  denial  of  the  existence  of  two 
series  beneath  the  Potsdam  of  the  south  shore  of  Lake  Superior. 


Leslie,  J.  P.  The  iron  manufacturer's  guide  to  the  furnaces,  forges,  and 
rolling  mills  of  the  United  States,  etc.    New  York,  1859.    Pages  480-489. 

This  author  gives  a  very  generalized  account  of  the  ores  of  Marquette, 
the  descriptions  of  their  geology  being  taken  mainly  from  the  reports  of 
Foster  and  Whitney  and  of  Rivot.  The  ores  are  said  to  be  in  a  gangue 
rock,  consisting  of  a  mixture  of  quartz  and  a  silicate  of  iron,  alumina,  and 
lime. 

Whittlesey,  Charles.  On  the  origin  of  the  Azoic  rocks  of  Michigan  and 
Wisconsin.    Proc.  Am.  Ass.  Adv.  Sci.,  Vol.  XIII,  1859,  pages  301-308. 

Charles  Whittlesey,  in  this  article,  deals  with  the  Marquette  district  only 
as  a  portion  of  a  larger  Azoic  area.  From  the  results  of  chemical  analyses  of 
many  rock  specimens  collected  from  northern  Wisconsin  and  Upper  Michi- 
gan he  concludes  that  there  are  two  metamorphic  series  in  the  Azoic,  both 
older  than  the  Potsdam,  one  characterized  by  the  presence  of  the  alkalis. 


GEOLOGICAL  EXPLOEATIONS   AND   LITERATURE— ISGl.  39 

sodium  and  potassium,  and  tlie  other  destitute  of  these  elements.  From  a 
careful  reading  of  Whittlesey's  article  it  appears  that  the  author  is  inclined 
to  doubt  the  sedimentary  and  metamoi-phic  origin  of  the  Huronian  ores 
and  to  advocate  an  eruptive  origin  for  them,  as  well  as  for  the  schists 
associated  with  them. 

1861. 

Hunt,  T.  S.  On  some  points  in  American  geology.  Am.  Jour.  Sci.  (2),  VoL 
XXXI,  ISGl,  pages  393-414. 

Hunt  reviews  the  conclusions  reached  by  studies  in  the  older  rock 
formations  of  America,  and  announces  that  Mr.  Alexander  Murray,  after  an 
examination  of  the  south  shore  of  Lake  Superior,  had  found  that  the  Mar- 
quette ores,  together  with  the  quartzites,  conglomerates,  limestones,  slates, 
and  the  "great  beds  of  diorite  which  we  are  disposed  to  regard  as  altered 
sediments,"  all  belong  to  the  Huronian  series  as  defined  by  the  Canadian 
survey,  and  to  that  portion  of  it  which  is  equivalent  to  Murchison's 
Cambrian  in  Scotland  (p.  394). 

WiNCHELL,  Alexander.  First  biennial  report  of  the  progress  of  the  geolog- 
ical survey  of  Michigan.     Lansing,  1861.    339  pages. 

By  an  act  approved  February  15,  1859,  the  State  of  Michigan  decided 
to  finish  its  geological  survey  begun  under  Dr.  Houghton.  Alexander 
Winchell  was  appointed  State  geologist.  He  published  one  report,  which 
is  devoted  almost  exclusively  to  the  geology  of  the  Lower  Peninsula.  In  a 
few ,  sentences  the  geology  of  the  Upper  Peninsula,  as  outlined  by  Foster 
and  Whitney,  is  described. 

This  report  constitutes  about  all  of  the  results  of  the  revivified  survey. 
It  was  evidently  abandoned  at  the  opening  of  the  war,  and  nothing  else 
was  done  by  the  State  in  the  way  of  geological  work  in  the  Upper 
Peninsula  until  the  second  survey  was  established  in  1869. 

1863. 

BiGSBY,  J.  J.  On  the  Cambrian  and  Huronian  formations.  Quart.  Jour.  Geol. 
Soc,  Vol.  XIX,  1863,  pages  36-52. 

Bigsby  correlates  the  Azoic  rocks  of  the  south  shore  of  Lake  Superior 
with  the  Huronian  of  Canada.     He  places  in  this  group  not  only  the 


40  THE   MAEQUETTE    IRON-BE  A  RING   DISTRICT. 

schists,  slates,  quartzites,  and  limestones,  but  also  "the  extraordinary  and 
extensive  intermixture  of  the  beds  of  greenstone  and  granite  which  defy 
description  and  classification." 

186.5. 

Kimball,  J.  P.  On  the  iron  ores  of  Marquette,  Michigan.  Am.  Jour.  Sci.  (2), 
Vol.  XXXIX,  18C5,  pages  290-303. 

In  the  year  1865  Kimball  published  the  most  imi)ortant  article  on  the 
iron  district  of  Marquette  that  appeared  between  the  rept)rt  of  Foster  and 
Whitney  and  that  of  Brooks,  In  it  the  author  contradicts  Whitney's 
notion  that  the  Azoic  of  the  Marquette  region  is  nondivisible.  Following 
Hunt,  he  divides  the  rocks  underlying  the  Lake  Superior  sandstone  into 
two  series,  the  -Laurentian  and  the  Huronian.  He  calls  attention  to  the 
fact  that  the  granites  are  separated  from  the  Azoic  schists  by  Foster  and 
Whitney  on  lithological  rather  than  structural  grounds,  and  therefore  that 
the  relations  of  the  schists  to  the  granite  have  not  been  established  upon 
sufficient  data.  From  his  own  observations  made  in  the  Huron  Mountains 
and  elsewhere  he  concludes  that  the  granites  and  the  associated  rocks  are 
metamorphic  and  indigenous  (were  formed  in  their  present  positions),  and 
are  not  exogenous  (intrusive),  as  Foster  and  Whitney  declared  the  granite 
to  be.  It  is  true  that  Mr.  S.  W.  Hill,  working  with  Foster  and  Whitney, 
discovered  a  granite  dike  intrusive  in  slates,  and  therefore  younger  than 
the  latter;  but  Kimball  explains  this  as  an  independent  dike,  not  in  any 
way  connected  with  the  greater  masses  of  granite.  On  account  of  the 
lithological  similarity  of  this  rock  with  the  Laurentian  granites  of  Canada, 
and  in  accordance  with  the  author's  notion  as  to  its  origin,  the  granites  and 
gneisses  of  the  Marquette  district  are  placed  in  the  Laurentian  series,  which 
is  older  than  the  Huronian. 

This  conclusion  is  correct,  but  the  granite  is  nevertheless  intrusive,  as 
Foster  and  Whitney  supposed. 

South  of  the  granite  and  its  associated  gneisses  lie  the  great  greenstone- 
schist  areas  of  later  authors.  These  are  described  by  Kimball  as  dark- 
colored  hornblende-schists,  which  represent  the  "base  of  the  Azoic  or  Huro- 
nian series."     They  are  separated  from  the  gneisses,  so  far  as  we  can  learn, 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE-! 865.  41 

simply  because  of  the  great  litliological  differences  between  the  two  sets 
of  rocks.     Following  these  schists  to  the  south  is — 

a  series  of  Mugitic  rocks  a'.ul  schists,  interstratilied  with  inagiiesian  hydrous  rocks 
aiKl  slates,  the  two  kinds  of  rocks  beiag  represented  on  the  one  hand  by  hypersthene, 
pyroxene,  and  bedded  diorite  passing  into  dioriteslates,  and  on  the  other  by  talcose 
and  chloritic  schists.  The  former  character  of  the  rocks  prevails  to  such  an  extent 
as  to  impart  to  the  lower  members  of  the  Huronian  series  a  distinctively  augitic 
aspect.  The  several  rocks  composing  this  augitic  zone  are  commonly  of  a  greenish 
color,  and  vary  in  this  respect  chiefly  as  to  shade,  resembling  in  this  particular  the 
lower  slate-conglomerate  which  marks  the  base  of  the  series  in  Canada,  and  from 
which  they  seem  to  differ  only  in  the  absence  of  pebbles  and  bowlders  from  the 
subjacent  Laurentian  rocks,  which  there  form  a  distinguishing  feature.     (P.  294.) 

From  this  quotation  it  is  plain  that  the  author  regards  these  green 
schists  as  of  sedimentary  origin,  and  as  forming  the  lower  division  of  the 
Huronian  series.  South  of  these  are  the  quartzites,  slates,  and  other 
plainly  fray-mental  rocks,  which  "are  associated  with  greenish  hornblendic 
slates  and  m^re  or  less  crystalline  diorite,  and  at  their  base  with  bands  of 
dolomite,  somewhat  siliceous  and  highly  altered."  Overlying  the  quartz 
zone  are  the  oi'es  of  the  region,  associated  with  talcose,  argillaceous,  and 
siliceous  schists.  These  are  thought  to  be  the  equivalents  of  the  upper 
slate-conglomerate  of  the  north  shore,  which  is  Upper  Huronian.  The 
ores  are  specular  schists  and  conglomerates  and  earthy  red  hematites. 
The  conglomerates  are  described  as  resting  upon  diorites  and  chlorite- 
schists  in  some  cases,  and  upon  dark  quartzites  in  others.  The  crystalline 
rocks  of  the  Huronian  are  regarded  as  metamorphic  as  well  as  the  schists. 
No  "trappean  overflows"  were  observed,  as  were  reported  by  Foster  and 
"Whitney.  Thus  Kimball  seems  to  place  himself  alongside  Rivot  in  denying 
the  igneous  origin  of  any  of  the  larger  masses  of  greenstone  in  the  region, 
while  at  the  same  time  he  acknowledges  the  igneous  nature  of  the  smaller 
dikes. 

In  addition  to  his  discussion  of  the  general  relations  of  the  Marquette 
rocks,  the  author  attacks  the  problem  of  the  genesis  of  the  ores,  and 
declares  very  strongly  in  favor  of  their  sedimentary  origin.  This  is  the 
first  time  any  definite  statement  had  been  made  in  contradiction  to  Foster 


42  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

and  Whitney's  declaration  that  the  g-reater  portion  of  the  specular  jasper 
is  eruptive. 

The  author  describes  the  entire  series  of  tlie  iron-bearing  rocks  as 
occurring  in  great  flexures,  with  a  uniform  trend  east  and  west. 

The  position  of  the  beds  of  specular  iron  ore  [is]  *  *  *  at  the  top  of  the 
Hurouian  series  as  developed  in  the  Marquette  region,  and  *  *  *  they  are  inter- 
stratifled  with  talcose  and  argillaceous  schists.  Sharing  the  plications  of  the  entire 
series,  these  specular  schists,  as  they  may  properly  be  called,  are  accordingly  folded 
into  synclinal  basins  and  anticlinal  crests,  of  which  the  axes  in  the  case  of  the  former 
lie  below  drainage,  in  the  bottoms  of  the  valleys,  and  in  the  case  of  the  latter  are 
commonly  obliterated  through  the  erosion  of  elevated  outcrops.     *     *     * 

The  bosses  of  specular  iron,  the  iron  knobs,  *  *  *  are  the  most  striking 
examples  of  exception  to  the  general  effects  of  denudation  already  noticed.  They  are 
instances  of  the  preservation  of  the  anticlinal  crest.     *     *     *     (P.  299.) 

The  oi-es  were  observed  to  be  associated  with  the  schists  in  these  folds, 
and  hence  they  must  have  been  "under  the  same  conditions  of  deposit  and 
secondary  modification"  as  these  latter,  which,  it  is  believed,  are  meta- 
morphosed sediments. 

The  hard  ores  of  the  region  are  shown  to  be  genuine  sediments. 
We  quote  the  author's  own  words  (pp.  301-302): 

Beds  of  specular  conglomerate  are  of  frequent  occurrence  throughout  the  iron 
region  of  northiern  Michigan,  consisting  of  a  paste  of  specular  peroxyd  of  iron, 
through  which  are  disseminated  fragments  of  jasper  and  rounded  pebbles  of  specular 
iron  ore,  which  usually  differ  from  the  paste  in  texture,  a  difference  very  perceptible 
among  ores  of  any  one  class,  eveu  within  narrow  limits  of  distribution.  These  con- 
glomerates not  unfrequently  resemble  breccia  in  the  angularity  of  the  jasper  frag- 
ments which  they  contain;  but  the  pebbles  of  specular  peroxyd,  although  sometimes 
obscure  in  a  matrix  of  the  same  material,  commonly  serve  to  indicate  the  detrital 
origin  of  these  beds.  That  they  are  derived  from  local  detritus  is  evident  from  the 
fact  that  the  jasper  fragments  are  not  rounded,  while  the  particles  of  softer  specular 
iron  ore  are  worn  but  slightly.  They  seem  to  be  of  littoral  formation  and  to  have 
been  derived  from  dismembered  and  crumbled  deposits  of  successive  lamiute  of  jasper 
and  iron  ore,  similar  to  those  deposits  distinguishable  in  the  bosses  of  the  region.  The 
specular  conglomerate  invariably  exists  under  circumstances  of  true  bedding,  and  is 
traversed  by  parallel  joints  splitting  the  imbedded  pebbles.  It  occurs  interstratified 
with  talcose  and  argillaceous  schists  quite  as  regularly  as  the  homogeneous  ores. 
*  *  *  A  specular  conglomerate,  uncontaminated  with  any  considerable  portion  of 
jasper,  forms  the  bulk  of  the  schists  at  present  wrought  by  the  Lake  Superior  mine. 


GEOLOGICAL   EXPLOEATIONS    AND    LITERATURE— I8GG.  43 

The  author  couckides  his  discussion  of  the  origin  of  the  ores  by  the 
statement  that — 

the  iron  ores  of  the  Huronian  series  in  Michigan  are  essentially  schists  and  heavy 
bedded  strata,  in  which  none  of  the  phenomena  of  aqueous  deposits  formed  by  precipi- 
tation from  water  on  the  one  hand,  or  by  detrital  accumulation  on  the  other,  are 
wanting.  They  exhibit  not  only  stratification,  anticlinal  and  synclinal  folds,  but  are 
invariably  traversed  by  systems  of  joints,  and  at  many  points  exhibit  a  perfect  slaty 
cleavage.    (P.  302.) 

As  for  the  greenstone-schists  and  greenstone-slates,  he  declares  that 
they— 

are  intermediate  in  composition  between  clay-slate  and  hornblende-slate,  and  together 
with  the  talcose  and  chloritic  slates,  with  which  they  are  interstratifled,  are  probably 
products  of  such  a  decomposition  in  the  wet  way  of  the  same  crystalline  sediments 
which,  entirely  or  less  undecomposed,  have  gone  to  form  those  greenstones  which 
constitute  members  of  the  same  series.     (P.  303.) 

This  means,  we  suppose,  that  the  greenstone-schists  were  deposited  as 
crystallized  sediments,  and  were  afterward  metamorphosed  in  the  presence 
of  water.     The  paper  concludes  (p.  303) : 

From  a  stratigraphical  point  of  view,  while  evidence  is  elsewhere  often  obscure, 
the  Huronian  greenstone,  schists,  and  iron  ores  of  Northern  Michigan,  in  the  absence 
of  close  attention  to  their  special  chemical  conditions,  exhibit  sedimentary  and  meta- 
morphic  phenomena  adequate  to  render  quite  untenable,  it  is  believed,  the  theory  of 
the  exotic  character  of  any  portion  of  them. 


Daddow,  S.  H.,  and  Bannan,  Bbnj.  Coal,  iron,  and  oil;  or  the  practical 
American  miner.     Pottsville,  Pa.,  1866,  pages  5J;G-550. 

In  connection  with  a  discussion  of  the  iron  ores  of  the  United  States, 
Daddow  and  Bannan  describe  those  of  the  Marquette  region  at  some 
length.  The  productive  magnetic  masses  of  the  district  are  believed  to  be 
the  result  of  precipitation.     They  are  said  to  be  uniformly  stratified. 

The  rocks  which  are  intercalated  with  the  ores  are  of  volcanic  origin,  and  though 
not  now  reposing  in  the  form  of  dikes,  they  are  true  volcanic  rocks,  disintegrated  by 
coming  in  contact  with  water  while  in  a  molten  state. 

The  authors  quote  a  private  report  by  Foster  to  the  Iron  Cliffs  Mining 
Company,   in    which,    however,    there    is    nothing    recorded    new    to    the 


4-1:  THE    MAKQUETTE    IKOiNBEARING   DISTRICT. 

literature  of  the  subject.  The  ores  are  divided  into  the  magnetites,  red 
hematites,  and  brown  hematites,  all  of  which  seem  to  be  regarded  as 
sedimentary  in  manner  of  deposition,  but  as  volcanic  in  origin.  Even  the 
conglomerates  of  the  region  are  explained  as  volcanic : 

Mucli  of  the  specular  ore  contains  fragiueuts  of  angular  jasper  iu  the  shape  of 
breccia,  evidently  the  disintegrated  portions  of  trappean  rocks  which  were  precipitated 
with  the  ores  when  the  molten  mass  was  thrown  into  the  surrounding  waters,  proving 
that  these  accumulations  of  ore-beds  and  intercalated  schist  owe  their  origin  to  local 
causes,  or  that  they  are  not  the  results  of  distant  formations,  but  that  they  are  true 
beds  formed  by  the  flow  of  molten  lava,  highly  impregnated  with  iron,  into  the  waters 
that  existed  around  and  perhaps  over  the  volcanic  vents.     (P.  548.) 


Credner,  Hermann.  Die  Gliederung  der  eozoischen  (vorsilurischeu)  Forma- 
tionsgruppe  Nord-Amerikas.  Zeits.  gesammt.  Naturw.,  Giebel,  Vol.  XXXII,  1808, 
pages  353-105. 

Hermann  Credner,  during  his  visit  to  North  America,  made  a  rapid 
examination  of  the  pre-Silurian  rocks  of  Michigan  and  Minnesota,  and 
announced  his  conclusions  regarding  them  in  two  articles,  of  which  the 
first  deals  with  the  general  relations  of  the  pre-Silurian  formations  to  one 
another  and  to  the  younger  rock  series  for  all  the  explored  parts  of  the 
United  States  and  of  Canada. 

The  author  agrees  with  Kimball  in  dividing  the  pre-Cambrian  rocks 
of  the  Marquette  district  into  two  divisions.  The  basal  rocks  all  over  upper 
Michigan  he  declares  to  consist  of  a  series  of  gneisses,  mica-schists  and  horn- 
blende-schists, granites,  and  syenites,  which  are  included  together  as  the 
Laurentian  series.  Above  these  unconformably  are  the  Huronian  beds. 
The  principal  rocks  of  the  Laurentian  are  mica-schists.  These  are  inter- 
bedded  with  granites,  syenites,  hornblende-schists,  and  gneisses,  the  whole 
forming  a  conformable  series  20,000  feet  in  thickness.  Details  are  given  and 
localities  are  described  in  which  the  relations  between  the  Laurentian 
and  the  overlying  Huronian  series  were  made  out,  but  these  localities  are 
without  the  limits  of  the  Marquette  area,  so  they  do  not  directly  concern 
us  at  the  present  time.  It  may  be  mentioned,  however,  that  at  Sturgeon 
River,   in  the  Menominee  district,  the  author  observed  a  great  layer  of 


GEOLOGICAL   EXPLORATIONS   AND    LITERATUKE— ISO'J.  45 

conglomerate,  wliieli  lie  tliou;i-lit  was  embedded  in  the  gneisses.  Largely 
because  of  this  observation,  he  coiu-ludes  that  the  whole  Laurentian  series 
is  a  Ijedded  one  of  metamorphosed  fragmental  sediments. 

In  the  Marquette  area  the  oldest  member  of  the  Huronian  series  is  a 
quartzite  impregnated  with  iron  oxide.  In  the  northern  portion  of  the  dis- 
tinct siliceous  hematite  replaces  the  quartzite,  and  in  both  poi-tions  of  tlie 
region  limestone  is  the  next  succeeding  rock.  Serpentines,  chlorite-schists, 
talc-schists,  and  diorites  are  interbedded  with  the  quartzites.  This  series 
of  rocks,  according  to  the  author,  was  deposited  close  to  the  shore-lines, 
whereas  the  Menominee  series  to  the  south  was  a  deep-sea  deposit.  The 
ores  of  the  Marquette  district  were  examined  quite  closely.  Tliey  are 
described  as  consisting  of  magnetite,  martite,  and  hematite. 

Structurally  the  Marquette  Huronian  forms  a  great  synclinal  l)asiu 
made  up  of  minor  synclinal  and  anticlinal  folds,  of  which  there  are  six  in 
the  neighborhood  of  Marquette.  Underlying  this  folded  series  unconform- 
ably  is  the  Laurentian,  and  above  it  unconforraably  is  the  Lake  Superior 
sandstone. 

18G9. 

Credneb,  Hermann.  Die  vorsiluri.scheu  Gebilde  der  "oberen  Halbiusel  von 
Michigan"  iu  Nord-Amerika.  Zeits.  der  dentscbeu  geol.  Gesellschaft,  Vol.  XXI,  1860, 
pages  516-568.    With  map  and  three  plates  of  sections. 

In  the  second  article  Credner  gives  more  specific  details  concerning 
the  geology  of  the  Upper  Peninsula  of  Michigan.  The  geology  of  Smiths 
Mountain  (now  Republic  Mountain)  and  of  the  Negaunee  region  is  fully 
described.  The  sequence  of  rocks  in  both  districts  is  given  as  the  autlif)r 
saw  them,  and  inferences  are  drawn  from  the  observations.  The  schistose 
greenstones  associated  with  the  t^iuartzites  and  ores  are  regarded  as  inter- 
bedded chlorite-schists,  and  the  massive  greenstones  as  interleaved  diorites. 
In  the  vicinity  of  Negaunee  the  lowermost  beds  are  quartzites,  replaced 
locally  b)^  limestones  and  interbedded  with  chlorite-schists,  and  above  these 
is  a  schist  complex  of  a  white  and  brown  banded  quai-tzite,  red  jaspers,  and 
hematites,  and  beds  of  pure  hematite,  with  two  interbedded  diorite  sheets 
and  a  bed  of  chlorite-schist.  The  character  of  the  ores  is  discussed.  Limon- 
ites  replace  the  hematites  locally,  as  at  the  Foster  mine,  and  magnetites 


46  THE   MARQUETTE   IKON-BEAKHSTG   DISTRICT. 

replace  them  elsewhere.  In  some  of  the  mines,  as  at  the  Lake  Superior 
and  the  Cleveland,  the  ores  are  closely  associated  with  octahedral  crystals 
resembling  magnetite,  except  that  they  possess  a  red  streak.  The  magnetites, 
hematites,  and  limouites  are  believed  to  represent  different  stages  of  devel- 
opment in  the  alteration  of  the  same  mineral  substance.  The  hematite  is 
regarded  as  an  oxidation  product  of  magnetite,  as  is  indicated  by  the  pseu- 
domorphs  of  the  former  after  the  latter  mineral  in  the  chlorite-schists.  The 
limonite  is  hydrated  hematite.  As  for  the  origin  of  the  magnetite,  it  is 
thought  possible  that  this  was  precipitated  from  a  solution  produced  by  the 
action  of  cai'bonated  waters  on  carbonate  of  iron,  and  that  the  entire  cycle 
of  changes  from  the  carbonate  to  limonite  was  completed  before  the  begin- 
ning of  Cambrian  time. 

The  lower  portion  of  the  Huronian,  represented,  according  to  the 
author's  view,  by  the  "chlorite-schists"  north  of  Marquette,  is  cut  by  dikes 
of  diorite,  and  north  of  Light-House  Point  by  a  dike  of  red  syenite  con- 
taining fragments  of  diorite,  aphanite,  quartzite,  chlorite-schist,  and  hematite. 
These  diorites  and  the  syenites,  he  declares,  are  the  only  eruptive  rocks 
observed  by  him  in  the  iron-bearing  region. 

The  chlorite-schists  referred  to  are  not  now  regarded  as  members  of 
the  iron-bearing  series,  so  that  the  iiorites  and  the  syenite  cutting  them  are 
not  necessarily  younger  than  the  iron  series.  The  syenite  dike  containing 
fragments  of  quartzite  and  hematite  has  not  been  seen  by  any  one  but 
Credner. 

,  A  geological  map  and  several  geological  sections  accompany  the  article. 

The  work  on  the  Marquette  district  up  to  the  close  of  the  sixties  and 
the  establishment  of  the  geological  survey  of  Michigan  had  been  concerned 
with  the  general  relations  of  the  rocks  to  one  another  and  their  separation 
into  large  groups  or  series.  Foster  and  Whitney  had  succeeded  in  differ- 
entiating the  pre-Cambrian  rocks  from  later  formations,  calling  the  former 
the  Azoic  series.  The  granites  associated  with  the  Azoic  rocks  were  claimed 
to  be  intrusive  into  these.  The  greenstones  so  abundantly  present  in  the 
Marquette  area  were  regarded  as  metamorphic  rocks,  with  the  exception  of 
the  small,  well-defined  dike  masses,  and  the  jasper  ores  were  thought  to 


GEOLOGICAL   EXPLORATIONS   AND    LITERATURE— 18G9.  47 

be  largely  eruptive.  The  ores  bedded  with  conglomerates  were  recognized 
as  sediments.  Hunt  added  the  next  important  suggestion  in  the  study  of 
the  region  by  declaring  the  ore-bearing  rocks  Huronian,  and  hence  younger 
than  certain  other  portions  of  the  Azoic,  which  represented  the  Laurentian. 
Kimball  tlien  made  as  thorough  a  study  of  the  Marquette  region  as  the 
conditions  allowed,  and  reached  conclusions  directly  at  variance  with  those 
of  Foster  and  Whitney.  Kimball  found  the  Azoic  divisible  into  the  Lau- 
rentian and  Huronian,  to  the  latter  of  which  series  the  ore  beds  belong. 
Both  the  Huronian  and  the  Laurentian  series,  together  with  the  green- 
stones in  the  Huronian,  were  concluded  to  be  metamorphic,  Avhile  the 
gi-anites  were  thought  to  represent  metamorphosed  sediments  older  than 
the  rest  of  the  Laurentian.  This  conclusion,  of  course,  was  directly 
opposed  to  that  of  Foster  and  Whitney,  who  believed  the  granites  to  be 
eruptive.  The  ores  of  the  Marquette  district  were  likewise  thought  to 
be  sedimentary  exclusively.  No  e^adence  of  an  eruptive  origin  of  the  ores 
was  found. 

The  remainder  of  the  publications  on  the  district,  up  to  the  time  of  the 
appearance  of  the  abstract  of  this  monograph,  were  confined  largely  to  the 
following  problems:  (1)  The  divisibility  of  the  "Azoic,"  (2)  the  origin  of  the 
gi-anites  and  greenstones,  and  (3)  the  origin  of  the  ores.  The  fii'st  problem 
could  not  be  solved  until  a  very  detailed  examination  of  the  entire  district 
had  been  made.  The  solution  of  the  second  problem  awaited  the  intro- 
duction of  the  microscope  as  a  working  tool  of  the  geologist.  The  third 
problem  became  the  principal  bone  of  contention. 

The  establishment  of  the  geological  survey  of  Michigan  and  the 
appointment  of  Maj.  T.  B.  Brooks  to  investigate  the  iron  district  were 
important  steps  in  the  solution  of  the  three  problems  referred  to.  Since 
the  appearance  of  Foster  and  Whitney's  report  in  1851  almost  nothing  had 
been  added  to  our  knowledge  of  the  geology  of  the  Marquette  district 
except  what  had  been  contributed  by  Kimball  in  1865  and  by  Credner  in 
18G9.  The  problems  to  be  solved  in  the  district  were  so  peii^lexing  and 
the  physical  difficulties  to  be  overcome  in  sohdng  them  were  so  enormous 
that  it" demanded  the  aid  of  the  State  to  enable  geologists  to  study  the 
area  with  any  degree  of  completeness. 


48  THE   MAKQUETTE   IllOX-BEAEIKG   DISTRICT. 


WiNCiiELL,  Alexander.  Keport  on  the  progress  of  the  State  geological 
survey  of  Micbigaii.     Lausing,  1871.     G4  pages. 

Tlie  second  State  geological  survey  of  Michigan  was  established  in 
May,  1869,  with  Alexander  Winchell  as  State  geologist.  Under  its  opera- 
tion several  valuable  reports  were  made,  and  a  new  era  in  the  history  of 
geological  work  in  the  ^larquette  district  was  ushered  in — an  era  of  activity 
such  as  had  not  been  known  since  the  days  of  Burt,  Hubbard,  Jackson, 
and  Foster  and  Whitney. 

In  his  report  of  progress  the  State  geologist  mentions  the  condition  of 
the  work  intrusted  to  his  care,  and  outlines  the  contents  of  the  proposed 
volumes  intended  to  be  issued  by  the  survey.  With  relation  to  the 
Marqviette  district,  he  states  that — 

tlie  rich  masses  of  magnetic  and  liematitic  ores  of  iron  are  found  not  to  be  those 
erupted  outbmrsts  which  the  older  geologists  were  inclined  to  regard  them.  They 
are  simply  constituents  of  the  system  of  sedimentary  deposits  which  make  up  the 
Hnrouian  system  of  Michigan.  The  diorites  of  the  region  appear  to  be  equally  of 
sedimentary  origin,  and  are  found  strictly  interstratitied  with  chloritic,  silicious, 
talcose,  argillaceous,  micaceous,  and  hematitic  schists.     *     *     *     (Pp.  26-27.) 

A  few  other  references  are  made  to  the  geology  of  the  Marquette  district, 
and  a  scheme  of  superposition  for  the  rocks  found  there  is  given;  but  the 
same  subjects  are  treated  more  fnlly  in  Brooks's  report. 


Beooks,  T.  B.  Iron-bearing  rocks  (economic).  Geol.  Surv.  of  Michigan,  Vol. 
I,  18(59-1873,  New  York,  1873,  Part  I.     319  pages.     With  maps. 

In  this  report  the  author  first  gives  a  histor}'  (if  the  de^'elopment  of 
the  iron-ore  industry  on  the  U])])er  Peninsula,  and  then  briefly  character- 
izes the  different  systems  of  rocks  occurring  therein  and  outlines  their 
distribution.  He  recognizes  the  Laurentian,  Huronian,  Copper-bearing, 
and  Lower  Silurian  series,  and  after  a  few  remarks  on  the  topograpliy  of 
the  region  underlain  by  the  rocks  of  each  series  he  proceeds  to  the  detailed 
description  of  the  Marquette  area.  It  is  with  this  portion  of  his  paper  that 
we  are  most  concerned. 


GEOLOGICAL   EXPLORATIONS   AXD   LITERATURE— 1S73.  49 

The  major  lithological  groups  recognized  in  the  Marquette  district  are 
the  ores;  the  ferruginous,  sihceous,  and  jaspery  schists;  the  diorites,  diorite- 
schists,  and  related  rocks;  the  magnesian  (chloritic)  schists;  the  quartzites, 
including  conglomerates,  breccias,  and  sandstones;  marble,  argillite  or  clay- 
slates,  and  related  rocks;  niica-schists ;  anthopliyllite-schists;  and  carljona- 
ceous  shales. 

The  ores  comprise  five  varieties,  viz:  the  red  specular,  including  slaty 
and  granular  aggregates  of  martite  and  magnetite;  the  magnetic  ores;  the 
mixed  ores,  consisting  of  interbanded  jasper  and  specular  ore;  the  soft 
hematites,  which  are  the  most  ferruginous  portions  of  a  limonitic  siliceous 
schist,  from  which  silica  was  probably  removed  by  tliermal  waters;  and  the 
flag  ores,  embracing  ferruginous  schists,  in  which  silicate  minei'als  are  often 
present.  These  latter  are  often  like  the  mixed  ores,  from  which  they  differ 
principally  in  geological  occurrence. 

The  diorites  and  their  schists  are  obscurely  Ijedded  rocks,  varying  in 
texture  from  aphanitic  to  coarse-grained  and  sometimes  porphyritic.  They 
are  composed  of  a  nonmagnesian  hornblende  and  a  plagioclase.  They 
occur  in  beds,  where  they  present  ' '  in  mass  precisely  the  same  phenomena 
as  regards  stratification  as  do  the  accompanying  schists  and  quartzites." 
In  the  Laurentian  area  rocks  similar  to  these  occur  as  dikes  and  veins,  and 
probably  as  beds.  In  the  Marquette  district  the  greenstones  are  abundant 
and  are  very  closely  associated  with  the  iron  ores. 

The  magnesian  schists  are  problematic  rocks,  consisting  largely  of  talc 
or  chlorite.  Their  cleavage  is  distinct,  but  their  bedding  planes  are  obscure. 
In  color  these  schists  vary  from  grayish  to  green.  They  are  intercalated 
with  the  pure,  hard,  and  mixed  ores  at  most  of  the  mines  worked;  Ijut  in 
a  few  of  the  mines,  and  in  the  quartzite  ridge  north  of  the  outlet  of  Teal 
Lake,  they  form  "slate  dikes."  The  author  finds  it  impossible  to  draw  the 
line  between  the  chloritic  schists  here  considered  and  certain  dioritic  schists 
like  those  mentioned  above.  At  the  Marquette  quarries  typical  chlorite- 
schists  are  found  bedded  with  granular  diorites. 

The  quartzites  are  recognized  as  occurring  in  three  principal  horizons. 
One  of  these  is  near  the  base  of  the  Huronian,  the  most  important  one  is  just 
above  the  hard-ore  formation,  and  the  third  is  near  the  top  of  the  series. 

MON  XXVIII i 


50  THE   MARQUETTE   lEOX-BEAEING   DISTRICT. 

The  first  is  known  as  the  h)wer  qnartzite  and  the  second  as  the  upper 
quartzite.  The  lower  quartzite  is  often  calcareous,  grading  in  places  into 
a  marble.  Sometimes  it  is  talcose.  Occasionally  it  is  interbedded  with 
aro-illite.  The  upper  quartzite  has  none  of  the  characteristics  of  the  lower 
bed,  but  on  the  other  hand  it  is  frequently  conglomeratic,  at  times  passing 
into  a  true  conglomerate.  The  lower  bed  is  rarely,  if  ever,  ferruginous, 
while  the  upper  one  is  composed,  at  several  places,  of  alternate  bands  of 
quartz  and  magnetite  sands.  The  marbles,  which  are  dolomitic  in  a  large 
measure,  are  regarded  as  a  phase  of  the  lower  quartzite,  which  they  overlie. 
This  marble  is  usually  siliceous,  and  is  filled  with  crystals  of  calcite  or 
dolomite  that  resemble  orthoclase  in  appearance. 

The  argillites  and  clay-slates  are  present  in  several  beds,  whose  relations 
to  other  beds  will  be  mentioned  later.  These  rocks  are  true  slates,  and 
many  of  them  are  above  the  upper  quartzite. 

The  mica-schist  group  embraces  a  number  of  different  rocks,  '\^dlose 
predominant  feature  is  the  possession  of  a  micaceous  constituent.  Some- 
times they  are  more  nearly  micaceous  quartzites  than  true  schists.  The 
mica-schists  often  contain  crystals  of  andalusite,  seams  of  black  hornblende, 
and  l)unches  of  white  quartz.  Three  horizons  of  the  schists  are  noted, 
the  most  important  of  which  is  near  the  top  of  the  series  in  the  western 
portion  of  the  district. 

Tlie  anthophyllite-schist  lies  immediately  Ixdow  the  highest  mica-schist 
horizon.  It  is  a  slightly  magnetic  rock,  varying  in  color  from  Ijrownish 
black  to  dull  slate.  It  shows  a  tendency,  in  some  places,  to  pass  into 
limonitic  schists,  and  so  may  pass  into  a  merchantable  ore.  Other  horizons 
of  the  schist  are  mentioned  and  their  places  in  the  series  fixed. 

The  carbonaceous  shale  may  be  a  carbonaceous  variety  of  the  clay- 
slate,  with  which  it  might  appropriately  be  placed.  It  contains  a  large 
quantity  of  graphite,  which  burns  off  when  the  rock  is  heated^  and  leaves 
it  white. 

After  characterizing  the  lithological  peculiarities  of  the  rocks  found  in 
the  district.  Brooks  describes  in  detail  each  of  the  principal  mines  worked  at 
the  time  the  survey  was  made. 


GEOLOGICAL   EXPLORATIONS  AND  LITEIIATURE    1873.  51 

Near  the  Republic  mine  outcrops  of  Huroniau  and  Laurentiau  rocks 
were  seen  in  such  relations  to  each  other  that,  althoug-li  no  contact  of  the 
two  series  was  observed,  the  author  nevertheless  concluded  that  the  former 
series  is  unconformably  upon  the  latter. 

The  Republic  and  Kloman  mines  are  described  in  more  detail  than 
most  of  the  others.  The  author  here  discovered  the  relations  of  the  differ- 
ent formations  to  one  another.  He  publishes  a  map  of  the  area  around 
these  mines,  which  is  so  accurate  and  comprehensive  that  the  district  has 
served  as  a  type  district  and  a  starting-  point  for  all  geologists  who,  since 
Brooks's  time,  have  worked  on  the  Marquette  iron  range.  We  quote  the 
author's  description  of  Republic  Mountain  (formerly  Smith's  Mountain) : 

The  numerous  outcrops  of  rock  aucl  ore  at  this  mouutaiu,  the  strong  magnetism 
possessed  by  three  of  the  beds,  the  remarkable  uniformity  iu  thickness  of  the  several 
formations,  and  the  bold  topographical  features  presented,  all  of  which  were  carefully 
surveyed  and  are  faithfully  represented  and  explained  on  the  accompanying  *  *  » 
maps  and  charts,  *  *  *  leave  but  little  more  to  be  said  in  this  place  regarding 
the  general  structure  of  Republic  Mountain. 

*  *  *  The  ten  formations,  represented  by  colors  on  the  map,  *  *  *  ^vjil 
now  be  enumerated,  commencing  with  the  lowest,  which  reposes  nonconforniably  on 
the  Laurentian  granites  and  gneisses. 

The  lowest  bed  of  the  .series  will  be  numbered  V,  for  reasons  which  will  liereafter 
appear.     (Pp.  125-126.) 

Then  follows  the  enumeration  of  the  beds,  which  is  given  here  in  more 
concentrated  form  than  appears  in  the  author's  report. 

At  the  bottom  of  the  series  is  a  quartzose  rock  (V),  followed  above  "by 
a  magnetic,  bright,  banded,  siliceous,  and  chloritic  schist"  of  various  colors 
(VI).  "The  greenish  layers  are  apparently  chloritic,  the  whitish  and  grayish 
are  quartz,  and  the  brown  and  dark  gray  are  siliceous  layers  of  the  red  and 
black  oxides  of  iron."  Following  the  schists  is  a  diorite  (VII),  and  above 
the  diorite  another  magnetic  siliceous  schist  (VIII)  like  VI.  A  diorite  (IX) 
again  appears  overlying  VIII,  and  .another  schist  (X)  similar  to  VI  and 
VIII  overlies  the  diorite.  This  schist  often  contains  enough  iron  to  make 
it  a  fairly  rich  flag  ore.  Formation  XI  is  a  coarse  diorite,  schist(ise  in 
places,  and  XII  is  a  reddish  quartz  or  jasper-schist.     The  iron  formation 


52  THE   MAEQUETTE  IKC)^^  BBAlilNG  DISTEICT. 

(XIII)  lies  above  the  jasper-schist.  It  consists  of  beds  of  mixed  ore  and 
jasper,  in  which  the  laminaj  are  contorted  and  twisted,  indicating- the  presence 
of  hxrger  folds  in  the  formation  as  a  whole.  Conglomerates  Avere  also  seen 
by  the  author  in  portions  of  tliis  belt.  Specular  ore,  magnetite,  and  a  bed 
of  magnesian  schist  make  up  the  balance  of  the  iron  formation.  Above  the 
iron  formation  is  the  upper  quartzite  (XIV),  which  near  the  contact  with 
the  ores  is  conglomeratic,  and  above  the  quartzite  is  another  bed  of  diorite 
(XY),  which  has  some  resemblance  to  the  micaceous  clay-slate  of  Spurr 
Mountain.  It  will  be  observed  that  the  author  has  fairly  good  evidence 
here  of  the  existence  of  an  erosion  interval  between  portions  of  the  iron 
formation  and  the  upper  quartzite;  but  unfortunately  he  regarded  the  pres- 
•ence  of  the  conglomerate  at  the  base  of  the  upper  quartzite  as  possessing 
little  significance.  This  conglomerate  is  the  same  as  that  reported  by 
Foster  in  1849.  It  will  be  referred  to  again  and  again  in  the  present 
volume,  for  it  is  largely  on  the  evidence  afforded  by  the  presence  of  such 
cono-lomerates  that  the  Huronian  within  this  district  has  been  divided  by 
later  authors  into  an  upper  and  a  lower  series. 

The  outcrops  of  the  above-mentioned  formations  present  on  the  surface 
-a  horseshoe-shaped  form,  which,  taken  in  connection  Avith  the  dip  of  the 
strata,  leaves  no  doubt  as  to  the  structure  of  Republic  Mountain.  "It  is 
evidently  the  southeast  end  of  a  synclinal  trough,  with  Smith's  Bay  in  the 
center,  under  Avhich,  at  an  unknown  depth,  all  the  rocks  represented  Avould 
be  found,  and  in  the  same  order."     (P.  129.) 

On  the  opposite  side  of  the  Michigamme  River  from  Republic  the 
'Continuation  of  the  Huronian  bands  was  not  found  where  expected,  and  so 
;a  fault  Avas  supposed  to  exist  through  the  bed  of  the  stream. 

The  account  of  the  geology  of  Republic  Mountain  includes  descrip- 
tions of  Formations  V  to  XV.  At  the  Spurr  mine  the  formations  from  XVI 
to  XIX  were  observed  as  folloAvs:  The  loAvermost  (northerly,  since  the 
beds  here  dip  south)  bed  at  Spurr  Mountain  is  a  clay-slate  (XV),  followed 
to  the  south  by  a  soft,  brownish,  ferruginous  rock  (XVI),  Avhich  may  be  a 
.decomposed  variety  of  the  anthophyllite-schist  (XVII)  Avhich  OA'erlies  it. 
The  ferruginous  rock  is  soft  and  is  not  found  in  outcrops  at  this  place, 
but  it  is  seen  in  a  ledge  east  of  Champion,  near  the  Keystone  mine.     The 


GEOLOGICAL   EXPLOEATIONS   AND   LITEEATURE— 1873.  53 

autliophyllite-scliists  are  exposed  at  the  Champion  furnace,  where  they 
lie  above  the  ferruginous  schist.  Formation  XVIII  is  not  seen  at  Spurr 
Mountain,  hut  it  is  found  at  the  west  end  of  Lake  Michigamme,  where  it 
appears  to  he  between  the  anthophylhte-schist  (XVII)  and  the  mica-schists 
(XIX)  exposed  on  the  south  shore  and  on  the  islands  of  the  lake.  The 
bed  is  a  gray  quartzite,  the  supposed  third  quartzite  of  the  region,  to  which 
reference  has  already  been  made.  The  mica-schists  are  the  youngest  mem- 
bers of  the  iron-bearing  series,  and  are  very  abundantly  developed. 

With  respect  to  the  position  of  the  strata  below  V  the  author  is  not  so 
confident.  He  thinks  that  the  iron  ores  and  the  associated  rocks  of  the 
Magnetic,  Cannon,  and  Chippewa  locations  in  the  vicinity  of  Republic 
belong  here.  These  rocks  are  different  from  any  of  those  described  as 
occurring  in  Formations  XV-XIX.  From  their  proximity  to  the  Laurentian 
they  are  supposed  to  be  the  oldest  members  of  the  Huronian.  They  are 
beds  of  siliceous  ferruginous  schists,  alternating  with  chlorite-schists  and 
diorite. 

The  geological  structure  in  the  mines  of  the  western  portion  of  the 
district  is  simple.  It  becomes  more  complicated  in  those  in  the  vicinity  of 
Ishpeming  and  Negaunee.  Beyond  these  towns  the  iron-bearing  horizons 
are  lost.  At  the  Lake  Superior  mines  the  ores  are  in  a  series  of  troughs 
with  east-and-west  axes.  Above  and  below  these  are  beds  of  chloritic 
schists.  On  the  east  side  of  the  mine  the  relations  of  the  rocks  are  so 
complicated  that  the  author  does  not  attempt  to  explain  them. 

The  reiiuiikable  features  are  the  great  masses  of  liglit  grayish-greeu  chloritic 
schist,  haviug  a  vertical  eastantl-west  cleavage,  no  disceruible  bedding  planes,  and 
holding  small  lenticular  masses  of  specular  ore,  which  conform  in  their  strike  and  dip 
with  this  cleavage,  and  which  seem  to  have  no  structural  connection  with  the  main 
deposits.  They  appear  like  dikes  of  ore  squeezed  out  of  the  parent  mass,  which  we 
may  suppose  to  have  been  in  a  comparatively  plastic  state  when  the  folding  took 
place;  or  they  may  have  been  small  beds,  contained  originally  iu  the  chloritic  schist, 
and  brought  to  their  present  form  and  condition  by  the  same  causes,  which  produce[d] 
the  cleavage  in  the  schist.     (Pp.  130-140.) 

In  the  hanging  wall  of  this  mine  Brooks  found,  instead  of  the  usual 
quartzite,  a  magnesian  schist,  similar  to  the  schist  associated  Avith  the  ore. 


54  THE   MARQUETTE   IKON-BEAKING  DISTRICT. 

After  the  rapid  survey  of  the  most  important  mines  is  concluded,  the 
author  writes  as  follows  (p.  143): 

Lookiug  back  over  the  field  we  have  now  hastily  surveyed,  *  *  *  it  will  be 
seen  that,  while  there  are  mauy  irregularities,  on  the  whole  the  ore  basin  gradually 
widens  toward  the  west,  from  a  mere  point  at  the  Jackson  niiuc  to  a  width  fully  5 
miles  at  the  west  end  of  Michigamme  Lake,  beyond  which  too  little  is  known  to 
enable  us  to  accurately  define  its  limits.  It  follows,  therefore,  that  all  the  Huronian 
rocks  north,  east,  aud  south  from  the  Jackson  mine  are  below,  or  older  than  the  ore 
formation  (XIII),  and  all  the  rocks  to  the  westward  aud  inside  of  the  ore  basin  are 
younger,  hence  above  it. 

The  country  southeast  of  the  Jackson  mine  jn-oduces  dark-colored, 
earthy  hematite. 

I  believe  these  ores  all  belong  to  one  formation.  No.  X,  in  which,  up  to  this  time, 
no  merchantable  ores,  except  the  Lake  Angeline  hematite,  have  been  meutioned  as 
occurring.  It  is  at  least  certain  that  they  are  older  than  Formation  XII,  which 
embraces  the  Lake  Superior  and  Winthrop  deposits.     (P.  143.) 

The  Cascade  range  lies  south  and  east  of  Negauuee,  extending  east 
and  west  through  the  southern  portion  of  T.  47  N.,  R  26  W.  The  ores  here, 
are  jaspery  oxides  with  a  "flaggy  structure."  They  are  near  the  Laurentian, 
and  the  whole  series  is  overlain  by  a  talcose  quartzite,  believed  by  the  author 
to  be  the  equivalent  of  No.  V  of  the  Republic  series,  and  to  be  a  continuation 
of  the  same  bed  that  forms  the  hills  north  of  Teal  Lake  and  becomes  calca- 
reous at  Morgan  furnace.  On  this  supposition  the  Cascade  ores  are  older 
than  those  of  Republic,  and  are  the  equivalents  of  the  ores  of  the  Magnetic, 
Cannon,  and  Chippewa  mines.  The  absence  of  ores  north  of  the  Teal  Lake 
quartzite  is  thought  to  require  investigation,  since  the  ores  of  the  Cascade 
range  are  supposed  to  be  inunediateb^  beneath  the  continuation  of  this 
quartzite. 

The  most  prominent  of  all  the  formations  of  the  Marquette  district  is 
this  lower  quartzite.  It  is  so  uniformly  present  and  at  such  a  constant 
horizon  that  an  account  of  its  distribution  is  largely  an  account  of  the 
structure  of  the  entire  iron-l)earing  series. 

A  brief  description  *  *  *  of  the  great  geological  basin  formed  by  this 
quartzite,  which  embraces  within  its  folds  the  great  mass  of  the  Huronian  rocks,  and 
nineteen-tweutieths  of  all  the  ore,  will  possess  interest.    Like  the  ore  horizon  (XIII), 


GEOLOGICAL   EXPLORATIONS  AND   LITEKATURE— 1873.  55 

which  we  saw  came  to  a  i)oiut  at  the  Jackson  mine,  and  widened  to  the  west,  so 
the  opposite  croppiugs  of  this  quartzite  converge  to  the  east  and  come  together 
at  the  Chocolate  Flux  quarry  [on  the  shore  of  Lake  Superior].  *  *  *  From  this 
starting  point  the  south  rim  of  the  basin  bears  away  toward  Goose  Lake,  where  some 
minor  folds  and  low  dips  make  it  the  surface  rock  for  a  large  area  northeast  of  the 
lake.  From  the  south  end  of  the  lake  west,  the  formation  has  a  prevailing  taleky  char, 
acter,  often  argillaceous  and  sometimes  conglomeratic;  it  has  a  great  thickness  and 
strikes  west  by  south.  West  of  the  Cascade  it  seems  to  assume  more  the  character 
of  a  chloritic  gneiss  and  protogine,  or  at  least  a  well-defined  bed  of  protogine  rock 
occupies  the  position  in  which  we  would  expect  to  find  the  quartzite. 

The  northerly  rim,  starting  also  from  the  Chocolate  quarry,  maintains  a  nearly 
due-west  course,  crossing  the  railroad  at  Morgan  Furnace,  *  *  *  passes  north 
of  Teal  Lake  and  south  of  Beer  Lake,  [is  seen]  occasionally  at  various  points  further 
west,  and  last,  so  far  as  I  know,  north  of  the  Spurr  Mountain,  nearly  40  miles  west 
of  Lake  Superior.     (Pp.  149-150.) 

The  g-eneral  geology  of  the  entire  district  covered  by  his  report  is 
described  b}'  Brooks  in  a  chapter  on  the  magnetism  of  the  iron-bearing 
rocks.     This  we  quote: 

Rocks  of  the  four  oldest  geological  epochs  yet  made  out  on  this  continent  are 
represented  on  the  Upper  Peninsula  of  Michigan ;  two  belonging  to  the  Azoic,  one  to 
the  Lower  Silurian,  and  one  between  these,  of  questioned  age.  The  equivalency  of 
these  with  the  Canadian  series  has  not  been  fully  established,  but  the  nomenclature 
of  the  Canadian  geologists  will  be  employed  ijrovisionally. 

The  Laureutiau  of  the  Upper  Peninsula  is  like  that  of  Canada  in  being  largely 
made  up  of  granitic  gneisses,  but  differs  in  containing  no  limestone  so  far  as  I  have 
seen,  and  little,  I  may  say  practically  no  iron  ore,  and  very  little  disseminated 
magnetite.  Next  above  the  Laurentian,  and  resting  on  it  noncouformably,  are  the 
Huronian  or  iron-bearing  rocks;  these  are  also  called  by  the  Canadian  geologists 
"the  lower  copper-bearing  series."  This  series  comprises  several  plainly  stratified 
beds  of  iron  ore  and  ferruginous  rock,  varying  in  the  percentage  of  metallic  iron  from 
15  to  67  per  cent,  interstratified  with  greenish  tough  rocks,  in  which  the  bedding  is 
obscure,  which  appear  to  be  more  or  less  altered  diorites,  together  with  quartzites 
(which  pass  into  marble),  clay-slates,  mica-schists,  and  various  obscure  magnesian 
schists.  The  maximum  thickness  of  the  whole  in  the  Marquette  region  is  not  far 
from  5,000  feet. 

While  the  great  Huronian  area  of  Canada  north  of  Georgian  Bay  bears,  so  far 
as  I  am  aware,  little  or  no  workable  iron,  and  derives  its  economic  importance  from  its 
ores  of  copper,  the  Marquette  series,  supposed  to  be  of  the  same  age,  are  eminently 
iron-bearing,  and  have  as  yet  produced  no  copper.     It  is  doubtful  if  in  the  same  extent 


56  THE  MAEQTTETTE   lEON-BEAEING   DISTRICT. 

and  thickness  of  rocks,  anywhere  in  the  world,  there  is  a  hirger  percentage  of  iron 
oxide  than  in  the  Marquette  series.  In  the  order  of  relative  abundance,  so  far  as  made 
out,  the  ores  are  the  flag,  the  red  specular  hematites,  soft  or  brown  hematites,  and 
magnetites.  These  all  exist  in  workable  beds,  and  all  as  disseminated  minerals  in 
rocks  usually  siliceous.  *  *  *  So  far  there  seems  to  be  the  greatest  conceutra  - 
tion  of  magnetic  ores  in  the  Michigamme  district  of  the  Marquette  region.  From  this 
the  relative  proportion  of  magnetite  seems  to  decrease  as  we  go  east,  north,  west, 
and  south. 

Next  younger  than  the  Huronian  are  the  copper-bearing  rocks  of  Keweenaw 
peninsula,  *  *  *  the  age  of  which  has  led  to  much  controversy.  *  *  *  The 
relations  of  the  copper-bea  ing  rocks  to  the  Huronian  are  not  fully  made  out.  In 
tracing  the  dividing  line  from  Bad  River  in  Wisconsin  to  Lake  Gogebic,  Michigan, 
last  fall  *  *  *  we  found  them  nearly,  if  not  precisely  conformable,  but  widely 
different  in  lithological  character    *     *     *. 

The  next  series  of  rocks  in  ascending  order  are  the  horizontally  bedded  Lower 
Silurian  sandstones,  which  skirt  the  south  shore  of  Lake  Superior  nearly  its  whole 
length,  called  by  Foster,  Whitney,  and  Dr.  Eominger,  Potsdam,  and  assigned  by  the 
Canadian  geologists,  under  the  name  St.  Mary's,  to  a  later  period.    *     *     * 

We  will  now  return  to  the  Huronian  or  highly  magnetic  series,  taking  up  its 
structure  in  some  detail.  About  nineteen  lithologically  distinct  beds  or  strata  make 
up  the  series;  of  these,  six  and  probably  seven  are  so  magnetic  as  to  cause  con- 
siderable variations  in  the  needle.  These  beds  vary  from  40  to  several  hundred 
feet  in  thickness,  and  strike  and  dip  in  all  directions  and  at  all  angles.  The 
prevailing  strike,  however,  is  easterly  and  westerly,  and  the  dip  at  high  angler 
often  vertical.     *     *     *     (Pp.  215-218.) 

The  sequence  of  the  strata  in  the  Marquette  series  is  outlined  as  follows: 

I,  II,  III,  IV  are  composed  of  beds  of  siliceous  ferruginous  schist,  alternating 
with  chloritic  schists  and  diorites,  the  relations  of  which  have  not  been  fully  made 
out;  V  is  a  quartzite,  sometimes  containing  marble  and  beds  of  argillite  and  novacu- 
lite;  VI,  VIII,  and  X  are  siliceous  ferruginous  schists;  VII,  IX,  and  XI  are  dioritic 
rocks,  varying  much  in  character;  XIII  is  the  bed  which  contains  all  the  rich  spec- 
ular and  magnetic  ore,  associated  with  mixed  ore  and  magnesian  schist;  XIV  is  a 
quartzite,  often  conglomeratic;  XV  is  argillite  or  clay-slate;  XVI  is  uncertain;  it 
contains  some  soft  hematite;  XVII  is  anthophyllitic  schist,  containing  iron  and 
manganese;  XVIII  is  doubtful;  XIX  is  mica-schist,  containing  staurolite,  andalusite^ 
and  garnets.    *     *     * 

These  beds  appear  to  be  metamorphosed  sedimentary  strata,  having  many  folds 
or  corrugations,  thereby  forming  in  the  Marquette  region  an  irregular  trough  or 
basin,  wliich,  commencing  on  the  shore  of  Lake  Superior,  extends  west  more  than  40 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 1873.  57 

miles.  *  *  *  AVhile  some  of  tbe  beds  present  lithological  characters  so  constant 
that  tbey  caji  be  identified  wherever  seen,  others  undergo  great  changes.  Marble 
passes  into  quartzite,  which  in  turn  graduates  into  novaculite;  diorites,  almost  por- 
phyritic,  are  the  equivalents  of  soft  magnesian  schists.  *  *  *  Tlie  total  thickness 
of  the  whole  series  in  the  Marquette  region  is  least  at  Lake  Superior,  where  only 
the  lower  beds  exist,  and  greatest  at  Lake  jMichiganime,  where  the  whole  nineteen  are 
apparently  present,  and  may  have  an  aggregate  thickness  of  5,000  feet.  (Pp.  83-84.) 
With  regard  to  the  associations  of  the  various  ores,  it  may  be  said  that  magnetic 
and  specular  ores  are  often  found  together,  as  are  also  the  specular  and  soft  hematite 
ores;  but  so  far  the  magnetites  and  hematites  have  not  been  found  in  juxtaposition. 
If  we  suppose  all  our  oi-es  to  have  once  been  magnetic,  and  that  the  red  specular  was 
first  derived  from  the  magnetite,  and  tbe  hydrated  oxide  (soft  hematites)  in  turn  from 
it,  we  have  an  hypothesis  which  best  explains  many  facts,  and  which  will  be  of  use  to 
the  explorer.     (Pp.  220-22L) 

Besides  the  magnetic  charts,  three  geoh^gical  maps  pertaining-  to  the 
Marquette  district  accompany  the  report.  One  represents  the  general  geol- 
ogy of  the  entire  Upper  Peninsula  (see  PI.  Ill),  the  second  is  a  detailed 
map  of  the  whole  of  the  Marquette  district,  and  the  third  is  a  large-scale 
map  of  Republic  Mountain.  The  northern  boundary  of  the  iron  rocks  is 
placed  much  farther  north  by  Brooks  than  it  is  in  this  monograph.  Brooks 
included  with  his  Huronian  all  of  the  gi-eenstone-schists  north  of  the  iron- 
bearing  rocks,  and  made  their  contact  with  the  granite  the  boundary  line 
between  the  Marquette  iron-bearing  rocks  and  the  Laurentian.  These 
schists  are  Group  XIII  of  Brooks's  seiies,  and  are  regarded  bv  him  as 
high  in  the  series.  In  this  volume  they  are  placed  below  the  whole  of  the 
iron  series. 

Several  appendixes  are  added  to  Brooks's  report  and  published  as 
Vol.  II  of  the  Michigan  State  survey.  Some  of  them  are  of  great  scientific 
interest.  Those  of  Julien  and  of  Charles  E.  Wright  are  the  first  articles  in 
which  the  lithological  features  of  the  Marquette  rocks  are  described  in  detail. 

Julien,  Alexis  A.  Lithological  descriptions,  etc.,  of  259  specimens  of  the 
Huronian  and  Laurentian  rocks  of  the  Upper  Peninsula.  Geol.  Surv.  of  Michigan, 
1809-1873,  Appendix  A,  Vol.  II,  New  York,  1873,  197  pages. 

The  aim  of  Julien's  report,  in  the  words  of  its  author,  as  given  in  the 
letter  of  transmittal  addressed  to  Brooks,  "is  but  a  provisional  one,  viz,  to 


58  THE   MAEQUETTE  IRON-BEARING  DISTRICT. 

give  a  somewhat  popular  description  of  such  characteristics  of  the  common 
varieties  [of  the  Marquette  rocks]  as  may  be  easily  discerned  (with  a  very 
few  exceptions)  in  the  field,  *  *  *  and  also  to  propose  a  temporary 
nomenclature  and  classification  for  the  present  use  of  your  report." 

The  rocks  are  divided  into  three  great  divisions,  the  simple  rocks,  the 
mixed  crystalline  rocks,  and  the  fragmental  rocks.  The  first  division  is 
subdivided  into  calcareous,  quartzose,  silicate,  iron  ore,  and  carbonaceous 
rocks,  and  the  mixed  crystalline  rocks  into  older  and  younger  feldspathic 
rocks.  The  former  includes  granites,  gneisses,  mica-schists,  greenstones,  and 
trappean  diorites,  and  the  latter  only  diorite-aphanite.  The  fragmental  rocks 
examined  were  sandstone-schists. 

The  greenstones  comprise  diorite,  am])hibolite,  serpentine,  chloritic  dio- 
rite,  diorite-wacke,  diorite- schist,  amphibole-schist,  diorite-greenstone,  mica- 
ceous greenstone-schist,  schalstone,  aphanite-schist,  and  chlorite-potstone. 
All  these  varieties  are  supposed  to  be  derivatives  of  diorite,  and  none  of 
them  were  believed  to  be  derived  from  diabase.  "I  am  decidedly  of  the 
opinion,"  writes  Julien,  "that  no  augite  occurs  in  these  rocks,  and  that  there 
is  no  diabase  whatever  in  this  region"  (p.  42).  The  trappean  diorite  and 
the  diorite-aphanite  are  intrusive  basic  rocks,  but  according  to  the  author 
they  contain  no  pyroxene. 

The  simple  silicate  rocks  determined  are  amphibolite,  amphibole-schist, 
hornblende-schist,  anthophyllite-schist,  chloritic  schist,  argillite,  and  talcose 
schist.  The  argillites  are  placed  in  this  di\asion  rather  than  among  the 
fragmental  rocks  because  some  of  them  were  believed  to  be  composed  of 
greenstone  (diorite)  ash. 

A  few  mineralogical  notes  close  the  report. 

Beooks,  T.  B.,  aud  Jtjlien,  A.  A.  Catalogue  of  tbe  Michigan  State  collection 
of  the  Huroniau  rocks  and  associated  ores.  Geol.  Sarv.  of  Michigan,  1869-1873, 
Appendix  B,  Vol.  II,  New  York,  1873,  pages  199-212. 

The  second  appendix  is  a  classified  list  of  the  collections  of  Michigan 
rocks  distributed  l)y  the  State  to  certain  colleges  and  institutions  in  this 
country  and  abroad. 


.  GEOLOGICAL  SURVEY 


MONOOBAPH  XXVJII     PL.  I 


|Laurentian  ^  |  |  Huronian  ^^^^ Potsdam  or  St.Marys  Sandstone 

Fig.  1.— portion  OF  BROOKS'S  MAP  OF  THE  UPPER  PENINSULA  OF  MICHIGAN. 


^^^^^^B^^ 

1 

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^^^^^^^^^^^^^^^A 

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s 

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>,Vp''?:C^Vs',^--\ii^ 

I. 

V^^^VV^V^^^^^E?^^'''iiii '^/''''  '^JJ^'  "'/  ^'hv^i'""^'  ^•"'  *  '  '■'^i-'^'i^l^^f^r'f^ 

J^'\  -,  -ji^-'^t^J^f'i.y^S^^^^^'^^^w^ 

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^^^^^^^^M^^^^M^^m 

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k 

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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^1 

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V — ^                               -  N-;/,x.^i  ',  _  '^  N/  /^^    v^ 

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l>^lv^) 


Lake  Superior  sand: 


Fig.    2.  — IRVING'S  OUTLINE   MAP  OF  THE   MARQUETTE    DISTRICT. 


GEOLOGICAL   EXPLORATIONS   AXD    LITEIIATITKE— 1873.  59 

Weight,  Charles  E.  jMicroscopic  cletermiuatious  and  descriptious  of  7S  speci- 
mens of  Huronian  rocks  and  ores.  Geol.  Surv.  of  Michigan,  1869-1873,  Appendix  C, 
Vol.  II,  Xew  York,  1873,  pages  L'13-231. 

In  tlie  third  a2)peiKlix  Wviiiht  classifies  for  the  first  time  the  rocks  of 
the  Marquette  area  in  accordance  with  their  microscopical  features.  Diorites, 
diorite  -  schists,  chlorite  -  schists,  hornblende  -  schists,  anthophyllite  -  schists, 
quartzites,  argillites,  and  various  specimens  of  ores  Avere  submitted  to  tlie 
author,  who  describes  them  1  briefly,  l)ut  with  suflicient  fullness  to  make  their 
recoo-nition  possible.  Wrig-ht  surmises  that  diabase  may  exist  among  the 
gi-eenstones  of  the  reg-ion. 

Houghton,  Douglass,  llemarks  on  rocks  between  Chocolate  Itiver  and 
Granite  Point,  embi'aeing  Marquette  Harbor.  From  unpublished  MSS.  Geol.  Surv. 
of  Michigan,  18G9-1873,  Appendix  E,  Vol.  II,  New  York,  1873,  pages  L'39-24(;. 

These  notes,  which  comprise  the  fifth  appendix  to  Brooks's  report, 
possess  only  a  historical  value  because  of  the  long  delay  in  their  publication. 
It  is  unnecessary  to  refer  to  them  further  than  to  state  that  the  author  dis- 
covered and  figured  the  unconformity  between  the  Lake  Superior  sandstone 
and  the  quartzite  near  the  mouth  of  tlie  Carp  Eiver. 

Brooks,  T.  B.  The  lamination,  plication,  and  faulting  of  banded  ore  and 
jasper  (mixed  ore),  with  illustrations  (figs.  19  to  29).  Geol.  Surv.  of  Jlicbigau, 
1869-1873,  Appendix  K,  Vol.  II,  pages  283-292. 

The  final  appendix  is  one  added  by  Brooks  himself.  It  ct^mprises 
eleven  figures  of  banded  jasper  and  ore,  with  their  explanation.  Its  object 
is  to  illustrate  the  way  in  which  the  original  parallel  lamination  of  the  ore 
and  jasper  may  be  entirely  obliterated,  and  may  be  replaced  by  a  mechanical 
brecciated  structure.     The  figures  are  very  interesting  and  suggestive. 

EOMINGER,  C.  PaliBOZoic  rocks.  Geol.  Surv.  of  Michigan,  1S(;;I-1S73,  Vol.  1, 
Part  III,  New  York,  1873,  104  pages. 

In  the  same  volume  of  the  Michigan  survey  in  which  Brooks's  report 
occurs  is  one  by  Rominger  on  the  Paleozoic  rocks  of  the  Upper  Peninsula. 
The  only  portions  of  this  report  in  which  we  are  now  interested  are  those 
relating  to  the  unconformity  between  the  Huronian  and  the  Potsdam  beds 
near  Marquette,  and  the  description  of  the  Presque  Isle  rocks. 


60  THE    MARQUETTE    lEOI^-BEAEI^'G   DISTRICT. 

The  unconformity  mentioned  by  Foster  and  Whitney  as  occurring 
near  the  Carp  River  is  described  by  Rominger  in  these  words  (p.  90): 

We  find  here  vertically  erected  white  quartzite  beds  of  the  Hurouiau  group, 
projecting  into  the  lake,  which  have  preserved  their  granular  sandstone  structuie 
and  are  distinctly  ripple- marked.  They  are  surrounded  by  brown  sandstone  and 
conglomerate  ledges,  horizontally  abutting  against  them.  The  sandstones,  which 
are  of  very  irregular  discordant  stratification,  closely  adapt  themselves  to  all  inequali- 
ties of  the  cliffs,  which  exhibit  under  the  sandstone  covering  a  rounded,  water-worn 
surface,  indicating  their  long  exposure  before  they  were  enveloped  by  the  sandstones;. 

He  describes  Presque  Isle  as  formed  by — 
a  protrusion  of  peculiar  rock  masses,  differing  considerably  from  the  rock  beds  of  the 
Huronian  group  in  the  vicinity.  Lowest  is  a  black,  unstratified,  semicrystalline  mag- 
nesian  rock,  resembling  a  half-decomposed  basalt  or  a  highly  ferruginous  serpentine. 
It  forms  considerable  cliffs  at  the  north  end  of  the  spur;  more  to  the  south  we  find  it 
overlaid  by  a  more  light-colored,  once  stratified  rock,  which  is  involved  in  the  upheaval, 
with  its  ledges  bent  and  broken  up  in  great  confusion.  *  *  *  The  principal  rock 
mass,  which  is  found  iu  all  forms,  from  compact  crystalline  to  an  absorbent,  earthy 
condition,  is  chemically  a  dolomite.     *     *     * 

On  the  south  portion  of  Presque  Isle  this  dolomite  is  nnconformably  overlaid 
by  a  conglomerate  and  succeeding  sandstone  layers,  which  are  identical  with  the 
sandstones  of  the  Marquette  quarries.  The  sandstone  strata  some  distance  off  froni 
the  protrusive  rocks  [the  "dolomites"]  are  nearly  horizontal.  In  immediate  contact 
with  them  they  have  a  considerable  dip,  corresponding  to  the  convexity  of  the 
underlying  surface.  It  is  possible  that  the  strata  were  slightly  uplifted  after  their 
deposition,  but  I  am  more  inclined  to  explain  the  existing  dip  as  an  adaptation  of 
the  sediments  to  the  surface  on  which  they  were  deposited.  The  conglomerate  beds 
at  the  base  are  o  feet  thick  and  contain  numerous  fragments  of  the  underlying 
dolomitic  rocks  and  of  their  inclosed  jaspery  minerals.     (P.  92.) 

On  Light-House  Point  tlie  autlior  noted  tliat  the  bands  of  diorite, 
which  had  been  so  frequently  mentioned  by  earlier  authors  as  interstratified 
with  the  Huronian  schists,  "are  connected  among  themselves  by  transverse 
bands  cutting  across  the  strata  of  the  schists,"  and  therefore  the  diorites 
are  believed  to  be  intrusive  (p.  93). 

Rominger  thus  makes  the  Presque  Isle  dolomitic  rock  older  than  the 
sandstones  associated  with  it,  and  the  green  schists  of  Light-House  Point 
he  I'effards  as  intrusive. 


GEOLOGICAL   EXPLOEATIONS   AND    LITEKATURE— 1874.  61 

Again,  following'  the  reports  of  the  Michigan  survey,  thei'e  is  a  dearth 
of  articles  of  a  general  natui-e  on  the  Marquette  rocks.  This  period  of 
little  activity  was  brouglit  to  an  end  by  the  publications  of  the  Wisconsin 
survey  in  1879.  A  numl)er  of  volumes,  in  which  reference  is  made  to  the 
geology  of  the  Marquette  district,  were  published  in  this  interval,  but  the 
statements  in  them  are  largely  based  on  the  work  of  others  than  their  authors. 
A  number  of  short  articles  also  appeared  during  this  time,  but  they  are 
devoted  mainly  to  the  elucidation  of  special  points  in  the  discussion. 

1S74, 
Dana,  J.  D.  Manual  of  geology.  2d  ed.,  New  York,  1874. 
In  the  second  edition  of  his  Manual,  Professor  Dana  makes  the  same 
statements  regarding  the  origin  of  the  Marquette  ores  and  of  the  Archean 
rocks  in  general  as  were  made  in  1865,  when  he  accepted  Foster  and 
Whitney's  views.  With  regard  to  the  geological  position  of  the  beds,  we 
find  that  the  author  has  discovered  evidence  enough  for  separating  the 
Azoic  (now  Archean)  beds  into  two  series,  the  Laurentian  and  the  Huronian, 
but  he  is  not  satisfied  that  the  Marquette  ores  are  not  members  of  the  first 
series.  On  one  page  (p.  151)  he  places  them  with  the  Laurentian,  and 
on  another  page  (p.  159)  with  the  Huronian,  mainly  in  deference  to  the 
opinions  of  the  Lake  Superior  geologists,  who  emphasized  the  lithological 
diff"erences  between  the  acid  schists  of  the  Laurentian  and  the  basic  ones 
of  the  Huronian.  "The  evidence  as  to  age  is  far  from  conclusive,"  writes 
the  author.  "  The  extent  of  the  beds  of  iron  ore  affords  some  reason  for 
believing,  as  shown  by  Whitney,  that  they  are  true  Laurentian"  (p.  159). 
Exactly  the  same  statements  concerning  the  Marquette  region  are 
given  in  the  third  edition  of  the  Mauual,  published  in  1880. 

Newberry,  J.  S.  The  iron  resoarce.s  of  the  Uuited  States.  The  Interua- 
tioual  Review,  Vol.  I,  1874,  pages  754^780. 

This  author  gives  a  general  review  of  the  iron  resources  of  the  United 
States,  and  refers  to  the  Marquette  ores  as  magnetites,  hematites,  and 
hydrated  sesquioxides,  inclosed  in  Huronian  terranes.  The  "ore  beds 
were    once   horizontal    strata,   deposited  in   conformity   with  many   other 


62  THE   MAEQUETTE    IRON-BEARING   DISTRICT. 

stratified  sediments,  but  they  are  folded  and  broken  in  such  a  way  that 
their  true  nature  was  for  a  long  while  misunderstood  "  (p.  758).  The  dis- 
tribution of  the  ore  bodies  is  believed  to  be  "  dependent  upon  the  immense 
surface  erosion  which  this  region  has  suffered.  This  has  removed  by  far 
the  greatest  part  of  the  ore  that  originally  existed  here,  leaving  it  only 
where  it  formed  masses  of  unusual  magnitude  and  solidity,  which  have 
resisted  the  erosive  action,  or  Avhere,  in  synclinal  troughs,  it  has  been 
beyond  the  reach  of  the  glaciers,  which  have  ground  oif  all  the  more 
elevated  portions"  (p.  758). 

In  this  view  the  ores  are  manifestly  supposed  to  have  existed  as  an 
extensive  bed  over  the  Marquette  Huronian  area,  of  which  the  present  ore 
bodies  are  the  remnants.  This  we  now  know  to  be  contrary  to  the  facts. 
The  irregular  distribution  of  the  ore  bodies  in  the  region  is  not  the  result 
of  erosion,  but  is  the  result  of  the  action  of  secondary  processes  under 
especially  favorable  conditions,  prevailing  only  here  and  there  throughout 
the  district. 

1875. 

PuMPELLY,  Raphael.  On  pseudomorphs  of  chlorite  after  garnet  at  the  Spurr 
Mountain  iron  mine.  Lake  Superior.  Am.  Jour.  Sci.  (3),  Vol.  X,  1875,  jiages  17-21. 
With  plate. 

This  article  is  of  a  mineralogical  nature.  In  it  the  author  describes 
the  well-known  chlorite  pseudomorphs  of  garnet  occurring  in  the  chlorite- 
schist  overlying  the  ore  bed  at  the  Spurr  mine.  The  chlorite-schist  is 
composed  of  minute  flakes  and  needles  of  chlorite,  through  Avhich  .  are 
scattered  small  octahedra  of  magnetite  and  the  garnet  j^seudomorphs. 
From  the  result  of  his  investigation  of  the  rock  the  author  infers  that  the 
schist  was  originally  an  argillaceous  limestone  or  marl  that  was  changed 
by  metamorphic  j^i'ocesses  into  its  present  form. 

Whittlesey,  Charles.  Physical  geology  of  Lake  Superior.  Proc.  Am.  Ass. 
Adv.  Sci.,  Vol.  XXIV,  1875,  pages  CO-72.     With  map. 

Whittlesey  denies  the  existence  of  a  true  Laurentian  series  in  Michigan. 
The  granitic  rocks,  heretofore  regarded  as  belonging  in  this  series,  are 
eruptive,  as  shown  by  their  analyses.     In  some  instances  in  the  Marquette 


GEOLOGICAL   EXPLORATIONS   AND    LITERATURE— ISTC.  63 

reg-ion,  tlic  author  writes,  these  eruptives  "have  pushed  up  through  the 
Hurouiau  lieds,  cuttiiii:;-  tlieiu  and  the  Potsdam  at  the  same  time,"  but 
he  does  not  give  particulars.  The  ores  ot"  the  Marquette  area  and  their 
associated  rocks  are  lieheved  to  l)elong  iu  the  Huronian  system,  wliich  in 
Michigan  and  in  Canada  possess  a  remarkable  similarity.  Incidentally  the 
author  gives  a  sketch  of  the  relations  between  the  trap  and  sandstone  at 
Presque  Isle.  The  former  is  represented  as  penetrating  the  sandstone  and 
producing  ou  the  contact  a  friction  conglomerate. 

1S70. 

Brooks,  T.  B.  On  the  youngest  Hurouiau  rocks  south  of  Lake  Superior,  and 
the  age  of  the  copper-bearing-  series.  Am.  Jour.  Sci.  (3),  Vol.  XI,  1876,  pages 
20(3-211. 

In  his  ^Michigan  report  on  the  iron-liearing  series  Brooks  places  the 
granites  with  the  gneisses  of  the  Upper  Peninsula,  and  regards  them  all  as 
belonging  with  the  Laurentian,  unconformaljly  beneath  the  Hiu-onian  l)eds. 

In  1876,  however,  in  an  article  dealing  more  jjarticularh-  with  the 
Menominee  iron  district,  the  same  author  notes  the  discovery  of  granites 
cutting  micaceous  and  hornblendic  schists  similar  to  those  in  Fonnatioii 
XIX  of  the  Marquette  series.  If  this  formation  is  the  topmost  portion  of 
the  series,  as  is  supposed,  the  granite  must  be  Huronian.  Moreover,  the 
Huronian  series  is  so  different  lithologically  from  the  copper-bearing  series, 
and  the  movements  that  have  been  undergone  by  the  two  series  are  so  dif- 
ferent in  amount,  that  it  is  necessary  to  conclude  that  the  copper-bearing 
series  is  much  younger  than  the  Huronian.  "We  are  therefore  justified, 
I  think,  in  regarding  the  copper-bearing  rocks  of  Lake  Superior  as  a  dis- 
tinct and  independent  series,  marking  a  definite  geological  period  wliich 
separates  the  Silurian  from  the  Huronian  ages."  For  this  series  the  author 
proposes  the  name  "Keweenawian,"  unaware  that  Hunt^  two  )ears  earlier 
had  reached  tlie  same  conclusion  and  had  proposed  as  the  name  of  the 
series  "Keweenian." 

This  article,  though  dealing  almost  exclusively  with  a  region  outside 
the  ^larquette  area,  is  of  interest,  since  it  contains  the  first  definite  statement 

'Trans.  Am.  Inst.  Min.  Eng.,  Vol.  I   pp.  339-341. 


64  THE   MARQUETTE    lEON-BEAEING    DISTKICT. 

that  the  Huronian  of  Michigan,  inchidiug  of  course  the  Marquette  Huronian, 
is  a  distinct  and  older  series  than  that  to  which  the  copper-bearing  rocks 
bek>ng. 

The  question  ()f  the  age  of  the  Keweenawan  was  contested  long  and 
bitterh',  and  it  was  finally  decided  to  the  satisfaction  of  most  geologists 
that  the  ^iew  suggested  by  Hunt  and  Brooks  is  the  correct  one.  The 
copper-bearing  rocks  are  younger  than  the  iron-bearing  series  of  the  Upper 
Peninsula  and  entirely  distinct  from  them.  The  literature  of  this  subject 
need  not  l)e  referred  to  again.  The  subject  is  mentioned  here  merely  to 
simplify  and  make  clearer  the  geology  of  the  Marquette  rocks,  which  in 
this  district  had  been  included  with  the  copper-bearing  series  as  forming  a 
portion  of  Foster  and  Whitney's  Azoic. 

Brooks,  T.  B.  Classified  list  of  rocks  observed  in  the  Huronian  series  south 
of  Lake  Superior,  with  remarks  on  their  abundance,  transitions,  and  geographical 
distribution;  also  a  tabular  presentation  of  the  sequence  of  the  beds,  with  au 
hypothesis  of  equivalency.    Am.  Jour.  Sol.  (3),  Vol.  XII,  1876,  pages  194-204. 

In  another  article  in  the  same  year  Brooks  gives  a  revised  classification 
of  Huronian  rocks,  based  on  microscopic  examinations  of  thin  sections 
made  by  A.  Wichmann,  Charles  E.  Wright,  and  Frank  Rutle}-.  The  clas- 
sification includes  Huronian  rocks  from  the  then  known  iron-bearing 
areas  of  this  age  in  Michigan  and  Wisconsin.  We  are  concerned  only  with 
the  Marquette  series.  This,  exclusive  of  the  youngest  observed  member 
(the  granite  bed),  according  to  the  author,  has  a  thickness  of  not  more 
than  6,000  feet.  The  sec^uence  of  the  beds  is  shown  in  the  following  table, 
in  which  the  italicized  names  are  those  of  the  rocks  possessing  greatest 
lithological  interest : 

Lower  Silurian. 
Copper  series  (wanting). 

HURONIAN. 

XIX.  Grayish  black  inica-schist,  often  staurolitic,  and  holding  andahtsite  and 
garnets;  rarely  chloritic  schist.  Quartz  in  bunches  and  veins,  and  hornblende  seams, 
rare.    Quartzyte.  (?) 

Probably  soft  slate. 

XVII.  Antliophyllitic  (!)  schist,  usually  magnetic,  and  containing  manganese. 


GEOLOGICAL  EXPLORATIONS   AND   LITERATURE— 1S7G.  65 

XVI.  Banded  ocbrey  porous  quartz-schiat. 

XV.  Blackish  argillaceous  slate,  with  imperfect  cleavage,  rarely  micaceous,  and 
sometimes  holding  garnets. 

XIV.  Grey  arenaceous  qiiart-ite,  often  semi-schistose  and  sometimes  micaceous; 
quartz  conglomerate. 

XIII.  Pure  specular  hematite  and  magnetite  ores;  ferruginous  banded  jdspery 
schist,  with  interstratiiied  beds  of  chloritic  and  hydromicaceous  schists.  " Trap  dykes" 
at  Washington  mine. 

XII.  Red  arenaceous  quartz  schist,  banded  with  micaceous  iron;  quartzose  Umonitic 
ores. 

XI.  IHori/tc,  hornblende-schist,  chloritic  schist,  chloritic-looking  vrica  schist;  rarely 
hornblende  gneiss. 

X.  Siliceous  hematitic  and  Umonitic  scliistose  ores,  often  manganiferous ;  siliceous 
schists;  garnetiferous  anthophyllitic  (eklogyte)  schists;  obscure  compact  chloritic  (?) 
magnetic  schists,  with  conchoidal  fracture. 

IX.  HornbleniUc  roch  and  related  diorite  and  diabase,  often  micaceous. 

VIII.  Ferruginous  quartzose  flags ;  clay-slate ;  quartzite ;  rarely  chloritic  and 
anthophyllitic  (?)  schist. 

VII.  Hornhlendic  rocks,  with  related  greenstones.  ( ? ) 

VI.  Ferruginous  quartzose  schist;  clay  and  chloritic  slates. 

V.  Quartzite  graduating  into  protogine,  with  interstratifled  beds  of  dolomitic 
marble;  noraculite ;  rarely  chloritic  and  micaceous  schist,  and  dikes  of  chloritic  schist. 

Syenite  (quartzless),  diorite,  diabase,  hornblende-schist,  and  obscure  chloritic 
slates,  conglomeratic  quartzite  and  various  quartzose  iron  ores.  Well-characterized 
tote-schist  is  found  in  this  horizon  only  at  Marquette. 

Nonconfornnible  with  Laurentian. 

Diabases  are  recognized  as  occurring'  among  the  greenstones,  contrary 
to  tlie  earlier  opinion  of  the  author.  These  rocks,  together  with  the  diorites 
and  the  rehited  schists,  are  still  regarded  as  "metamoi-phic." 

Lithologically  the  division  is  into  (1)  fragmental  rocks,  exclusive  of 
limestone;  (2)  melamorphic  rocks,  not  calcareous;  (3)  calcareous  rocks; 
and  (4)  igneous  rocks.  Among'  the  fragmental  rocks  are  noted  quartz- 
conglomerates  in  the  middle  liorizon  of  tlie  series.  The  second  ilivisi(m 
includes  granite,  the  gneisses,  schists  and  slates,  syenite,  diorite,  gabbro, 
diabase,  and  the  pyroxenic,  the  garnetiferous,  the  olivinitic,  and  the  chloritic 
and  talcose  rocks,  besides  quartzite,  jasper,  chert,  and  the  iron  ores.  The 
eruptive  rocks  are  the  granites,  traps,  and  the  hydrous  maguesian  scliistose 
MON  XX vm 5 


66  THE  MARQUETTE    IRON-BEARING  DISTRICT. 

rocks  observed  in  dike-like  masses.  Sometimes  these  latter  rocks  are 
believed  to  have  been  "formed  from  the  abraded  material  of  the  walls  of  a 
fault." 

Wichmann,  Tornebohm,  and  Zirkel,  according  to  the  author,  all  agree 
in  regarding  many  of  the  greenstones  as  diorites  and  diabases  of  eruptive, 
and  not  of  metamorphic,  origin.  Among  them  they  would  place  also  many 
of  the  dioritic  schists  and  chloritic  diorites.  The  author,  however,  still 
regards  them  as  metamorphic,  and  so  includes  them  with  the  metamorphic 
rocks. 

1878. 

Hunt,  T.  Sterry.  Special  report  on  trap  dykes  and  Azoic  rocks  of  south- 
eastern Pennsylvania.  Part  I,  Historical  introduction.  2d  Geol.  Surv.  of  Pennsyl- 
vania, E,  1878,  253  pages. 

Hunt  refers  to  the  Marquette  rocks  as  belonging  partly  in  the  author's 
White  Mountain  or  Montalban  series  and  partly  in  his  Green  Mountain  or 
Huronian  series.  Those  belonging  to  the  Montalban  are  the  micaceous  and 
hornblende  schists  of  Brooks's  Formation  XIX  and  the  granites  associated 
with  them  (Formation  XX).  The  Huronian  rocks  underlie  the  Montalban. 
In  Michigan  they  include  the  greeustones,  diorites,  serpentines,  carbona- 
ceous argillites,  and  porphyries.  The  author  agrees  with  most  of  the  earlier 
writers  on  the  district  in  regarding  the  greenstones  as  indigenous,  that  is, 
as  formed  in  their  present  position  by  the  metamorphism  of  sediments. 


Wright,  Charles  E.    First  annual  report  of  the  Commissioner  of  Mineral 
Statistics  of  the  State  of  Michigan  for  1887-88  and  previous  years.    Marquette,  1879, 


In  this  report  Wright  briefly  sketches  the  geology  of  the  Upper  Penin- 
sula. The  author  accepts,  with  some  modifications,  Brooks's  view  as  to  the 
sequence  of  rocks  in  the  Marquette  district.  He  believes  that  there  are  two 
metamorphic  granites  in  the  district — one  Laurentian  and  the  other  occup)^- 
ing  a  position  in  time  between  the  Marquette  or  Lower  Huronian  rocks  and 
the  members  of  the  copper-bearing  or  Upper  Huronian  series.  The  green- 
stones are  believed  to  exist  in  beds,  and  to  be  represented  in  some  places 


GEOLOGICAL   EXPL0RATI0:N^S   AND    LITERATUEE— 1879.  67 

by  slates  and  maguesiaii  schists.  Bed  X  of  Brooks  is  a  layer  of  siliceous 
ferruginous  schists,  from  which,  locally,  silica  has  been  removed,  leaving 
deposits  of  soft  ores.  These  ore  bodies  are  noticed  to  be  in  those  portions 
of  the  series  where  the  disturbance  has  been  greatest.  The  quartzite  and 
jasper-conglomerates  were  also  observed  in  many  mines  forming  the  hang- 
ing walls  of  the  ore  bodies,  but  then-  significance  was  not  realized  liy  tlie 
author  any  more  fully  than  it  was  by  Brooks.  Below  Bed  V,  which  is 
the  lowest  identified  by  the  latter  geologist,  Wright  places  a  quartzite  and 
a  garnetiferous  mica-schist. 

The  author  also  reports  the  discovery  of  a  series  of  sharp  ridges 
composed  of  serpentine,  marble,  magnesian  schists,  etc.,  about  3J  miles 
northeast  of  Ishpeming.  The  rocks  are  similar  to  those  occurring  at 
Presque  Isle. 

WiCHMANN,  Akthue.  A  microscopical  study  of  some  Hnronian  clay-slates. 
Quart.  Jour.  Geol.  Soc.,  Vol.  XXXV,  1879,  pages  15G-1G4. 

In  this  paper  Wichmann  gives  a  detailed  description  of  the  slates  of 
the  Marquette  district.  He  divides  them  into  clay-slates,  "  deposited  on  the 
upper  strata  of  quartzite  "  (forming  Bed  XV  of  Brooks  and  Bed  VIII  of 
Credner),  and  occurring  also  in  the  marble  series  (Bed  V  of  both  Brooks 
and  Credner),  novaculites,  and  carbonaceous  shales.  The  slates  are  simi- 
lar to  those  of  later  periods.  Often  the}^  contain  small  tourmaline  and 
hematite  crystals.  The  novaculites  are  hard,  brittle  clay-slates  containing 
garnets  and  quartz.  In  the  carbonaceous  slates  carbonaceous  material  is 
present  in  large  quantities,  and  crystalline  components  are  absent.  The 
paper  ends  with  a  discussion  as  to  the  processes  of  formation  of  slates, 
but  readies  no  decided  conclusions. 

Irving,  E.  D.  Note  on  the  stratigi-apliy  of  the  Huroniau  series  of  uortlieru 
Wisconsin;  and  on  tlie  equivalency  of  the  Huroniau  of  the  Marquette  and  Penokee 
districts.     Am.  Jour.  Sci.  (3),  Vol.  XVII,  1879,  pages  393-398. 

In  1879  Professor  Irving  began  his  series  of  valuable  contributions  to 
the  geology  of  the  Marquette  district  with  a  note  in  which  he  calls  attention 
to  errors  in  Brooks's  scheme  of  equivalency  for  the  strata  of  the  difierent 


68  THE   MARQUETTE   IRON-BEARING    DISTRICT. 

Huronian  districts  in  the  Lake  Superior  region,  and  proposes  a  new  scheme, 
which  he  claims  shows  clearly  the  equivalency  of  the  Penokee  and  the 
Marquette  series. 

Ckosby,  W.  O.  Ou  a  possible  origin  of  petrosiliceous  rocks.  Read  March  5, 
1879.     Proc.  Boston  Soc.  Nat.  Hist,  Vol.  XX,  1878-80,  pages  160-160. 

Crosb}'  compares  the  felsites  and  "petrosilexes"  with  the  siliceous  red 
clays  di'edged  by  the  Challenger  Expedition  from  the  deep  seas.  With 
respect  to  the  Marquette  jaspers  and  ores  he  says  (p.  1G8): 

Oue  of  the  most  interesting  rocks  in  the  Marquette  iron  district,  in  the  Upper 
Peninsula  of  Michigan,  and  the  one  most  closely  associated  with  the  iron  ore,  is  a 
brownish  or  reddish  jasper;  it  sometimes  becomes  chloritic  or  micaceous,  passing  into 
chlorite- schist,  etc.,  but  for  the  most  part  it  is  a  distinct  and  beautiful  stratified  jasper. 
This  Lake  Superior  jasper,  like  all  the  iietrosiliceous  rocks  so  far  as  known,  belongs 
to  the  Huronian  formation,  and  may,  apparently',  be  fairly  taken  to  represent  the 
petrosilex  and  felsite  characterizing  many  other  Huronian  areas,  but  apparently 
wanting  here.  Its  association  with  the  iron  ore  is  usually  very  intimate;  the  two 
substances  being  interlaminated  in  such  a  manner  as  to  give  rise  to  a  banded  structure 
which  matches  in  all  important  i>articulars  the  banding  of  the  petrosilex  of  eastern 
Massachiisetts  and  other  regions,  the  hematite  simply  taking  the  place  of  the  feldspar. 
The  extreme  irregularity  of  the  banding  in  many  cases  makes  it  not 'only  proper  but 
necessary  for  us  to  conclude  that,  as  in  the  case  of  the  petrosilex,  it  is  largely  the 
result  of  a  segregation  process,  the  two  constituents,  hematite  and  jasper,  having  been 
originally  more  intimately  mixed. 

With  A'ery  few  exceptions  this  INIarquette  ore  always  contains  some  oxide  of 
manganese,  usually  from  1  to  2  per  cent,  though  the  ore  ^rom  one  l>ed  contains  nearly 
7  per  cent. 

Here,  then,  so  far  as  clic-mical  composition  is  concerned,  we  have  a  formation 
almost  identical  with  some  of  the  siliceous  oozes  of  the  deep  sea;  while  the  chief 
structural  distinction  consists  in  the  different  forms  of  the  segregated  masses  of  the 
iron  and  manganese  oxides,  lenticular  layers  taking  the  place  of  irregularly  rounded 
nodules,  certainly  a  distinction  of  no  great  imxiortance. 

The  author  does  not  claim  to  prove  that  the  jaspers  and  ores  origi- 
nated from  deep-sea  oozes,  but  only  to  show  the  close  analogy  existing 
between  these  ancient  rocks  and  the  deposits  now  being  formed  in  the 
ocean's   depths.      There   are   several   misinterpretations   in   the   description 


GEOLOGICAL   EXPLORATIONS   AND    LITERATUllE-lSSO.  69 

of  the  jaspers  given.  The  author  had  not  visited  the  Marquette  district, 
so  he  must  have  obtained  his  notions  of  the  rektions  of  the  jaspers  from 
the  literature  (.u  the  area.  The  jaspers  had  frequently  been  described 
as  interstratified  with  fhlorite-schist,  etc.,  but  nowhere  had  it  been  shown 
that  the  two  rocks  grade  into  each  other.  Moreover,  tlie  ]\Iarquette  ore 
does  not  usually  contain  from  1  to  2  per  cent  of  manganese. 


Brooks,  Thomas  Benton.  The  geology  of  the  Menominee  iron  region  (east  of 
center  of  Range  17  E.),  Oconto  County,  Wisconsin.  Geology  of  Wiscon.siii,  1S73-1S79, 
Vol.  Ill,  pub.  in  1880,  Part  VII,  pages  429-i599. 

The  Wisconsin  reports,  although  they  are  devoted  mainly  to  the  discus- 
sion of  Wisconsin  problems,  contain  numerous  references  to  the  geology 
of  Michigan.  In  Brooks's  report  on  the  geology  of  the  Menominee  iron 
range  are  several  references  to  the  Marquette  rocks.  There  is,  besides,  a 
new  table  of  the  formations  in  the  ]\Iarquette  Huronian.  The  principal 
difference  between  this  table  and  that  published  in  the  Michigan  report  is 
the  addition  of  Formation  XX,  which  embraces  the  "granites"  southwest 
of  Lake  Michigamme,  thought  to  be  younger  than  tlie  mica-schists  of  the 
vicinity.  The  references  to  the  Marquette  series  are  mainl}^  with  respect 
to  their  correlation  with  the  Menominee  series. 

As  an  appendix  to  this  report  (pp.  661-663)  Brooks  gives  a  brief  sketch 
of  the  Laurentian  rocks  in  Michigan.  This  series  includes  mica-gneisses, 
hornblende-gneisses,  hornblende-schist,  chloritic  gneisses,  chloritic  schists, 
often  derived  from  the  mica-gneisses  and  hornblende-gneisses,  and  granites. 
The  latter  rocks  are  the  massive  varieties  of  gneiss  "in  which  all  interior 
evidence  of  bedding  is  obliterated  by  metamorphic  action."  All  the  gran- 
ites of  the  Laurentian,  with  the  exception  of  the  dike  granites  "and  certain 
great  irregular  red  masses,"  are  thought  by  the  author  to  be  altered  sedi- 
mentary rocks. 

The  Laurentian  rocks  are  cut  in  all  directions  by  dike-like  masses  of 
granite  and  greenstone,  of  which  the  latter  are  "far  thicker,  more  regular 
and  persistent  than  those  of  the  granite." 


70  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

WiCHMANN,  Abthtir.  Microscopical  observations  of  the  iron-bearing  (Huro- 
nian)  rocks  from  tlie  region  south  of  Lake  Superior.  Geology  of  Wisconsin,  1873-79, 
Vol.  Ill,  ISSO,  pages  600-65G. 

A  series  of  about  500  tliin  sections  of  rocks  from  the  Peuokee,  the 
Menominee,  and  the  Marquette  iron  districts  was  submitted  hj  Brooks  to 
Arthur  Wichmann  for  microscopical  study.  Wichmann's  report  announces 
the  resuhs  of  the  study  and  the  conclusions  reached  by  him  concerning  the 
origin  of  some  of  the  rocks  investigated — conclusions  tliat  have  already 
been  referred  to  in  some  instances.  The  rocks  are  divided  into  lithoLtgical 
grouj)s  as  follows:  nonfragmental  rocks,  including  simple  rocks,  massive 
rocks,  and  schistose  rocks;  and  fragmental  rocks,  including  clay-slates, 
sandstone,  and  chert-breccia,  of  which  the  latter  occin-s  only  in  the  Ponokee 
district.  Among  the  simple  rocks  are  found  limestone,  dolomite,  quartzite, 
magnetite-schist,  jasper-schists,  chert  schists,  hematite-schist,  and  serpentine; 
among  the  massive  ones,  granite,  syenite,  diorite,  and  diabase;  and  among 
the  schistose  ones,  gneiss,  mica-schist,  hornblende-schist,  chlorite-schist, 
augite-schist,  talc-schist,  sericite-schist,  and  eklogite.  The  ser^ientine  is  an 
altered  olivine  rock.  Diabases  are  abundant  in  the  Marquette  district,  and 
are  eruptive  in  origin.  By  addition  of  hornblende  to  the  diabases,  diorites 
result;  hence  the  diorites  are  also  eruptive.  The  talc-schists  described  by 
Brooks  in  Bed  XIV  are  sericite-schists  of  sedimentary  origin. 

Hunt,  T.  Sterry.  Letters  and  notes  on  tlie  irou-bearing  and  associated 
rocks  of  the  ]\Iarquette  region,  and  comparisons  with  tlie  Archean  f)f  Canada  and 
of  the  eastern  United  States.  Appendix  A,  Geology  of  Wisconsin,  1873-79,  Vol.  Ill, 
1880,  pages  657-6C0. 

In  the  same  volume  in  which  Brooks's  report  is  published  are  several 
letters  and  notes  by  Hunt  on  the  iron-bearing  and  associated  rocks  of 
the  Marquette  district.  One  of  these  letters  is  from  Hunt  to  Alexander 
Winchell,  and  is  dated  November  26,  1869.  In  this  Hunt  mentions  the 
difficulty  of  assigning  names  to  the  greenstone-schists  of  northern  Michi- 
gan, which  include  altered  varieties  of  many  distinct  rocks.  In  a  letter  to 
Brooks  dated  February  22,  1871,  an  attempt  is  made  to  correlate  various 
members  of  the  Marquette  series  with  the  Huronian  and  Montalban  rocks 
of  New  England  and  the  Adirondacks.     In  a  third  letter  Hunt  expresses 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— ISSO.  71 

doubt  as  to  the  alteration  theory  with  respect  to  the  origin  of  the  rocks  of 
the  Marquette  district.  He  is  inclined,  apparently,  to  i-egard  these  rocks  as 
direct  sediments. 


Wadswokth,  j\r.  E.  Notes  on  the  geology  of  the  iron  and  copper  districts  of 
Lake  Superior.     Bull.  ^lus.  t'omp.  Zool.,  Vol.  YII,  ISSO,  l.")7  pages  and  6  plates. 

In  tills  paper  Dr.  Wadsworth  submits  In  great  detail  his  views  on  the 
origin  of  the  Marquette  ores.  As  a  preface  to  his  own  observations  he 
gives  a  sunnnary  of  the  work  done  in  the  region,  as  follows  (pp.  26-27): 

In  general,  tlien,  in  looking  over  the  views  advocated  by  past  observers,  we  find, 
in  brief,  the  Ibllowing  opinions  held. 

The  rocks  of  this  district  (excepting  the  sandstones)  were  all  taken  as  Azoic  by 
Foster  and  Whitney,  and  not  considered  to  be  capable  of  subdivision  into  geological 
periods.  We  nnist  also  notice  that  Prof.  H.  D.  Rogers  regarded  them  as  of  Primal 
or  Potsdam  age.  On  the  other  hand,  we  find  that  this  formation  is  divided  by 
Murray,  Hunt,  Kimball,  Winchell,  Credner,  Brooks,  and  Wright  into  the  Huroniau 
and  Laurentian.  This  division  is  based  upon  lithological  characters,  and  on  uncon- 
formability  said  to  exist  between  the  two.  Rivot  considered  the  whole  as  Potsdam. 
The  granite  is  regarded  as  an  eruptive  rock  by  Foster  and  Wliitney,  Bigsby,  and 
Whittlesey ;  and  as  of  sedimentary  origin  by  Rivot,  Kimball,  Brooks,  Hunt,  and 
Wright.  Tliese  latter,  with  Credner,  take  it  as  being  older  than  the  schistose  rocks 
associated  with  the  ii'on  ores,  and,  excepting  Rivot,  with  its  accompanying  gneissoid 
rocks  composing  the  Laurentian  formation.  Foster  and  Whitney  and  S.  W.  Hill 
regarded  the  granite  as  younger  than,  and  eruptive  in,  the  schists. 

The  gneisses  and  schists  were  taken  by  all  the  observers  as  being  of  sedimen- 
tary origin,  except  possibly  Whittlesey,  whose  language  is  as  obscure  as  the  formations 
about  which  he  wi'ites. 

The  metamorphism  of  the  schists  is  supposed  by  Hubbard,  Rivot,  Kimball, 
Hunt,  Brooks,  and  Wright  to  be  occasioned  by  chemical  agencies,  accompanied,  as 
part  thought,  by  galvanism.  Foster  and  Whitney  and  Bigsby  considered  that  the 
metamorphism  was  brought  about  by  the  presence  of  eruptive  rocks,  and  their 
accompanying  chemical  agencies.  Foster  and  Whitney  regarded  the  "diorites"  of 
this  region  as  eruptive  rocks,  but  Rivot,  Kimball,  Hunt,  Winchell,  Credner,  Brooks, 
and  Wright,  as  sedimentary  ones  and  interstratifled  with  the  schists.  The  iron  ores 
are  regarded  as  all  of  sedimentary  origin  by  Foster,  Kimball,  Dana,  Hunt,  Winchell, 
Credner,  Brooks,  Newberry,  and  Wright,  but  are  believed  for  the  most  part  to  be  of 
eruptive  origin  by  Whitney,  and  by  Foster  and  Whitney.    These  ores  were  said  to  be 


72  THE   MAEQUETTE   IRONBEARINCi    DISTRICT. 

in  the  upper  i>ortion  of  the  Huroiiiau  series  by  Kimball,  Brooks,  ami  Wright,  with 
the  "diorites"  underlying  them. 

It  will  thus  be  seen  that,  while  Foster  and  Whitney  regarded  certain  of  the 
rocks  in  the  Hnronian  as  eruptive,  Hubbard,  Rivot,  Kimball,  Hunt,  Creduer,  Brooks, 
and  Wright  regarded  all,  with  a  few  slight  exceptions,  as  sedimentary;  and  Hough- 
ton, Uubbai-d,  Locke,  Kimball,  Rivot,  and  Brooks  teach  that  they  pass  by  gradual 
transition  into  one  another. 

The  most  important  points,  then,  about  which  there  has  been  or  is  difference  of 
opinion,  are  the  age  and  relation  of  the  granite  and  schists,  the  origin  of  the  diorites 
and  iron  ores,  the  passage  of  one  rock  into  another,  and  the  presence  or  absence  of 
eruptive  rocks.  These  and  other  questions  relating  to  this  district  admit  in  many 
cases  of  no  middle  ground;  one  or  the  other  party  must  be  mistaken  in  their  obser- 
vations or  conclusions,  or  both. 

After  reviewing  the  literature  on  the  i\Iarquette  area  the  author  proceeds 
at  once  to  attempt  the  solution  of  the  jn-oblems  that  present  themselves. 
He  studied  the  district  ininutely,  and  so  compelled  others  who  disagreed 
with  his  conclusions  to  jnake  a  similar  close  study  of  it.  Consequently, 
Wadsworth's  paper  may  he  regarded  as  the  opening  chapter  of  a  new 
volume  on  the  geology  of  the  district.  As  Ave  sliall  see,  several  of  tlie 
author's  conclusions  were  subsequently  proved  untenable,  but  the  work 
required  to  prove  them  wrong  waa  necessary  before  a  correct  knowledge 
of  the  geology  of  the  district  became  jDossible.  The  article  is  especially 
valuable  for  its  detailed  description  of  tlie  relations  existing  between  the 
variovis  rocks. 

The  contacts  of  the  jasper  and  ore  with  the  schists  associated  with 
them  are  shown  to  be  like  those  of  an  eruptive  with  an  older  rock.  At 
the  Lake  Superior  ]uine,  for  instance,  the  jasper  and  ore  were  seen  in  contact 
with  the  chlorite-schist. 

The  jirnctiou  of  the  two  is  very  irregular,  the  banding  of  the  jasper  and  ore 
following  the  irregularities  of  this  line,  while  the  schist  is  indurated  and  its  laminae 
bear  no  relation  to  the  line  of  contact.  Stringers  of  ore  project  into  the  schist,  which 
near  the  jasper  is  filled  with  octahedrons  of  magnetite.  The  schist  loses  its  green 
color  generally,  and  becomes  apparently  an  indurated  argillite.  The  contact  and 
relations  of  the  two  rocks  are  not  such  as  are  seen  when  one  sedimentary  rock  is  laid 
down  upon  another,  but  rather  that  observed  when  one  rock  is  intrusive  through 
another;  and  in  this  case  the  intrusive  one  is  the  jasper  and  its  associated  ore.     (P.  30.) 


GEOLOGICAL   EXPLORATIONS  AND   LITERATUUE-188().  73 

Observations  of"  the  same  character  were  made  at  the  New  York,  Jack- 
son, and  other  mines,  and  in  all  of  these  the  contacts  of  the  jasper  and  ore 
with  the  surrounding  schists  were  found  to  have  the  peculiarities  of  eruptive 
contacts.  The  "bosses"  of  schist  so  frequently  met  with  in  mining-  opera- 
tions are  likewise  regarded  as  proofs  of  the  eruptive  origin  of  the  ore. 

This  structure  evidently  is  cousouaut  with  the  theory  of  the  eruptive  origin  of 
the  jasper  and  ore.  They  break  obliquely  up  through  the  schist,  and  send  otf 
branches,  which,  pursuing  the  same  general  course,  leave  wedge  shaped  masses 
between  them  and  the  trunk.     (P.  31.) 

At  the  Home  mine,  on  the  Cascade  range,  at  the  Pittsburg  and  I^ake 
Superior  mine,  on  the  same  range,  at  the  Lake  Superior  mine,  Ishpeming, 
and  at  a  number  of  other  places  dike-like  masses  of  the  jasj^er  are  reported 
as  cutting  schists  and  ferruginous  sandstones  overlving  the  ores.  At  the 
Pittsburg  and  Lake  Superior  mine,  "while  in  g-eneral  these  little  dikes 
follow  approximately  the  bedding  [of  the  quartzite],  they  are  seen  not  to 
exactly  do  this,  but  cut  the  laminae  obliquely  through  much  of  their  course." 

In  order  t(  >  determine,  if  possible,  what  was  the  original  state  of  the 
ore  and  jasper,  the  author  examined  thin  sections  of  both.  Of  the  jasper 
he  writes  (p.  33): 

Microscopically  this  section  is  composed  of  a  fine  grapular  aggregate  of  quartz 
and  hematite,  and  a  more  coarsely  crystallized  portion  made  up  of  octahedrons  of 
magnetite  or  martite,  and  of  quartz  of  secondary  origin.  The  quartz  in  the  first  part 
is  largely  tilled  with  uiiuute  globules  and  grains  of  ore,  which  also  occur  in  ii-regular 
masses  and  in  octahedrons.  The  qua,rtz  associated  with  the  more  coarsely  crystallized 
portion  is  water  clear,  and  shows  the  usual  fibrous  granular  polarization  of  secondary 
quartz.    Wherever  the  iron  is  in  a  distinguishable  crystalline  form  it  is  in  octahedrons. 

Of  other  sections  of  jasper  and  ore  he  writes  (p.  33): 
The  structure  of  the  quartzose  portion  is  like  the  devitrification  structure  of  the 
rhyolites  and  felsites.    *     *     *    The  jaspery  iwrtion  is  finely  banded,  and  shows  an 
apparent  fluidal  structure.     We  are  inclined  to  regard  the  structure  as  tluidal,  but  in 
a  rock  so  deeply  colored  it  is  difticult  to  make  satisfactory  examinations. 

It  was  impossible  to  determine  whether  or  not  the  ores  were  all  origi- 
nally magnetic.  In  some  cases  the  magnetites  are  believed  to  be  secondary, 
since  the  hematite  where  in  contact  with  diorite  dikes  is  often  changed  to 
this  mineral. 


74  THE   MAKQUETTE   lEON-BEAKlNG  DISTKICT. 

At  the  Republic  mine  magnetite  and  raartite  (hematite  pseudomorphs 
after  magnetite)  are  frequently  found  near  the  "quartzite"  of  Brooks  (For- 
mation XIV).  For  this  reason,  and  because  the  quartzite  is  firmly  welded 
to  the  ore,  and  breaks  across  its  laminae,  cutting  them  and  sending  tongnies 
into  the  "mixed  jasper  and  ore,"  the  rock  is  supposed  to  be  eruptive 
(intrusive).  In  one  place  (pp.  54-55)  the  author  describes  it  as  greisen. 
The  quaiitity  of  magnetite  present  at  any  place  is  thought  to  be  dependent 
largely  upon  the  abundance  of  eruptive  dikes  and  their  proximity  to  the 
ore  deposit. 

The  question  as  to  the  origin  of  the  basic  massive  rocks  of  the  region 
the  author  answers  decidedly.  He  finds  many  examples  of  fresh  and 
altered  diabases  occurring  in  dike-like  forms,  cutting  the  green  schists  near 
Marquette,  Ishpeming,  and  Negaunee,  and  traversing  a  "breccia  or  con- 
glomerate" near  Deer  Lake  and  other  places.  The  material  of  the  dike 
in  the  Deer  Lake  conglomerate  "is  so  altered  that  it  resembles  a  chlorite 
schist,  and  in  the  thin  section  is  seen  to  be  composed  of  chlorite,  quartz, 
and  mica.  It  holds  some  ferruginous  masses  resembling  the  product  of  the 
decomposition  of  titaniferous  iron,  as  well  as  one  or  two  that  prolialily 
resulted  from  the  decomposition  of  olivine  or  brown  hornblende.  *  *  * 
We  regard  the  rock  simply  as  a  more  highly  metamorphosed  condition  of 
the  diorites  of  the  region"  (pp.  42-43).  Other  dikes  were  seen  at  the  Jack- 
son and  the  Washington  mines,  cutting-  the  ore,  and  south  of  the  Champion 
mine,  traversing  the  granites  and  gneisses.  In  many  cases  the  dikes  show 
their  intrusive  character  in  the  field,  and  exhibit  under  the  microscope  the 
features  usually  regarded  as  appertaining  to  crystalline  rocks. 

The  "magnetic  siliceous  schist"  of  Brooks  is  learned  to  be  composed 
of  actinolite,  hornblende,  magnetite,  and  garnet,  and  together  with  other 
similar  rocks,  including  Wichmann's  eklogite,  is  believed  to  be  eruptive. 
The  actinolite-schist  south  of  Humboldt  passes  into  a  quartzite  rock  made 
up  principally  of  alternating  layers  of  quartz  and  actinolite,  and  is  therefore 
sedimentary.  The  author  thinks  that  the  actinolite-schists  were  formed  of 
the  detritus  of  the  garnetiferous  actinolite  rocks,  which  are  intrusive. 

From  these  studies  it  is  plain  that  the  eruptive  natui-e  of  many  of  the 
massive  beds  of  "diorite"  described  by  earher  writers  as  interstratified  with 


GEOLOGICAL  EXPLORATIONS  AND  LITERATURE— 1880.  75 

the  Huronian  schist  is  ijroved  conckxsively.  They  were  regarded  as  sed- 
imentary by  the  earher  g-eologists,  because,  as  stated  by  them,  they  were 
found  to  grade  insensibly  into  the  "green  schists"  associated  with  them. 
Wadsworth,  however,  declares  that  this  is  not  the  case.  The  massive  beds 
are  distinct  from  the  schists.  The  contact  between  the  two  rocks  is  often 
sharp,  and  the  one  rock  does  not  grade  into  the  other.  The  large  masses 
of  diorite,  like  that  south  of  Teal  Lake,  are  not  interstratified  beds,  but  are 
true  dikes.  Thus  the  origin  of  these  massive  beds  is  set  at  rest.  As  to  the 
origin  of  the  green  schists  so  frequently  associated  with  the  dikes,  ncjthing 
is  said.  It  is  true  that  the  author  found  some  of  the  diabase  and  "diorites" 
becoming  schistose,  and  others  passing  into  typical  chlorite-schists  and 
hornblende-schists,  but  the  origin  of  the  older  schists,  through  whieli  all 
the  dike  rocks  were  supposed  to  cut,  has  been  left  unsettled. 

With  respect  to  the  origin  of  the  soft  hematites,  the  author  is  in  accord 
with  the  majority  of  those  who  had  studied  them.  He  places  them,  how- 
ever, in  the  same  formation  with  the  jasper  ores,  and  not  in  a  different  and 
lower  formation,  as  does  Brooks.  The  soft  ores  are  believed  to  have  been 
formed  through  the  decomposition  of  ferruginous  schists  by  thermal  waters. 
The  geology  of  the  Salisbury  mine  and  its  situation  seem  to  the  author  to 
lead  to  this  view.  The  ores  are  most  abundant  where  the  schists,  jaspers, 
etc.,  are  most  fractured  and  shattered,  and  hence  are  found  in  the  acute 
angles  between  interpenetrating-  diorite  dikes,  provided,  of  course,  the 
"diorite"  is  younger  than  the  jaspers  and  ores,  as  is  supposed  to  be  the 
case.  Near  Ishpeming  and  Negaunee  "the  dip  of  the  jasper  increases  as 
it  approaches  the  'diorite,'  sometimes  standing-  nearly  vertical.  It  was  not 
observed  in  contact  with  the  'diorite,'  but  we  feel  that  the  constant  ujstilting 
of  the  jasper  and  associated  schist  when  near  these  inti-usive  rocks  is  good 
evidence  that  the  'diorite'  eruption  was  later  than  that  of  the  jasper" 
(pp.  51-52). 

The  next  problem  attacked  is  the  relation  of  the  granite  to  the  Huro- 
nian schists.  If  it  is  intrusive  in  the  schists,  it  is  younger  than  they  and  can 
not  be  of  Laurentian  age,  as  had  been  thought  by  earlier  writers.  The 
author  describes  a  number  of  localities  where  granite  veins  traverse  gneisses 
and  micaceous  and  green  schists,  and  where,  consequently,  the  granite  is 


76  THE   MARQUETTE  lEON-BEAEING   DISTRICT. 

younger  than  the  schists.  If  these  schists  are  Huronian,  as  was  beheved 
to  be  the  case,  then  the  granite  is  not  Lauren tian.  More  significant,  if 
found  con-ect,  are  the  author's  observations  that  the  granite  is  intrusive  also 
in  quartzite.  On  the  hne  of  the  Chicago  and  Northwestern  Railway,  south 
of  Ishpeniing,  the  granite  is  mentioned  as  cutting  a  quartzite  "that  resembles 
the  ordinary  'Huronian'  quartzites."  Southeast  of  Champion  it  is  said  to 
cut  a  sedimentary  micaceous  and  magnetite  schist. 

If  these  observations  are  correct,  some  of  the  granites  of  the  Mar- 
quette region  are  younger  than  the  quartzites  associated  with  the  ores;  but 
apparentl)-  later  obser^^ers  could  not  anywhere  find  quartzite  intruded  by 
granite. 

The  microscopic  features  of  the  various  granites  mentioned  by  Wads- 
worth,  and  of  many  of  their  associated  rocks,  are  described  by  him.  At 
Republic  a  fine-grained  rock,  composed  essentially  of  quartz  and  mica,  was 
found  in  actual  contact  with  tyjDical  granite  or  very  near  the  latter  rock. 
This,  together  with  the  "quartzites"  of  Formation  XIV,  at  the  same  place, 
is  regarded  as  the  modified  edge  of  the  granite,  and,  from  a  purely 
petrographical  standpoint,  as  gneiss. 

After  describing  the  characteristics  of  the  Potsdam  sandstone,  the 
author  discusses  the  nature  of  the  Presque  Isle  trap  and  its  relations  to 
the  sandstone  associated  with  it.  By  microscopic  examination  he  finds  the 
"trap"  to  be  a  peridotite  composed  of  oli^dne,  enstatite,  and  diallage  in  its 
freshest  portions,  and  a  serpentine  elsewhere.  The  serpentine  has  evidently 
been  derived  from  both  the  olivine  and  the  diallage  of  the  original  peridotite. 
With  it  is  always  a  large  quantity  of  dolomite,  so  that  it  seems  probable 
that  Rominger's  stratified  dolomite  at  this  place  is  simply  a  very  much 
decomposed  portion  of  the  peridotite.  With  respect  to  this  latter  rock  the 
author  says  (p.  62): 

We  regard  this  ]iericlotite  as  an  eruptive  rock,  younger  than  the  sandstone  over- 
lying it,  and  agree  in  this  particular  with  Dr.  Houghton.  The  portion  filled  with  veins, 
that  was  taken  by  him  as  a  sedimentary  rock  belonging  to  the  sandstone,  or  a  mixture 
of  sandstone  and  trap ;  as  a  volcanic  sand  or  ash,  by  Messrs.  Foster  and  Whitney ;  and 
as  a  dolomite,  older  than  both  trap  and  sandstone,  by  Dr.  Rominger,  we  regard  as 
simply  the  upper  portion  of  the  intrusive  mass,  modified  by  its  contact  while  heated 
with  the  overlying  sandstone,  and  by  the  percolating  waters  since. 


GEOLOGICAL    EXPLOliATIONS   AND   LITERATURE— ISSO.  77 

He  gives  as  his  reasons  for  this  t-ouclusion  the  observations  that  the 
sandstones  were  "found  to  conform  in  their  stratification  to  the  contour  of 
the  whole  mass"  of  ])eridotite;  that  the  hjwer  portions  of  the  overlying 
rock  are  altered,  as  though  by  the  action  of  heat  and  heated  waters;  the 
absence  of  pebbles  and  fragments  of  the  peridotite  from  the  conglomerates 
of  the  sandstones.  Tlie  serpentine  northwest  of  Ishpeming,  first  mentioned 
by  Wright,  from  its  microscopic  features  is  thought  by  Wadsworth  to  be 
an  altered  peridotite. 

Tlie  author  summarizes  his  work  and  coni-lusions  in  a  few  pages,  fi'om 
which  A^e  extract  these  passages: 

The  observations  and  tigiires  giveu  iu  the  precediug  text  show  conclusively  that 
the  statements  of  Messrs.  Daua,  Kimball,  Hunt,  Brooks,  and  others,  that  the  iron  ore 
is  interstratifled  in  the  associated  schists,  are  incorrect,  and  only  return  to  the  view 
advocated  by  Mr.  Foster  in  his  early  publication.  So  far  as  geologic  science  has  now 
advanced,  the  facts  observed  can  only  be  explained  by  the  eruptive  origin  of  both  the 
ore  and  jasper,  as  they  make  the  same  formation.  The  only  escape  from  this  conclu- 
sion is  the  supposition  that  the  ore  and  jasper  have  been  rendered  plastic  in  situ,  while 
the  chlorite-schist  has  not  been.  *  *  *  That  the  ore  and  jasper  have  been  thus 
rendered  plastic,  while  the  schists,  quartzites,  and  other  associated  rocks  have  not 
been,  is  too  absurd,  chemically  or  geologically,  to  be  tolerated  for  a  moment  as  an 
hypothesis.  *  *  *  The  ore  and  jasper  show  that  tliey  are  the  intrusive  bodies  by 
their  breaking  across  the  lamination  of  the  schists  and  other  rocks,  by  the  changes 
that  take  place  in  the  latter  at  the  line  of  junction,  by  horses  of  schist  being  inclosed 
in  the  ore,  by  the  curvature  of  the  lamination  produced  by  the  intrusion  of  the  ore 
and  jasjier,  etc.  Not  the  slightest  sign  of  the  plasticity  or  intrusion  of  the  schists 
relative  to  the  ore  or  jasper  was  seen.  That  the  present  lamination  of  the  schist 
existed  prior  to  the  intrusion  of  the  ore  and  jasper  is  shown  by  the  effect  of  the  latter 
upon  and  its  relations  to  it.  That  this  lamination  is  the  original  plane  of  deposition  is 
for  part  of  the  schists  not  known ;  *  *  *  The  lamination,  however,  coincides  with 
many  of  the  well- strati  tied  rocks  adjacent,  and  in  some  of  these  the  ore  and  jasper 
were  unmistakably  intrusive.  *  *  *  lu  the  finer-grained  detritus  composing  some 
of  the  schists  it  is  quite  likely  true  that  the  lamination  does  not  coincide  with  the 
original  bedding;  but  if  it  does  not,  then  the  breaking  of  the  ore  across  any  chosen 
plane  whatsoever,  except  the  lamination  plane,  can  be  shown  more  easily  than  in  the 
former  case.  *  *  *  We  are  well  aware  that  objections  from  a  metallurgical  or 
chemical  standpoint  have  been  raised  against  the  theory  of  the  eruptive  origin  of 
hematite  and  silica  together,  in  such  forms  as  we  now  find  them.  If  the  ore  was 
magnetic  at  the  time  of  eruption,  and  has  since  been  altered,  this  objection  is  then 


78  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

done  away  with.  The  secondary  changes  that  have  occurred  in  the  rock  since  eruption, 
as  shown  by  microscopic  examination,  may  also  help.  It  is  well  known  that  there  are 
facts  in  every  science  that  it  is  not  able  to  explain  at  any  one  jriven  time ;  but  the  facts 
exist  the  same,  and  the  science  in  time  rises  to  meet  them.  So  in  this  case  the  fact  is 
they  are  eruptive,  and  the  burden  of  chemical  explanation  rest«  upon  the  chemist,  not 
ui)on  us.  He  must  explain  it  sooner  or  later,  uuless  he  disproves  our  observations. 
Crystals  of  hematite  crystallizing  from  the  molten  magma  of  trachytes  and  rhyolites 
have  long  been  known,  and  are  described  in  all  the  standard  works  of  micro-lithology. 
These  then  offer  the  same  problem,  and  prove  that  hematite  can  be  crystallized 
directly  out  of  the  same  molten  magma,  and  at  the  same  time  with  the  silica  and 
silicates.  It  is  the  business  of  the  chemist  to  meet  the  facts,  and  not  for  iis  to  make 
the  facts  conform  to  his  knowledge  or  theories. 

We  have  found  that  a  large  proportion  of  the  rocks  said  to  be  iiiterstratified, 
and  to  pass  by  insensible  (or  any  other)  transitions  into  the  adjacent  rocks,  are 
eruptive,  and  do  not  so  pass  into  the  country  rock.  The  assumption  that  they  were 
stratified  was  based  on  their  foliation  being  parallel  to  their  walls,  on  their  being 
intrusive  approximately  parallel  to  the  lamination  of  the  schists,  [and  on]  their  gen- 
eral resemblance  to  the  country  rock  of  similar  composition  *  *  *.  The  intrusive 
rocks  belong  in  general  to  the  basalts,  but  are  of  course  old,  and  in  the  majority  of 
cases  greatly  altered.  One  probable  andesite  as  well  as  intrusive  felsites  (rhyolites) 
was  discovered.    *     *     * 

The  "soft  hematites"  are  doubtless  produced  by  the  decomposition  of  the  jasper 
and  its  ore,  brought  about  by  the  fracturing  of  the  rocks  by  the  intrusives  and  by  the 
secondary  action  of  water,  presumably  hot,  on  account  of  the  microscopic  characters 
of  the  quartz  deposited  by  it.  Besides  the  ''  soft  hematites"  there  occur  the  quartzites 
and  conglomerates  derived  from  the  ore  and  jasper,  as  well  as  the  sandstones  and 
schists  impregnated  by  iron,  which  are  sometimes  mixed  to  a  slight  extent. 

We  have  heretofore  seen  that  the  view  that  the  "Huronian"  iinconformably  over- 
lies the  "Laurentian"  has  beeu  only  supported  by  the  fact  that  the  foliation  of  the 
latter  did  not  conform  in  its  dip  to  the  lamination  of  the  former.  Tliis  proof  is  of  no 
value  unless  it  can  be  shown  that  both  rocks  are  stratified  and  in  situ.  That  the  latter 
is  not  so,  we  have  seen  in  numerous  localities.  Heretofore  the  two  systems  have  not 
been  observed  in  contact,  but  recently  statements  have  been  published  that  their 
junctions  have  been  seen  in  other  regions.     *     *     * 

So  far  as  the  Marquette  district  is  concerned  we  have  shown  very  much  stronger 
and  more  abundant  evidence  to  prove  that  the  "Laurentian"  granite  is  younger  than 
the  "Huronian,"  and  an  eruptive  rock,  than  has  been  advanced  by  Mr.  Brooks  (the 
only  man  who  has  advanced  anything  called  proof)  to  show  that  it  is  older.  *  *  * 
(Pp.  OG-70.) 

The  general  structure  of  the  counti-y  would  seem  to  be  as  follows.     The  schists, 


GEOLOGICAL   EXPLORATIONS   AND   LITERATUKE— 1881.  79 

sandstones,  etc.,  having  been  laid  down  in  tlie  usnal  way,  were  tlien  disturbed  by  the 
eruption  of  the  jasper  and  ore;  this  formed  the  knobs  of  jasper,  the  banding  belong- 
ing to  the  fluidal  structure,  and  not  to  sedinient.ation.  Besides  occurring  in  bosses, 
the  jasper  was  spread  out  in  sheets,  and  intruded  through  the  rock  in  wedge-shaped 
masses,  sheets,  and  dikes.  Mucli  of  the  original  rock  still  remained  horizontal,  and 
new  sedimentary  deposits  continued  to  be  formed  out  of  the  jasper  and  the  other 
rocks.  Nest  came  the  eruption  of  "diorites,"  which  comj)Ieted  most  of  the  local 
folding  and  tilting  of  the  strata.  Finally,  the  granite  eruption  took  place  on  both 
sides  of  the  "Huronian,"  uplifting  and  contorting  the  strata  near  it,  and  perhaps 
laterally  compressing  the  inclosed  iron-beai'ing  rocks.  No  basis  exists  so  far,  then, 
for  the  scheme  of  formations  laid  down  by  Mr.  Brooks,  as  it  was  founded  on  the 
supposition  that  all  the  rocks  were  sedimentary. 

Although,  in  deference  to  the  common  custom  we  have  employed  the  term 
jasper  in  writing  of  the  siliceous  eruptive  rocks  associated  with  the  ore,  in  reality  it 
it  not  properly  called  so.  *  *  *  It  is  more  acid  than  the  rhyolites,  the  silica  being 
above  80  per  cent.  *  *  *  We  would  propose,  therefore,  that  all  the  acidic 
eruptive  rocks,  whose  chemical  and  physical  constitution  carries  them  above  the 
rhyolites  should  be  designated  as  jrts^th7es,  *  *  #  in  accordance  with  a  sugges- 
tion of  Professor  Whitney.     (Pp.  75-76.) 


Wadsworth,  M.  E.  On  the  origin  of  the  iron  ores  of  the  Marquette  district, 
Lake  Superior.  Read  March  17,  1880.  Proc.  Boston  Soc.  Nat.  Hist.,  Vol.  XX, 
1878-1880,  pages  470-479. 

The  banded  ores  of  the  Marquette  district,  as  will  be  remembered, 
are  regarded  as  eruptive  by  Foster  and  Whitney  and  as  sedimentary  by 
Kimball.  Since  1865  Kimball's  notion  regarding  them  had  been  generally 
accepted;  at  any  rate  it  was  not  seriously  questioned  until  Wadsworth 
reopened  the  discussion  as  to  their  origin. 

The  author  first  states  that  the  ores  in  question  do  not,  except  in  some 
few  cases,  present  the  characters  of  vein-stones.  The  question  to  be  decided 
is  as  to  whether  "the  ore  and  jaspilite  were  deposited  as  sediments  in  situ 
or  are  of  eruptive  origin." 

The  grounds  upon  which  their  sedimentary  origin  had  been  advocated 
are  these: 

(1)  Bogiron  ores  are  forming  at  the  present  day. 

(2)  On  account  of  the  banding  or  lamination  of  the  ore  and  jaspilite. 


80  THE   MARQITETTE   IRON-BEAEING   DISTRICT. 

(3)  The  bankings  show  foldings  and  coutortious. 

(4)  The  jaspilite  and  ore  are  jointed  and  show  cleavage. 

(5)  The  associated  rocks  are  sedimentary,  and  on  account  of  the  alternation 
with  schists,  the  ore  and  jaspilite,  as  well  as  the  schists,  must  be  metamorphosed 
sedimentary  rocks. 

(G)  The  presence  of  phosphoric  acid. 

These  arguments  are  then  taken  up  and  discussed  separately.  That 
numbered  (1)  is  summarily  dismissed  as  no  argaiment.  "We  will  let  the 
author  himself  I'eply  to  the  others. 

(2)  The  banding  and  lamination  of  the  jaspilite  and  ore  do  not  appear  to  us  to 
be  proof  of  sedimentary  origin,  since  a  similar  banding  is  strongly  marked  in  the 
rhyolites  the  modern  lavas  approaching  nearest  the  jaspilite,  in  dikes  of  felsite,  in 
furnace  slags,  etc.  *  *  *  This  structure  is  common  to  both  sedimentary  and 
eruptive  rocks,  hence  per  se  is  of  no  value  either  way.  The  structure  of  the  banding 
does  often  show  the  origin  of  the  rock  when  it  has  been  studied  with  care.  Those 
advocating  the  sedimentary  origin  of  the  above  mentioned  ore  have  rested  their  claim 
on  the  simple  fact  that  the  rock  was  "striped,"  and  not  on  the  character  of  the 
banding.  We  have  studied  the  banding  and  can  find  nothing  in  it  that  proves 
sedimentation  or  is  inconsistent  with  that  repeatedly  seen  by  us  in  known  eruptive 
rocks. 

(3)  The  folding  and  contortion  of  the  banding  would  take  place  in  any  rock 
whatever  its  origin,  after  it  was  in  position,  if  subjected  to  proper  conditions.  *  *  * 
Hence  folding  and  contortion  of  banding  in  rocks,  like  the  banding,  is  common  to 
both  sedimentary  and  eruptive  rocks,  and  like  the  latter  (banding)  is  no  proof  of 
either  origin. 

(4)  Joints  and  cleavage  planes  are  well  known  to  be  common  to  both  sedimen- 
tary and  eruptive  rocks,  hence  their  i)resence  can  not  be  taken  as  proof  of  either 
origin. 

(5)  Whoever  advanced  the  view  that  since  the  associated  rocks  were  sedimentary, 
therefore  the  jaspilite  and  ore  musfr  be,  *  *  *  must  have  been  aware  that  this 
principle  would  prove  the  great  majority  of  dikes  and  veins  to  be  sedimentary.  A  dike 
])assing  through  slate  must  be  sedimentary  because  the  slate  is  sedimentary.    *    *    * 

(6)  The  presence  of  phosphoric  acid  could  only  have  been  taken  as  proof  of 
sedimentary  origin  by  those  who  had  no  knowledge  of  eruiitive  rocks,  since  it  is  well 
known  to  occur  in  many  of  the  latter.     *     *     * 

We  have  now  taken  up  all  the  evidence  which  we  are  aware  has  been  used  to 
prove  the  sedimentary  origin  of  the  jaspilite  and  ore.  The  characters  used  as  proof 
seem  to  be  such  as  are  common  to  both  sedimentary  and  eruptive  rocks  or  are  of  no 
weight.     (Pp.  473-475.) 


GEOLOGICAL   EXI'LOEATIONS   A^D    LlTEKATUltE— 188L  81 

The  evidence  g-iven  in  beliulf  of  the  eruptive  oi'igin  of  the  jaspiHte 
and  ore  are  the  eruptive  relations  that  are  shown  to  exist  between  them  and 
the  associated  rocks  at  their  contacts.  "The  jaspihte  and  ore  are  found  to 
break  in  various  directions  across  the  lamination  of  the  associated  rocks, 
to  indurate  them  at  the  line  of  junction,  to  send  string'ers  and  tongues  into 
them,  to  cut  the  lamina;  in  every  direction;  in  short,  to  behave  always 
like  an  eruptive  rock  and  never  like  a  sedimentar}-  one"  (}).  47 (i).  This 
theory  assumes  the  sedimentary  origin  of  the  schists  associated  with  the 
ores  and  jaspilite. 

The  author  thinks  that  if  the  ores  were  originally  magnetite,  or  if  they 
have  always  been  hematite,  there  is  no  chemical  difficulty  in  the  way  of 
believing  in  their  eruptive  orig'in,  for  magnetite  is  present  in  all  eruptive 
rocks,  and  hematite  in  many  of  them. 

We  rest  our  conclusion  that  the  jaspilite  and  iron  ore  in  the  Marquette  district 
are  eruptive  upon  the  fact  that  they  possess  characters  which  eruptive  rocks  exliibit, 
especially  in  relation  to  other  rocks,  and  which  no  sedimentary  rock,  proved  to  be 
such,  has  been  known  to  have.  They  offer  no  characters  inconsistent  with  those  that 
known  eruptive  rocks  have,  but  they  do  exhibit  those,  as  said  before,  that  no  strati- 
fled  rock  has,  so  far  as  our  present  kuowledge,  uot  theory,  goes.     (P.  477-478.) 

The  paper  closes  with  a  statement  of  the  conditions  demanded  by  the 
sedimentary  and  the  eruptive  theories.  It  is  pointed  out  that  the  conglom- 
erate over  the  ore  would,  according  to  the  sedimentar}-  theory,  necessitate 
the  belief  in  a  time  interval  between  the  ores  and  the  overlying  rocks, 
whereas  accoi'ding  to  the  eruptive  theory  this  would  not  be  required. 

RoMtNGER,  C.  Marquette  iron  region.  Geol.  Surv.  of  Michigan,  Vol.  IV, 
Part  I,  New  York,  18S1,  pp.  xiv  and  154.     With  map. 

In  the  year  1881  C.  Kominger,  who  had  Ijeen  appointed  State  geolo- 
gist of  Michigan  to  complete  the  survey  begun  under  Alexander  Winchell, 
published  a  report  based  on  three  seasons'  field  work.  In  this  time  its 
author  was  able  to  accomplish  an  immense  amount  of  geological  work, 
and  to  accumulate  a  great  mass  of  facts  concerning  the  geolog}'  of  the 
district  studied.  This  report  is  intended  as  a  supplement  to  that  of  Major 
Brooks.      It   deals   solely  with  the   scientific   aspects   of  the   case,  while 

MON  XXVIII 6 


82  THE   MAEQUETTE   IKOX  BEARING    DISTKICT. 

Brooks's  report  considered  tlie  district  principally  from  the  economic  stand- 
point. The  map  fnrnished  by  Rominger  is  extremely  accurate  in  its 
delimitation  of  the  various  formations  recognized  by  its  author.  It  has 
proved  of  incalculable  value  to  the  present  writers  in  their  field  work  in 
that  portion  of  the  district  covered  b}-  it.  Its  topography,  as  well  as  its 
g-eology,  shows  evidence  of  the  immense  amount  of  careful  labor  put 
iipon  it. 

In  general  the  author  regards  the  Marquette  iron-bearing  rocks  as 
Huronian  and  as  lying  in  a  synclinal  trough  formed  by  the  upheaval  of  the 
edges  of  the  granite  basin  in  which  they  rest.  By  the  rising  of  these  edges 
the  inclosed  sedimentary  rocks  were  uplifted  and  compressed  into  parallel 
folds.  The  upheaved  granitic  and  sedimentary  rocks  are  traversed  hj  rock 
belts,  which  represent  lava  streams  that  ^vere  intruded  from  below  at  different 
periods  after  the  formation  of  the  traversed  rocks.  The  author  declines  to 
regard  the  granitic  rocks  of  the  region  as  Laurentian,  since  as  a  series  they 
do  not  correspond  lithologically  with  the  Canadian  Laurentian,  and  since 
the  discordances  described  as  existing  between  them  and  the  Huronian  rocks 
are  not  discordances  between  the  two  divisions  at  their  immediate  contacts. 
Even  if  discordances  do  exist  they  would  pro^•e  nothing,  according  to  the 
author,  in  beds  so  much  disturbed  as  are  tliose  in  the  Marquette  district 

As  far  as  my  own  observations  go,  I  have  never  been  able  to  discover  any  posi- 
tive proof  of  an  existing  discordance  between  the  granites  of  Marquette  and  the 
adjoining  Huronian  beds ;  on  the  contrary,  outcrops  of  the  two  kinds  of  rock  supposed 
to  represent  the  contact  of  the  two  formations  exhibit  everywhere  a  remarkable 
liarallelism  in  strike  and  dip,  and  in  a  good  many  localities,  where  belts  of  granite  are 
found  interlaminated  between  the  Hurouiau  schists,  the  conformity  is  perfect;  but  I 
am  far  from  believing  that  these  conformably  interstratified  bands  of  granite  ever  had 
been  formed  there  as  regular  members  of  the  sedimentary  series;  I  consider  them  as 
Intrusive  masses  *  *  *  which  came  to  the  surface  after  the  Huronian  beds  were 
already  formed,  and  by  their  eruption  caused  not  only  the  great  dislocations  of  the 
Huronian  formation,  but  the  half  molten  plastic  granite  masses  induced  by  their  con- 
tact with  the  Huronian  rock  beds,  also  their  alteration  into  a  more  or  less  perfect 
crystalline  condition,  and  commingled  with  them  so  as  to  make  it  an  embarrassing  task 
to  tiiid  a  line  of  demarcation  between  the  intrusive  and  the  intruded  rock  masses. 
(P.  6.) 


GEOLOGICAL   EXPLORATIONS   AND   LITEEATUIiE— 1881.  83 

Rominger  thiuks  Brooks's  subdivisions  are  more  numerous  than  is 
necessary  in  discussing  the  formations  present  in  the  Marquette  series.  He 
recognizes  only  six  "groups,"  as  follows,  beginning  with  the  lowermost: 
the  granitic,  the  dioritic,  the  iron,  the  quartzite,  the  arenaceous  slate, 
and  the  mica-schist.  The  members  of  the  "granitic  groups"  are  confined 
to  the  northern  and  southern  limits  of  the  Marquette  area,  where  they  exist 
as  the  predominant  rocks,  intimately  associated  with  diorites,  green  schists, 
etc.,  with  which  they  often  seem  to  be  interlaminated. 

The  granites  on  both  sides  of  the  synclinal  basin  are  similar  in  compo- 
sition and  structure.  They  consist  essentially  of  red  orthoclase,  quartz, 
and  a  micaceous  mineral  that  often  resembles  chlorite.  Sometimes  this  is 
replaced  by  a  hydromicaceous  substance  which  imparts  to  the  granite  a 
subschistose  cleavage.  Such  granites  are  found  at  a  few  places  in  the 
northern  granite  belt  and  on  the  south  side  of  the  synclinal  l^asin,  on 
the  line  of  contact  between  the  normal  granites  and  the  lower  quartzite 
formation.  Here  they  appear  to  be  metamorphosed  quartzites,  "as  we  find 
all  degrees  of  transition  from  the  ordinary  quartzite  into  a  regular  granitic 
rock  mass"  (p.  15). 

Associated  with  the  granites  are  also  belts  of  gneissoid  rocks,  consisting 
of  dark  mica  or  a  dark-green  hornblende,  feldspar,  and  quartz. 

This  stratified  banded  rock,  in  contiguity  witli  the  granite  and  alternating  with 
it  in  parallel  belts,  often  becomes  completely  intermingled  and  entangled  with  it.  The 
granitic  masses  intersect  the  gneissoid,  enter  wedge-like  between  them  in  the  direc- 
tion of  the  lamination  or  transversely,  inclosing  strips  of  the  gneissoid  ledges 
between  the  loops  of  the  anastamosing  granite  seams,  and,  moreover,  frequently  the 
so-intermingled  masses  are  curved  into  the  most  curious  coils  and  serpentiue  flexions,, 
which  evinc3s  their  almost  liquefied,  plastic  condition  at  the  time  their  intermixture 
took  place.     (Pp.  16-17.) 

The  granites  and  gneisses  are  cut  by  large  and  small  dikes  of  horn- 
blende-rocks, by  diorites,  and  by  seams  of  quartz.  In  the  northern  halves 
of  sees.  20,  21,  22,  and  23,  T.  48  N.,  R  26  W.,  is  a  range  of  genuine  sye- 
nitic  rock  composed  of  dark  hornblende  and    a  reddish-gray  orthoclase. 

From  these  descriptions  it  is  seen  that  the  granite  is  considered  an 
eruptive  rock  intruding  certain  dioritic  and  hornblendic  schists  that  must  be 


84  THE    MAEQUETTE   IllON-BEAKING   DISTRICT. 

older  than  the  granite,  and  yet  the  author  places  the  granite  in  the  oldest 
formation  of  the  series.  Naturally,  if  the  granites  as  eruptive  I'ocks  pro- 
duced the  folding  of  the  Huronian  beds,  they  must  be  the  youngest  of 
all  the  rocks  occurring  in  the  series,  and  can  not  possibly  be  the  oldest  in 
their  present  position.  The  author  explains  this  anomaly  in  the  following- 
words  (pp.  22-23): 

The  granites,  coasidered  iu  their  present  surface  position,  are,  in  relation  to 
the  stratified  sedimentary  rocks  of  the  Huronian  series,  actually  the  younger  rocks. 

*  *  *  The  hypothesis  of  their  contemporaneous  eruption  is  therefore  well  admis- 
sible. But  supposing  this  to  have  been  the  case,  one  may  ask,  Of  what  nature,  then, 
was  the  substratum  on  whicli  the  Huronian  sediments  were  deposited?  I  answer, 
Nothing  contradicts  the  possibility  of  their  deposition  on  a  surface  of  granite  already 
formed;  it  is  even  probable  to  me  that  it  has  been  so;  but  if  we  reflect  upon  the  high 
degree  of  plasticity  and  the  almost  perfect  liquefaction  which  the  concerned  rocks 
subsequently  underwent,  and   upon    the    dislocating  forces,    causing  the   softened 

*  *  *  masses  to  intermingle  almost  chaotically,  we  can  no  more  wonder  that  the 
traces  of  the  originally  existing  former  relative  iiosition  of  the  rocks  among  themselves 
;are  greatly  obliterated. 

The  meaning  of  this  is  that  the  Huronian  rocks  were  laid  down 
upon  a  crust  of  granite  which  had  not  become  rigid.  Subsequently 
the  granite  rose  and  was  erupted  through  the  l^eds  that  had  been  piled 
vipon  it. 

The  "granitic  group"  contains  a  great  many  beds  of  green  schists, 
hornblende-rocks,  and  diorites,  but  the  granites  are  the  predominant  rocks. 
'The  "dioritic  group,"  on  the  other  hand,  is  made  up  of  a  large  succession  of 
;Schistose  beds  interstratified  with  massive  belts  of  diorite  almost  identical 
chemically  with  the  schistose  beds.  The  lines  of  demarcation  between  the 
granitic  and  the  dioritic  "groups"  are  not  sharp;  indeed,  as  the  author 
declares,  the}'  are  "  artificial  lines  of  demarkation  for  the  convenience  of 
■description."  "I  do  not  intend  to  indicate  by  these  subdivisions  (into 
•groups)  separate,  distinct  epochs. ' 

The  various  crystalline  hornblende-rocks  associated  with  the  granites 
are  considered  to  be  remelted  Huronian  sediments.  Those  more  remote 
from  the  eruptive  are  much  altered,  but  the  author  believes  that  he  can 
detect  in  them  the  sedimentary  structure.    Through  them  have  been  intruded 


GEOLOGICAL   EXPLORATIONS   AND    LITEKATUllE— ISSl.  85 

the  lower  inelteil  jxn-tious  of  the  series,  either  as  dikes  or  as  sheets  of  a 
dioritie  character. 

The  schists  coiistitiMing-  the  greater  part  of  the  "dioritie  g-roup  "  are 
dark-g-ray  or  blackish-green  rocks,  composed  of  horiil)lende,  chlorite,  and 
mica,  with  feldspar  and  quartz.  Chlorite  frequenth'  replaces  the  horn- 
blende and  often  seems  to  be  a  product  of  its  decomposition.  A  part  of  the 
schists  belonging-  to  the  group  are  composed  largely  of  a  hydromicaceous 
constituent. 

The  ilioritiu  rock-belts  are  usually  imbedded  conformably  with  the  schists,  and 
not  rarely  an  insensible  gradation  from  the  schistose  condition  to  the  massive  dioritie 
can  be  observed.  In  the  exi)osures  the  massive  body  of  diorite  generally  forms  a 
nucleus  around  which,  eccentrically,  the  inclosing  rock  masses  assume  more  and  more 
a  perfect  schistose  structure.  *  *  *  Other,  generally  narrower,  diorite  belts  inter- 
sect the  schists  transversely,  which  differ  little  in  composition  from  the  conformably 
interstratifled  masses,  and  may,  as  I  previously  intimated,  represent  the  lowest,  more 
coinjiletely  liquefied  portions  of  the  rocks  in  progress  of  alteration.     (P.  24.) 

The  schists  of  this  "group"  are  described  as  occupying  an  area  south 
of  the  northern  granite  belt,  as  far  west  as  they  were  examined,  viz,  the 
west  side  of  the  eastern  tier  of  sections  in  R.  28  W.  On  Dead  River, 
above  Bancroft's,  a  succession  of  schists  and  diorites,  measuring  3,000  feet 
in  aggregate  thickness,  was  traversed,  but  these  figures  are  not  regarded  as 
indicating  positively  that  the  tliickness  of  the  series  is  as  great  as  this. 
There  may  be  folds  in  the  rocks  at  this  point,  causing  a  repetition  of  the 
same  beds  on  opposite  sides  of  the  axis  of  folding.  They  were,  however, 
not  detected. 

In  its  northern  portion  the  schist  belt  comprises  dark  rocks  with  a 
delicately  laminated  schistose  structure.  At  Marquette  there  is  interstratifled 
with  them  a  belt  of  banded  quartz-schist,  which  in  places  becomes  a  lean 
maginetite  ore.  Farther  south  the  schists  are  lighter-colored  and  their 
structure  is  more  fissile  and  slate-like.  Some  of  the  layers  ai'e  tinged  witli 
red  oxide  of  iron,  and  irregular  belts  of  hematitic  iron  ore  are  interstratified 
with  them,  as  at  the  Harlowe  or  Eureka  mine.  Argillite-like  beds  are  met 
with  on  the  hillside  south  of  Ridge  street,  in  Marquette,  and  immediately 
north  of  these  is  a  belt  of  novaculite  more  than  100  feet  in  width.     South 


86  THE    MAEQUETTE   IRON-BEAEING   DISTRICT. 

of  the  argillites  are  again  diorite-schists,  and  .south  of  these  is  found  a  repe- 
tition of  the  argiUites.  Still  farther  south  the  rocks  have  an  inverted  dip, 
and  the  novaculites  and  argillites  are  found  just  beneath  the  "quartzite 
group."  From  the  facts  observed  with  respect  to  the  distribution  of  the 
schists  in  the  schist  belt,  the  author  concludes  that  "we  must  necessarily 
infer  the  existence  of  repeated  plications  of  the  strata  exposed  *  *  * 
and  an  overturned  position  of  the  northern  part  of  the  layers,  as  the 
novaculites  and  argillites  underlying  them  are  beyond  doubt  the  equiva- 
lents of  those  seen  on  the  south  side  next  below  the  quartzite,  and  upon 
the  dioritic  layers,  and  represent  the  uppermost  beds  of  the  dioritic  rock- 
group"  (p.  32). 

Very  many  detailed  descriptions  of  individual  exposures  are  given  by 
the  author,  but  they  are  not  necessary  to  an  understanding  of  his  discussion 
of  the  district.  It  is  interesting,  however,  to  mention  the  discovery  of  great 
beds  of  conglomeratic  schists  in  the  upper  part  of  the  "dioritic  group"  in 
the  neighborhood  of  Deer  Lake.  These  are  spoken  of  as  conglomeratic 
or  brecciated  seams,  because  a  part  of  the  pebbles  are  angular  and  a  part 
rounded.  "The  majority  of  them  consists  of  granular,  somewhat  ^Kirous 
feldspathic  substance,  which  on  fresh  fractures  contrasts  little  from  the 
surrounding  schistose  mass,  but  sliows  itself  very  plain  on  weathered 
surfaces,  on  which  the  pebbles  turn  white  or  pale  reddish"  (p.  36).  Other 
conglomerates  exist  in  which  the  schists  are  the  groundmass  and  the 
pebbles  granite.  Some  of  these  are  in  the  same  horizon  as  are  the  Deer 
Lake  conglomerates,  while  others,  iuterstratitied  with  the  schists,  are  much 
lower  in  the  sei'ies. 

The  greenstones  with  the  iron  formation,  like  those  near  Ishpeming 
and  Negaunee,  were  regarded  by  Brooks  as  interstratified,  metamorphosed 
sedimentary  beds.  Rominger,  on  the  contrary,  regards  them  as  the  lower, 
fused  portions  of  the  dioritic  series  that  were  forced  up  into  their  present 
position.  In  other  Avords,  he  looks  upon  them  as  eruptives  which  were 
forced  between  the  beds  of  the  iron  formation  as  sheets,  but  which  in  origin 
are  fused  sedimentary  rocks,  as  indicated  above. 

The  quartzite  formation  succeeds  the  "iron  group"  in  age,  but  precedes 
it  in  the  author's  discussion  of  the  two  series  in  question.     A  number  of 


GEOLOGICAL    EXPLORATIONS    AND    LITEKATUKE— 1881.  87 

sections  are  described  across  the  tormatioii.  Tliey  are  so  nearly  alike  in 
general  features  that  we  need  refer  to  only  two  of  them  in  this  place.  In 
particulars,  however,  they  vary  widely. 

The  quartzites  of  the  Mesnard  range,  near  Marquette,  are  interliedded 
with  argillites  and  hydromicaceous  schists  or  slates,  and  with  ferruginous 
and  siliceous  slaty  seams,  all  dipping  south  nearly  vertically.  Conformably 
superimposed  upon  the  quartzite  is  a  series  of  siliceous  limestones,  inter- 
laminated  with  bands  of  novaculitic  slaty  seams  of  a  purplish  color.  Tlie 
limestone  is  folded  and  corrugated.  Farther  south  quartzites  again  appear, 
and,  folio  whig  these,  a  band  of  conglomerate  inclosing  quartzite  pebbles  and 
novaculitic  schistose  fragments,  which  "are  cemented  tog-ether  bj^  a  paste 
of  similar  schistose  material,  and  intermingled  with  quartz-sand  and  octa- 
liedric  crvstals  of  martite."  South  of  the  conglomerate  belt  is  a  recurrence 
of  the  entire  series,  but  in  reversed  order,  though  the  dip  continues  to  be 
southerly. 

At  the  west  end  of  Teal  Lake  is  another  wide  exposure  of  quartzites, 
whose  description,  given  in  the  author's  words,  will  express  his  notion  of 
the  relation  of  the  "(piartzite  group"  to  the  underh-ing  diorites: 

Large  and  very  iustructive  exposures  of  the  quartzite  formation  and  of  its 
connectiou  with  the  underlying  dioritic  series  are  observable  at  the  west  end  of  Teal 
Lake.  The  .south  slope  of  the  hills  is  formed  by  a  thick  belt  of  compact,  heavy- 
bedded,  whitish  quartzites,  which  project  in  long  rock  walls,  dipping  southward 
under  an  angle  of  G.jo  to  70°;  north  of  this  belt  we  find  dark-colored  slaty  rocks 
richly  impregnated  with  minute  crystals  of  martite  in  connection  with  seams  of  lighter 
colored,  not  ferruginous,  argillitic  or  novaculitic  schists,  amounting  to  considerable- 
thickness;  another  thick  body  of  quartzite  ledges  follows  on  their  north  side,  which 
forms  the  edge  of  the  northern  slope  of  the  hillside,  on  which  a  descending  .section 
through  the  lower  portion  of  the  formation  down  to  the  dioritic  series  is  well  exposed; 
novaculitic  seams  alternating  with  bands  of  quartzite  form  the  upper  part  of  the 
slope;  beneatli  them  follows  a  large  succession  of  silky,  shining,  hydromicaceous 
slate-rocks,  with  a  crystalline,  granular,  aluminous  groundmass  in  different  shades  of 
color,  and  in  some  of  the  seams  charged  with  large  proportions  of  granular  crystals 
of  martite  and  magnetite.  These  slaty  rock-beds  are  on  their  north  side  conform- 
ably adjoined  by  chloritic  and  dioritic  schists  with  inclosed  massive  diorite  belts, 
which  rock  series  composes  all  the  hill  ranges  farther  north  to  the  valley  of  Carp 
Eiver  and  those  on  the  north  side  of  the  valley.     (P.  45.) 


88  THE   MARQUETTE    lEOX-BEArjNCT    DISTRICT. 

Near  Lake  Cooper  "the  quartzite  formation  forms  *  *  *  the  basal 
girdle  of  a  much  higher  mountain  body  of  dioritic  rooks,  which  occupies 
the  central  part  of  the  above-named  section;  and  here  frequent  opportunities 
are  offered  to  see  the  intimate  connection  existing  between  the  two  groups, 
linked  together  by  uninterrupted  succession,  and  l)y  gradations  in  the 
change  of  the  material  of  the  rock  beds"  (p.  46).  Thus  there  is  no 
unconformity  ))etweeu  the  dioritic  schists  and  the  overlying  quartzites,  but, 
on  the  other  hand,  the  rocks  grade  into  each  other;  hence  the  diorites  are 
but  little  older  than  the  quartzites.  At  the  contact  of  the  quartzites  with 
the  granite  the  former  rock  is  often  altered.  Its  structure  becomes  schistose 
and  in  composition  it  approaches  granite.  The  author  quotes  several 
occurrences  of  this  character  as  illustrations  of  "the  transformation  of 
sedimentary  strata  into  a  granite-like  rock"  by  exposure  to  contact  with 
eruptive  granite.  From  the  descriptions  so  carefully  given  it  is  clear  that 
the  quartzite  formation  is  folded  into  a  large  synclinorium,  composed  of 
numerous  small  synclines  and  anticlines.  Tlie  upper  portion  of  the  quartzite 
is  calcareous,  as  has  been  remarked.  It  comprises  a  series  of  siliceous 
limestones  and  calcareous  slates,  which  is  called  the  "marble  series." 

Conglomerates  and  breccias  are  often  to  be  found  associated  with  the 
quartzites,  especially  when  the  latter  are  near  granite.  In  the  soutli  half 
of  sec.  22,  T.  47  N.,  R.  26  W.,  certain  hills  are  "composed  of  a  very  coarse 
granite  breccia,  inclosing-  blocks  of  large  size,  several  feet  in  diameter, 
cemented  together  Ijy  an  arenaceous  and  chloritic  interstitial  mass  of  a  lami- 
nated, evidently  sedimentary  structure,  wliieh  exliibits  itself  plainly  in  some 
portions  of  the  rock,  in  which  the  rock  fragments  are  more  thinly  scattered 
through  the  cementing  groundmass.  In  certain  portions  of  these  knobs  the 
granite  appears  in  solid  masses,  too  large  to  be  taken  for  fragments  pertain- 
ing to  the  breccia  which  fact  induces  me  to  suggest  the  nucleus  of  all  these 
hills  to  be  a  solid  granite  mass,  whose  shattered  surface  portions  are  rece- 
mented  on  the.  spot  by  sedimentary  debris  washed  into  the  interstices" 
(p.  62).  A  little  farther  south  in  the  same  section  are  other  conglomerates, 
whose  "inclosed  waterworn  grains  are  in  part  granite,  in  part  slate  frag- 
ments." Near  the  southwest  corner  of  the  section  are  other  conglomerates, 
whose  matrix  is  a  slate  and  whose  pebbles  are  waterworn  granites.     In 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— 1881.  89 

all  these  cases  tin-  cc mg'lomerates  are  associated  with  the  members  of  the 
quartzite  formation  near  the  granite. 

(Ither  couiilomerates  of  a  different  nature  were  found  above  the  iron- 
bearin;^'  fnnnation.  Tliese  are  generally  coarse  quartzite-conglomerates, 
composrd  parth'  of  rounded  waterworn  pebbles,  partly  of  angular  frag- 
ments of  flint^-  (puirtzite,  of  red-banded  ferruginous  jasper,  of  novaculitic 
or  argillitic  schist,  and  of  other  rocks.  At  the  Cascade  mine  the  conglom- 
erate has  ''partly  the  structure  of  a,  very  coai-se  sandstone  w^ith  a  micaceo- 
ferruginous  cement,  being  partly  formed  of  an  agglomeration  of  large 
pebljles  and  angular  fragments,  some  of  which  have  the  size  of  a  man's 
head,  whicli  are  all  iirmly  cemented  together  l)y  a  micaceo-chloritic,  sandy, 
interstitial  mass,  often  disseminated  with  granules  of  inartite.  The  pebbles 
are  glassy  or  flinty  quartzite,  jasper-banded  siliceous  iron  ores,  novaculitic 
and  argillaceous  slate  fragments,  and  dioritic  rocks.  These  conglomerates 
have  the  thickness  of  from  50  to  60  feet,  and  can  Ije  followed  in  one  con- 
tinuous sheet  all  the  way  east  to  the  Gribben  mines"  (pp.  66-67).  Among 
the  other  occurrences  of  this  conglomerate  and  breccia  described  are  those 
of  the  Home,  Jackson,  Cleveland,  Gibbon,  Salisbury,  Lake  Superior, 
Champion,  Saginaw,  Goodrich,  Keystone,  Republic,  and  ^Michigamme 
mines.  Althougli  the  full  significance  of  the  widespread  occurrence  of 
this  conglomerate  aljove  the  ore  formation  was  not  realized,  Rominger 
nevertlieless  was  led  Ijy  its  existence  to  suggest  "that  disturbances  of  not 
only  a  local  extent  must  have  occurred  nt  the  end  of  this  era  of  iron 
sediments." 

In  discussing  the  iron-bearing  formation  tlie  author  declines  to  admit 
the  existence  either  of  the  two  iron  formations  of  Ih-ooks  or  of  his  two 
quartzites.  Only  one  iron-bearing  foririatinn  is  recognized  l)y  the  author, 
and  onlv  one  quartzite,  and  this  is  the  nppei-  (piartzite  of  Brooks. 

The  surface  rock  of  the  envirous  of  Negauiiec  and  Ishpeniing  is  ahiio.st  exclu- 
sively formed  of  the  iron-bearing  rock  series.     *     * 

The  strata  are  in  an  extremely  disturbed  condition,  folded  and  distorted  in  every 
possible  way,  usually  without  causing  a  rupture  of  the  beds,  but  in  some  other 
instances  the  laminated  banded  seams  composing  the  thicker  ledges  have  ruptured, 
and  the  ends  often  came  in  a  faulted  position  to  each  other,  and  w6re  so  recemented 
by  the  siliceous  groundraass.    *     #     *     These  disturbed  beds  lie,  in  every  instance, 


90  THE   MARQUETTE   lEON-BEAEING    DISTRICT. 

directly,  but  very  often  inconformably,  on  chlorito-hydro- micaceous  schists,  or  on 
crystalline  dioritic  masses  wliicli  are  constant  associates  of  these  chloritic  schists,  or 
sometimes  dioritic  schists,  as  hornblende  and  chlorite  substitute  each  other,  or  are 
both  components  of  them. 

Overlooking  the  extremely  plicated  and  corrugated  condition  of  the  strata, 
they  form,  considered  in  their  totality,  a  synclinal  basin  hemmed  in  between  dioritic 
ridges.     (Pp.  72-73.) 

After  describing  the  general  structural  features  (if  the  ore  formation 
Rominger  proceeds  to  describe  the  dilTerent  mines,  giving  details  too 
numerous  to  mention  in  this  review.  He  begins  with  the  Jackson  mine, 
which  he  savs  is  in  the  upper  part  of  the  formation  whose  lower  portion 
is  found  on  the  diorite  hills  south  of  Negaunee.  The  ores  are  in  a  banded 
jasper  rock,  and  are  usually  associated  with  "belts  of  argillitic  schists  called 
soapstone  by  the  miners."  The  ore  bodies  are  irregularly  distributed 
through  the  jasper-heni.ititi'-scliists,  except  that — 

a  very  rich  seam  of  ore  is  almost  invariably  found  on  top  of  this  jasper-banded  rock- 
series,  immeiliately  beneath  the  quartzites  which  form  the  terminal  strata  iu  all  these 
exposures.  This  upper  ore  belt  is  almost  regularly  brecciated  iu  its  upper  jiart,  aud 
the  same  is  true  of  the  lower  quartzite  beds,  which  often  are  a  mixture  of  ore  frag- 
ments with  quartzite  pieces  held  together  by  an  arenaceous  cement.  As  this  is  the 
case  in  nearly  all  the  mines  of  the  district,  we  must  suggest  that  great  disturbances, 
of  not  only  a  local  extent,  must  have  occurred  at  the  end  of  this  era  of  iron  sedi- 
ments.    (P.  74.) 

In  another  part  of  the  series,  it  is  declared,  there  is  a  great  thickness 
of  argillitic  rocks,  all  impregnated  with  red  oxide  of  iron,  and  in  these  are 
intercalated  ore  seams  in  such  quantity  as  to  make  this  one  of  the  most 
productive  fields  for  the  miner. 

These  ore  dei^osits  are  not  regular  sedimentary  layers,  originally  formed  of  iron 
oxide  in  this  state  of  purity,  but  are  evidently  the,  product  of  decomposition  of  the 
impurer  mixed  ferruginous  ledges  by  percolating  water,  leaching  out  the  siliceous 
matter  and  replacing  it  by  deposition  of  oxide  of  iron  held  iu  solution.     (P.  7r>.) 

The  ores  are  dark  or  yellow  soft  ores,  composed  parth"  of  hydrated 
and  partly  of  nonhydrated  oxide,  and  containing  often  globular  and  con- 
cretionary masses  with  a  radiating-  or  fibrous  structure  or  "with  the  granular 
crystalline  form  of  goethite."     Pyrolusite  and  other  manganese  compounds, 


GEOLOGICAL   EXPLOEATIONS   AND    LITERATURE— 188L  91 

quartz,  barite,  and  nodular  masses  of  a  soft  aluminous  silicate  are  associated 
with  these  ores.  Where  the  ore  beds  are  in  contact  with  the  diorites  there 
is  usually  an  unconformity  between  them. 

All  the  ore  beds  of  Negaunee  are  placed  together.  The  hard-ore  for- 
mation is  not  separated  from  the  lower  soft-ore  series,  the  latter  ores  being 
regarded  as  local  phases  of  the  former.  East  of  Negaunee  the  lower 
portion  of  the  iron  series  is  found. 

TLe  abseuce  of  the  uijper  ore-bearing,  red  jasper-banded  rock  series,  with 
inclosed  seams  of  hard  s^iecular  ore,  and  of  the  quartzites  incumbent  on  it,  from 
the  exposures  of  the  lower  part  of  this  group  on  the  east  side  of  Negaunee,  is 
remarkable,  because  younger  strata,  which  elsewhere  have  their  place  above  the.se 
eliminated  beds,  directly  succeed  the  others.  These  yoiniger  rock  beds  cover  most 
of  the  surface  east  of  this  place  for  many  square  miles,  and  iio  more  of  the  iron 
formation  can  be  discovered  in  that  direction.     (P.  80.) 

After  his  description  of  the  mines  the  author  declares  that  "every  one 
of  these  localities  differs  somewhat  from  the  other  in  the  character  of  its 
layers,  but  the  unity  of  all  these  deposits  Jis  coordinate  members  of  one 
formation  is  plainly  obvious"  (p.  87).  At  Teal  Lake  the  entire  series  is 
overturned  with  the  quartzite  under  the  ore  formation. 

The  actinolitic  schists  of  the  western  portion  of  the  iron  range  are 
placed  ill  the  iron  formation. 

At  the  Repul)lic  mine  the  diorite  beds  that  were  reported  by  Brooks 
as  interbedded  with  the  sedimentary  formations  are  found  by  Rominger  to 
be  "intrusive  belts  of  short  local  extension"  (p.  101).  At  the  Spurr  and 
Michigamme  mines,  however,  diorite  forms  the  base  of  the  series  and  this 
rock,  which  is  often  chloritic,  exhibits  sometimes  obscure  traces  of  former 
stratification. 

The  formation  lying  above  the  quartzite  is  termed  the  "arenaceous  slate 
group," because  so  many  of  its  members  are  "sandy,  siliceous  laj^ers,"  alter- 
nating with  "slaty  argillitic  beds."  The  character  of  the  strata  differs  in 
different  jilaces,  but  on  the  whole  the  nature  of  the  series  is  as  indicated. 
The  rocks  comprising  this  series  are  found  sometimes  lying  upon  the 
quartzites,  sometimes  upon  the  iron  formation,  and  often  directly  upon  the 
diorite  series.     The  most  easterly  exposures  of  them  are'  near  the  center 


92  THE   MARQUETTE   IROX-BEAEING  DISTRICT. 

of  the  SE.  4  sec.  6,  T.  47  N.,  R.  25  W.,  where  they  are  represented  by 
black  slates.  From  this  point  the  formation  extends  westward,  with  a  few 
interruptions,  to  Lake  Michigamme.  Sandy  micaceous  flagstones,  black 
carbonaceous  and  light  slates,  and  sand-rock,  interspersed  with  ferruginous 
layers,  referred  to  as  "flag  ores,"  are  all  included  in  the  "group."  They 
are  the  equivalents  of  the  great  slate  formation  at  L'Anse  and  on  Huron 
Bay.  All  the  slates  of  the  series  are  cleaved,  with  the  cleavage  planes 
usually  inclined  to  the  stratification.  The  sequence  of  the  different  members 
of  the  formation  ^vas  not  determined,  though  the  lighter-colored  layers  in 
the  west  appear  to  be  the  u})per  portion  of  the  series,  with  the  older  members 
farther  east. 

Above  the  "arenaceous   slate  group"  near  the  railroad,  west  of  the 
]\Iicliigamme  mine — 

are  outcrops  of  dark  blackisli-gray  rock  beds,  partly  of  slate  structure,  partly  in  well- 
lauiiiiated,  banded,  more  compact  seams,  wbicli  succession  of  beds  follows  immediately 
above  the  actiuolitic  rock  series,  dipping  in  conformity  with  that  southward.  These 
rock  beds  consist  of  a  subporous  groundmass,  formed  of  very  minute  granules  of  white 
translucent  quartz,  in  intermixture  with  a  large  proportion  of  brightly  glistening  black 
mica  scales,  and  not  rarely  also  with  chlorite.  In  the  softer,  quite  fissile  schistDse  or 
slaty  beds  the  mica  overbalances  the  granular  quartz,  and  they  have  a  silky  luster. 
In  the  compact  banded  ledges  the  quartzose  groundmass  prevails  and  their  aspect  is 
dull.  Certain  seams  inclose  an  abundance  of  brown  garnet  crystals,  from  the  size  of 
a  mustard  seed  to  that  of  a  pea.  These  beds  I  consider  as  representatives  of  the  upper 
horizon  of  the  fifth  group;  they  correspond  with  the  micaceous  schists  *  *  *  ou 
the  north  side  of  the  Keystone  mines.  *  *  *  On  the  south  shore  of  the  lake, 
[Michigamme]  opposite  Michigamme  village,  the  rock  beds  come  to  the  surface  near 
the  water's  edge;  we  find  there  silvery-shining  gray-colored  mica-schists,  some  smooth, 
even  bedded,  others  much  corrugated,  which  essentially  consist  of  the  same  minutely 
granular  quartzose  groundmass  mingled  with  lighter-colored  mica  scales,  which  com- 
poses the  schists  on  the  west  side  of  the  Michigamme  mines.  This  great  similarity  in 
the  sedimentary  material  is  an  evidence  of  the  close  connection  between  this  mica- 
schist  group  and  the  areuceous  slate  group,  which  proves  the  immediate  succession  of 
the  first  to  the  other  more  reliably  than  it  is  done  by  the  southern  dip  of  the  strata, 
conformable  with  those  of  the  Michigamme  mine.  *  *  *  The  dip  of  the  nearly 
vertical  rock  beds  is  almost  uniformly  to  the  south,  but  the  succession  of  beds  is  so 
immensely  large  that  there  must  be  suggested  a  frequent  doubling  up  of  the  strata, 
which,  in  a  belt  of  several  miles  in  width,  retain  from  one  end  to  the  other  almost  the 
same  general  rock  character."    (Pp.  131-132.) 


GEOLOGICAL   EXPLOUATIONS  AND   LITEEATUKE— 1882.  93 

Tliert'  are  inau}-  varieties  of  these  schists,  from  ahnost  black  to  silver- 
white  ill  color.  Some  are  garnetiferous ;  others  contain  andalusite.  All  are 
regarded  as  sedimentary. 

This  concludes  the  author's  observations  on  the  iron-bearing  series. 
The  remainder  of  the  report  is  devoted  to  the  serpentines  and  the  erup- 
tive dikes  met  with  in  his  explorations.  He  describes  for  the  first  time  in 
detail  the  serpentines  northwest  of  Ishpeming.  Both  the  Ishpeming  and 
the.Presque  Isle  serpentines  occur  in  "  nonstratified  masses,  which,  if  they 
ever  originated  from  mechanical  sedimentary  deposits,  are  by  chemical 
action  so  completel}'  transformed  as  to  efface  all  traces  of  their  former 
detrital  structure.  They  resemble  a  volcanic  eruptive  rock,  forced  to  the 
surface  in  a  soft  plastic  condition"  (p.  135).  The  (lolomitie  and  other 
phases  of  the  rock  are  all  accurately  described,  but  notliing  is  added  to 
our  knowledge  of  its  age  or  origin. 

The  eruptive  dikes  cutting  the  Huronian  deposits,  including  the  granite, 
consist,  in  the  order  of  their  age,  of  diorites,  dolerites,  and  certain  schistose 
rocks,  probably  originally  diorites.  The  diorite  dikes  vary  in  width  from  a 
foot  to  50  or  ()0  feet,  the  wider  ones  being,  as  a  rule,  coarser  than  the  narrow 
ones.  The  dolerites  are  found  only  in  the  highest  formation.  Most  of  these 
are  massive,  but  many  of  them,  cutting  granites,  are  schistose  through 
pressure. 

The  quartz  and  other  vein  rocks  of  the  distinct  are  described  as  fissure 
veins. 


Columbia  University.  The  Marquette  iron  region.  By  the  students  of  the 
Summer  School  of  Practical  Mining,  Lake  Superior,  1881.  School  of  Mines  Quarterly, 
Vol. Ill,  1882.  I,  November,  1881,  pages  35-48;  TI,  ibid.,  Jo,iiuary,  1882,  pages  103-117; 
III,  ibid.,  pages  197-207;  IV,  ibid.,  pages  243-233. 

In  the  summer  of  1881  the  students  of  a  class  in  the  Summer  School 
of  Mining  connected  with  Columbia  University  spent  a  few  weeks  in  the 
Marquette  district,  and  in  the  following  year  published  a  record  of  their 
observations  in  a  series  of  articles.  Parts  III  and  IV  of  the  series  and  a 
portion  of  Part  II  are  devoted  exclusively  to  descriptions  of  the  mining 


94  THE   MAKQUETTE   IKONBEAEIXG   DISTEICT. 

operations  of   tlie  district.     Part   I    and    the    remainder  of  Part  II    deal 
largely  with  a  theory  to  account  for  the  deposition  of  the  ore  Ijodies. 

The  general  geology  of  the  district  is  described  as  in  previous  articles. 
The  Laureutian  rocks  are  apparently  regarded  as  metamorphosed  sediments, 
for  it  is  related  that  "the  beds  of  rock  constituting  the  system  are  usually 
tilted  at  high  angles,  the  whole  series  having  been  upturned  and  flexed, 
broken  and  displaced,  until  little  evidence  of  the  original  deposition  in 
horizontal  strata  remains."  The  Huronian  rocks  were  observed  to  be  tilted 
nearly  vertical.  "They  have  been  raised  into  folds  or  crumpled  into 
groups  of  irregular  flexure,  forming  a  series  of  irregular  s}-nclinal  troughs." 
The  diorites  and  diorite-schists  are  believed  to  be  the  prevailing  rocks 
of  the  series,  and  apparently  are  regarded  as  sedimentary  in  origin.  Con- 
glomerates containing  pebbles  of  jasper  were  found  grading  into  (piartzite. 
The  ore  bodies  lie  in  the  strata,  forming  minor  folds. 

The  most  interesting  portions  of  the  paper  are  those  dealing  with  the 
character  and  origin  of  the  ore  bodies.  The  Champion,  j\Iichigamme,  Lake 
Superior,  New  York,  Cleveland,  and  Jackson  mines  are  described  liriefly, 
and  some  points  in  their  geology  are  touched  upon.  At  the  Michigamme 
mine  the  chlorite  pseudomorphs  of  garnet  were  found  by  Messrs.  Dawes  and 
Oothout  (pp.  46-47),  but  their  nature  was  regarded  as  doubtful,  since  most 
of  them  were  thought  to  be  monoclinic  in  crystallization.  This,,  of  course, 
is  an  effect  of  the  distortion  to  which  the  crystals  have  been  subjected. 
Messrs.  Crocker  and  Porter  examined  the  Jackson  mine  (pp.  107-1 09). 
Here  they  found  great  complications  in  the  stratigraphy.  The  "ore  masses 
were  deposited  in  the  jasper  by  some  action  of  water,  and  over  and  between 
them  layers  of  'soap  rock'  were  deposited." 

The  description  of  the  occurrence  of  the  ore  bodies  in  the  Champion 
mine,  by  C.  Q.  Payne,  gave  Prof  H.  S.  Munroe  an  opportunity  to  suggest 
a  theory  for  the  deposition  of  the  ore.  Payne  states  that  the  Champion 
mine  deposit  lies  in  one  of  the  minor  folds  of  the  iron-bearing  rocks.  It 
consists  of  a  number  of  overlapping  lenses,  with  the  west  end  of  one  lens 
usually  lying  to  the  north  side  of  the  next  western  lens,  and  overlapping 
the  underlying  lens  on  the  hanging- wall  side  (see  fig.  2).  "The  foot  wall 
of  the  ore  bed  is  dioryte  and  the  hanging  wall  quartzyte.     Chloritic  schist 


GEOLOGICAL   EXPLORATIONS   AND    LITEKATUKE— 1882.  95 

occurs  in  large  quantity  between  the  lenses  and  forms  in  nearly  every 
case  the  rock-bedding-  under  the  diflfereut  lenses.  *  *  *  Jasper  occurs 
mostly  on  the  foot-wall  side  of  the  lenses  and  back  of  the  chlorite-schist." 
The  author  then,  assuming  that  the  ore  was  accumulated  by  the  transport- 
mg  and  oxidizing  action  of  water,  proceeds  to  account  for  its  deposition  by 
supposing  the  original  drainage  of  the  area  to  have  had  an  east-west 
course  through  a  numljer  of  small  lakes.  In  tliese  hikes  precipitation  of 
limonite  may  have  taken  place,  which,  upon  metamorphism,  was  trans- 
formed into  hematite  and  magnetite.  The  different  lenses  of  ore  may 
represent  the  accunuilations  in  different  lake  basins,  or  portions  of  the 
accumulation  in  a  single  basin,  in  which  the  process  of  deposition  \\as 
interrujited  at  intervals.  On  this  theory  the  overlapping  of  the  lenses  is 
explained  by  slight  shifting  of  the  localities  of  deposition,  brought  aljout 
by  the  same  causes  as  produced  the  suspension  of  deposition.  The  secjuenee 
of  the  deposits,  viz,  ore,  quartzite,  and  slate,  is  thought  to  l)e  due  t(j  a  sep- 


Fia.  2. — Horizontal  section  of  ore  bodies  at  the  surface  of  the  Champion  mine. 

aration  of  the   material   in    the    lakes    through   the    mechanical    action   of 
wind-disturbed  water. 

Professor  Munroi^,  iu  referring  to  this  tlieorv  of  Payne's,  calls  attention 
to  the  fact  that  the  ^vater  of  a  lake,  disturbed  hj  the  action  of  the  wind, 
would  not  separate  a  mixture  of  substances  into  its  component  portions. 

The  deposit  formed  iu  tbe  bottom  of  a  lake  would  be  uo  richer,  ou  the  average, 
than  tbe  sediment  flowiug  iu.  There  might  be  local  patches  of  couceutration,  due  to 
wave  actiou  ou  the  shores  of  the  lake,  or  to  the  deposition  of  heavy  sediment  at  the 
mouths  of  small  streams,  but  the  deposit  would  be  like  the  immense  beds  of  "mixed 
ore  "  found  in  the  iron  regions.     *     *     # 

Munroe  believes,  however,  that  the  mechanical  action  of  water  has  in 
some  cases  played  an  important  part  in  the  concentration  and  ])urificati()ii 
of  the  rich  ores,  but  he  believes  that  the  water  was  in  the  form  of  running 
streams.  The  heavy  and  purest  ores  were  concentrated  above  the  lighter 
and  less  jjure   ones,    which  were  carried  farther  downstream,   while  the 


96  THE    MARQUETTE    IRON-BEAliING   DISTRICT. 

lig-litest  particles  were  washed  away  from  the  neighborhood  of  the  concen- 
trates. Thus  the  ores  upstream  grade  into  less  pure  ores  farther  down- 
stream, and  these  into  beds  containing  no  ore.  The  section  illustrated  in 
fig.  3  was  supposed  to  lend  aid  to  this  view. 

The  four  lenses  shown  in  the  section,  with  the  intercalated  seams  of  chloritic 
schist,  suggest  in  their  arrangement  the  false  bedding  or  cross  bedding  sometimes 
found  in  sandstone  strata.  This  false  bedding  is  due  to  the  action  of  running  water 
depositing  sediment  in  successive  layers  on  a  sloping  bank.  *  *  *  Por  example, 
during  and  after  a  freshet  any  stream  heavily  charged  with  sediment  will  deposit 
beds  of  sand  having  this  structure  in  the  pools  and  wider  jwrtions  of  its  bed. 

According  to  this  theory,  magnetitic  or  limonitic  sand  would  be  deposited 
even  where  the  current  was  swift,  while  the  more  common  sand  and  clay 
Avould  Ije  carried  onward.     After  tlie  freshet,  when  the  A'elocit^'  of  the  stream 


Direction  of  old  Cur  re  ntz^ 


^Siliceous  Ore   SOOFt. 


Fio.  3.— Cross-section  through  ore  Ijudies  at  the  Eibvarcls  miuc 

decreased,  the  lighter  substance  would  be  deposited  ujion  the  iron  sand,  and 
this,  when  metamorphosed,  would  give  rise  to  various  schists.  Dux'ing  sub- 
sequent freshets  most  of  the  lighter  material  would  be  washed  away,  leaving 
the  ore  bodies  covered  with  a  thin  film  of  mud  or  sand.  "The  lighter  'tails' 
of  the  de})0sit  being  swei)t  away,  there  would  lie  left  a  place  for  a  new 
deposit  of  ore  beyond  and  overlapping  the  older  one."  The  distribution  of 
the  ore  bodies  of  the  Edwards  mine  is  thought  to  correspond  in  all  respects 
to  the  distribution  expected  of  deposits  thus  formed;  "the  pure  ore  is  in 
each  case  at  the  head  and  the  mixed  ore  at  the  tail  of  the  natural  puddle,  as 
indicated  in  the  sketch."  When  the  ore  lenses  consist  partly  of  magnetite 
and  partly  of  specular  ore,  the  former  is  found  at  the  head,  the  latter  in  the 
middle,  and  mixed  ore  at  the  tail  of  each  lens.  This  theory  is  intended  to 
explain  more  particularly  the  manner  of  concentration  of  the  hard  ore  on 


GEOLOGICAL  EXPLOEATIONS   AND   LITEKATURE— 1883.  97 

tlie  Marquette  range,  tliough  nowhere  so  stated,  since  nearly  all  of  the 
observations  upon  which  it  is  based  were  made  in  the  hard-ore  mines.  It 
is  interesting  in  that  it  recognizes  the  sedimentary  origin  of  these  ores  and 
ascribes  their  concentration  to  the  action  of  moving  water,  an  agency  whose 
geological  importance  in  this  district  had  hitherto  been  largely  overlooked. 
The  large  regular  beds  of  ore  formed  in  the  district  are  thought  to  have 
been  deposited  originally  as  beds  of  bog  ore. 

Claassen,  Edo.  MiDeralogical  notes.  Am.  Jour.  Sci.  (3),  Vol.  XXIII,  1882, 
page.  G7. 

In  this  year  also  Edo  Claassen  jjublished  a  note  giving  the  record  of 
an  analysis  of  orthoclase  crystals  found  implanted  on  hematite  in  one 
of  the  Cleveland  mines.  He  also  describes  polyhedi-al  cavities  in  several 
micaceous  hematites,  supposed  to  have  been  produced  by  the  removal  of 
crystals  of  pyrite  from  the  midst  of  the  ore. 

1883. 

'  Irving,  R.  D.  The  copper-bearing  rocks  of  Lake  Superior.  Tliirrt  Ann.  Rept. 
U.  S.  Geol.  Survey  for  1881-82,  Washingtou,  1883,  pages  89-188.  Witli  geological  map 
of  Lake  Superior  region. 

The  year  1S83  marks  the  entrance  of  the  geologists  of  the  newly 
organized  United  States  Geological  Survey  into  the  discussion  of  the 
Marquette  problems.  Prof  R.  D.  Irving  was  appohited  to  take  charge 
of  the  work  in  the  Lake  Superior  region,  and  from  the  date  of  his  first 
report  under  the  auspices  of  the  Survey  until  his  death  in  1888  he  devoted 
himself  energetically  to  the  solution  of  the  problems  concerning  the 
correlation  of  the  Lake  Superior  formations  and  those  connected  with  the 
geology  of  the  iron-bearing  rocks,  more  particularly  those  relating  to 
the  Penokee  and  to  the  Marquette  series.  It  is  safe  to  say  that,  consider- 
ing the  short  time  in  which  he  worked,  no  one  has  added  more  to  the 
clear  understanding  of  the  relations  existing  between  the  various  rock 
series  in  the  Marquette  district  than  has  Professor  Irving. 

In  a  preliminary  account  of  the  Keweenawan  series  Irving  discusses 
the  relations  of  this  series  to  the  older  rock  series  associated  with  it.     In 

MON   XXVIII 7 


98  THE   MAKQUETTE   IKON-BEARING    DISTRICT. 

this  connection  he  describes  the  Marquette  Huronian  schists  as  occurring  in 
intricately  folded  troughs.  He  points  out  that  the  principal  difference 
between  the  Huronian  in  this  region,  as  described  by  Brooks,  and  that  on 
the  north  shore  of  Lake  Superior,  consists  in  the  presence  in  the  Marquette 
district  of  diorites  and  other  greenstones,  syenites,  granite,  gneisses,  sericite- 
schists,  chlorite-schists,  and  talc-schists,  jasper  and  chert-schists,  augite- 
schists,  and  amphibolites.  Otherwise  the  rocks  in  the  two  areas  are  alike. 
The  diorites  of  the  Marquette  district  are  uralitic  diabases.  Brooks's 
syenite  is  an  altered  diabase  rich  in  orthoclase.  The  chlorite-schists  fall 
into  two  classes,  in  one  of  which  the  rocks  are  altered  greenstones,  while 
in  the  other  they  are  related  to  hornblendic  and  micaceous  schists.  The 
gneisses  and  granites  rise  from  beneath  the  schists  that  are  associated  with 
the  iron-bearing  rocks.  Other  schists  which  are  interbedded  with  the 
gneisses  must  be  i-egarded  as  belonging  with  them.  The  Keweenawan,  or 
copper-bearing  series,  is  distinctly  younger  than  the  Huronian. 

On  the  map  accompanying  the  report  the  Marquette  Huronian  occu- 
pies approximately  the  same  area  as  it  does  on  Brooks's  map,  except  in  the 
area  southeast  of  Michigamme  Lake,  where  the  gneisses  and  granites  are 
colored  for  the  Archean  (pp.  166-173). 

Irving,  E.  D.  The  copper-beariug  rocks  of  Lake  Superior.  Mon.  U.  S.  Geol. 
Survey,  Vol.  V,  Washington,  1883,  xvi  and  464  pages.  With  maps.  The  Marquette 
and  Menominee  regions,  pages  393-409. 

The  same  year  that  saw  the  preliminary  report  on  the  Keweenawan 
series  saw  also  the  full  report  of  Avhich  the  preceding  paper  is  an  abstract. 
About  the  on\j  additional  idea  concerning  the  Huronian  rocks,  embraced 
in  the  detailed  report,  is  imparted  in  one  of  the  conclusions,  which  is  as 
follows: 

The  Huronian  sediments  are  metamorphic,  whatever  the  nature  of  the  meta- 
morphosing process  may  have  been — and  the  metamorphisni  has  always  been  greatest 
where  tlie  folding  has  been  greatest — while  the  Keweenawan  sediments  are  unaltered. 
The  metamorphism  and  folding  may  have  taken  place  before  or  during  the  period  of 
Keweenawan  eruptions  and  depositions,  or  both.  Our  present  knowledge  of  the 
Huronian  is  too  incomplete  to  allow  of  a  very  firm  opinion  as  to  this  point.    (P.  409.) 


GEOLOGICxiL   EXPLORATIONS   AND   LITERATURE— 1884. 


Whitney,  J.  D.,  and  Wauswouth,  M.  E.  TLe  Azoic  system  and  its  proposed 
subdivisions.  Bull.  Mas.  Comp.  Zool.  Harvard  Coll.,  Geol.  Ser.,  Vol.  I,  1884,  pages 
xvi  and  331-505. 

In  this  article  Whitney  and  Wadsworth  review  all  the  literatnre  relat- 
ing to  Foster  and  Whitney's  Azoic  system,  and  conclnde  that  no  basis 
exists  for  its  subdivision.  The  "diorites"  of  the  Marquette  district  are 
again  asserted  to  be  eruptive,  as  are  also  the  iron  ores  and  jaspilites  of  the 
area.  "The  only  evidence  that  the  Wisconsin  geologists  have  that  the 
Laureutian  and  Huronian  are  what  they  pui-port  to  be  is  lithological,  and 
they  have  advanced  no  sound  argument  showing  that  they  form  distinct 
ages  in  the  Azoic  system"  (p.  497). 

JuLiEN,  A.  A.  Genesis  of  the  crystalline  irou  ores.  Eng.  and  Min.  Jour., 
Vol.  XXVII,  February  2, 1884,  pages  81-83. 

The  author  maintains  the  sedimentaiy  origin  of  the  jasper  ores  of  the 
Marquette  region.  He  advances  the  view  that  they  were  originally  frag- 
mental  rather  than  chemical  sediments.  The  ores  are  supposed  to  have 
been  washed  as  fragments  from  preexisting  rocks,  and  to  have  become 
mixed  with  other  similarly  derived  detritus.  Their  present  condition  is 
thought  to  be  due  to  metamorphism. 

Smock,  John  C.  Geological  distribution  of  the  irou  ores  of  the  eastern 
United  States.  Eng.  and  Min.  Jour.,  VoL  XXXVII,  January-Juue,  1884,  pages 
217-218  and  230-232. 

Smock,  in  his  geological  classification  of  the  importnnt  iron-ore  deposits 
of  the  country,  places  the  magnetite  and  hematite  of  the  Manjuette  district 
in  the  "Huronian  period." 

Wadsworth,  M.  E.  Lithological  studies.  Menjoirs  Mus.  Com]).  Zool.  Harvard 
Coll.,  Vol.  XI,  Part  I,  Cambridge,  1884,  pages  136-139,  7  plates. 

In  his  classification  of  the  basic  terrestrial  and  meteoric  rocks  the 
author  describes  thin  sections  of  the  Presque  Isle  and  Ishpeming  serpentines. 


100  THE    MARQUETTE   IRON-BE AKING   DISTRICT. 

He  declares  that  in  both  locaHties  the  rock  was  originally  a  Iherzolite,  as  he 
had  already  some  time  earlier  declared  the  Presque  Isle  rock  to  be.  The 
limestones  associated  with  the  serpentine  are  also  believed  to  be  altered 
Iherzolites.  The  peridotite  is  still  believed  to  be  intrusive  in  the  overlying 
sandstones,  and  for  the  same  reasons  previously  given.  Fourteen  thin 
sections  of  rocks  were  examined.  Their  descriptions  leave  no  further  doubt 
as  to  the  correctness  of  the  author's  view  concerning  the  origin  of  the  ser- 
pentine. A  colored  plate  containing  three  lithographic  reproductions  of 
the  microscopic  appearance  of  thin  sections  of  Iherzolite,  serpentine,  and 
dolomite  illustrates  the  descriptions. 

luviNG,  R.  D.,  iiud  Van  Hisb,  G.  R.  Ou  secondary  eiilargenients  of  mineral 
fragments  iu  certain  rocks.  Bull.  U.  S.  Geol.  Survey  No.  8,  Washington,  1SS4.  "With 
plates  of  thin  sections. 

Irving  and  Van  Hise  announce  some  of  the  results  of  their  study  of 
thin  sections  of  Potsdam  and  St.  Peter's  sandstones — results  which  had 
alreadA'  been  announced  in  a  brief  form  by  Irving  in  an  article  published 
in  June,  1883,  in  the  American  Journal  of  Science — and  add  a  number  of 
new  facts  concerning  these  and  similar  rocks  of  other  horizons.  The  iiuthors 
show  conclusively  that  many  t)f  the  quartzites,  quartz-schists,  graywackes, 
etc.,  of  the  Lake  Superior  and  other  regions  are  sandstones  whose  inter- 
stices have  been  filled  with  secondary  quartz,  largely  by  deposition  of  this 
material  around  the  fragmental  quartz  grains  iu  optical  continuity  with 
their  suljstanee.  •  This  conclusion  is  important  in  tliat  it  indicates  that  a 
part  of  the  quartzites  ami  graywackes  that  make  u\)  a  considerable  pro- 
portion of  the  Marquette  iron-bearing  series  are  not  extremely  metamor- 
phosed rocks,  but  are  simply  sandstones  hardened  by  quartz  infiltration. 
]\Iuch  of  the  mica-schist  of  Brooks's  lower  quartzite,  noi'th  of  the  Michigamme 
mine,  and  of  Formation  XXI  in  the  Penokee  region  shows  enlarged  quartz 
grains,  and  the  jaspers  and  cherts  so  common  in  the  iron  formation  of  the 
Marquette  Huronian  sometimes  contain  enlarged  fragmental  grains  of  quartz 
in  a  matrix  of  chalcedonic  silica.  This  latter  fact  is  in  opposition  to  the 
view  of  an  eruptive  origin  for  these  rocks,  and  in  favor  of  that  which 
ascribes  to  them  a  sedimentary  origin.  A  large  number  of  thin  sections 
are  described  in  detail.     They  all  confirm  the  conclusions  outlined  above. 


GEOLOGICAL   EXI'LOKATIONS   AND   LITERATURE— LS85.         101 


Irving,  R.  D.  Preliminary  paper  on  an  investigation  of  the  Arclieau  forma- 
tions of  tbe  nortliwestern  States.  Fifth  Ann.  Rept.  U.  S.  Geol.  Survey,  for  1883-84, 
Washington,  1885,  pages  175-242.     Witli  maps. 

Ill  this  paper  we  liiul  for  the  tirst  time  a  dehnite  statement  of  the 
problems  to  be  attacked  in  working  out  the  structure  of  the  Lake  Superior 
region,  and  a  report  of  the  author's  success  in  solving  them. 

The  first  problem  which  he  attacks  is  the  stratigraphic  relation  of  the 
bedded  rocks.  "In  the  purely  granitic  areas  this  question  will  not  present 
itself,  and  it  is  very  doubtful  whether  anything  in  the  way  of  a  determination 
of  a  succession  of  layers  can  be  accomplished  in  the  regions  where  the 
rock  is  mainly  gneissic."  In  some  of  the  Huronian  areas  the  succession  of 
layers  can  easily  be  worked  out,  but  in  the  Manjuette  area,  "besides  the 
obstacle  of  frequent  lieavy  drift  covering,  an  additional  and  more  serious 
difficulty  is  met  with  in  the  complex  folding  to  which  the  rock  layers  have 
been  sul^jected  " 

The  second  problem  Avliich  presents  itself  for  solution  is  the  structural 
relations  of  the  i)laiuly  se<limentary  rocks  to  others  that  are,  or  may  l>e,  of 
eruptive  origin,  such,  as  the  greenstones  of  the  district  ))rovisiona]ly  called 
Huronian. 

The  third  problem  is  the  origin  of  the  gneisses  and  granites,  of  the 
schists  associated  with  these  rocks,  and  of  the  iron  ores  and  jaspers  among 
the  Huronian  rocks. 

The  fourth  problem  is  one  of  correlation  between  the  rocks  of  different 
portions  of  the  lake  region.     This  does  not  concern  lis  at  present. 

With  respect  to  the  Marquette  series,  the  author  writes  (pp.  189-11)0): 

The  roclvs  of  this  series  are  highly  fokled,  and  their  structure  is  often  very 
difficult  to  work  out.  Moreover,  the  metasomatic  changes  which  the  crystalline 
members  of  the  series  have  undergone  have  often  been  extreme;  added  to  which 
difflculties  are  frequent  interruptions  by  drift  covering. 

Although  studied  more  minutely  and  by  more  different  authorities  than  any 
other  portions  of  the  region,  the  divergencies  of  view  as  to  structure  and  genesis  in 
this  region,  even  among  the  later  writers,  have  been  very  great.     •     *     * 

After  having  obtained  a  thorough  acquaintance  with  the  rocks  of  this  region 
and  those  of  the  type  region  of  Lake  Huron,  no  doubt  remains  in  my  mind  as  to  the 


102  THE   MAEQUETTE   IRON-BEAKING   DISTRICT. 

correctness  of  the  iiositioii  of  those  who  have  heretofore  regarded  the  two  series  as 
equivalent.  I  came  away  from  the  Marquette  region,  indeed,  with  a  good  deal  of 
'doubt  as  to  whether  some  of  the  greenish  schists  included  by  Brooks,  Rorainger, 
and  others  within  the  lower  portion  of  the  iron-bearing  series,  might  not  really  belong 
with  the  older  gneissic  formation.  Excluding  these  schists,  the  remainder  of  the 
series  has  a  distinctly  Huronian  aspect.  Like  the  latter,  it  may  be  described  as,  in 
the  main,  a  fragmental  slate  and  quartzite  series,  including  a  large  proportion  of  basic 
eruptives.  *  *  *  Besides  the  quartzites,  magnetitic  scliists,  chert-schists,  iron  ores, 
limestones,  dolomites,  clay-slates,  mica-slates,  and  greenstones,  which  make  up  the 
bulk  of  the  Marquette  series,  a  number  of  other  less  abundant  kinds  have  been 
described  as  occurring.  It  is  among  these  kinds  that  the  lithological  difi'erences 
between  the  rocks  of  this  region  and  those  of  the  typical  Huronian  area  of  Lake 
Huron  are  found.     *     *     * 

A  considerable  proportion  of  these  unusual  kinds  are  plainly  but  the  results  of 
metasomatic  changes  upon  the  ordinary  basic  eruptives  of  the  series,  and  in  the  list 
of  rocks  so  produced  come,  I  think,  in  all  probability,  a  number  of  tlie  so-called 
hornblende-schists  and  actinolite-schists. 

These  schists  had  been  regarded  as  sedimentary  by  many  observers 
because  of  .their  schistose  structure,  but  to  Irving  they  seem  unquestion- 
ably eru2)tive. 

The  question  as  to  the  succession  of  beds  in  the  Marquette  disti'ict 
is  left  unsolved.  Neither  Brooks's  nor  Romingei-'s  scheme  is  accepted,  nor 
is  any  other  one  adopted.  The  greenstone  layers,  XI,  IX,  VII,  and  those 
below  V  of  Brooks's  divisions,  including  hornblendic  and  chloritic  schists, 
are  all  believed  to  be  eruptive,  some  contemporaneous  with  the  sedimentary 
layers,  and  others  intrusive  in  and  between  them. 

With  respect  to  the  origin  of  the  jasper  ores,  tlie  author  writes  (pp. 
192-193): 

My  studies  in  this  connection  are  as  yet  incomplete,  and  I  feel  unwilling,  therefore, 
to  advance  any  general  theory.  But  several  points  with  regard  to  these  ores  which 
have  impressed  themselves  upon  our  attention  may  be  mentioned.  In  the  first  place, 
I  may  say  that  I  am  quite  unable  to  accept  any  of  the  jaspery  ores  as  of  eruptive 
origin.  From  the  most  highly  contorted  and  confused  forms  we  have  every  gradation 
to  forms  in  which  the  sedimentary  lamination  seems  so  distinct  as  to  render  irresist- 
ible the  conclusion  that  all  are  of  one  origin.  *  *  *  Jaspery  and  quartzitic  ores, 
which  must,  I  think,  be  admitted  by  all  to  have  had  the  same  genesis  with  those  of 
the  Marquette  region,  occur  in  the  Penokee-Gogebic  belt,  and  again  in  the  Animikie 
formation  of  the  National  boundary,  in  a  relatively  undisturbed  position  and  under 


GEOLOGICAL   EXPLOEATIONS   AND   LITERATURE— 1885.        103 

such  circumstances  that  their  original  sedimentary  deposition  seems  to  be  placed 
beyond  doubt.  What  has  been  the  origin  of  the  iron  oxide  of  any  of  these  ores, 
whether  fragmental  or  chemical,  or  both,  I  do  not  undertake  now  to  discuss,  but  that 
much  of  the  quartzitic  material  mingled  with  them,  particularly  in  the  Penokee  belt, 
has  had  the  same  fragmental  origin  with  the  associated  quartzites  I  have  convinced 
myself  from  study  in  the  field  and  from  study  of  the  thin  sections.  Besides  this 
fragmental  siliceous  material,  however,  and  occurring  frequently  intermingled  with 
it,  and  again  at  times  almost  or  entirely  excluding  it,  is  a  chalcedonic  or  amorphous 
silica.  Much  of  the  jasper  of  the  Marquette  ores  seems  to  be  made  up  of  purely 
crystalline  quartz,  but  much  of  it  also  is  chalcedonic  or  amorphous.  *  *  *  So  far 
as  our  study  has  extended  it  has  seemed  evident  to  us  that  this  chalcedonic  silica  is 
of  original  formation,  or  at  least  that  it  existed  in  its  present  condition  pinor  to  the 
formation  of  much  of  the  series. 

Reference  is  made  to  the  fact  of  the  existence  of  fragments  of  ore  and 
jasper  in  the  qnartzite-conglomerate  overlying  the  ore  formation,  which,  it 
is  admitted,  proves  that  the  ores  are  older  than  the  quartzite,  and  that  they 
wei'e  in  their  present  condition  prior  to  the  deposition  of  the  quartzite,  but 
this  is  not  regarded  as  proof  that  the  ores  and  associated  jaspers  are 
eruptive. 

At  the  time  the  paper  was  published -work  was  being  done  on  the 
schistose  and  gneissic  areas  of  the  district,  but  it  had  not  progressed 
sufficiently  to  allow  of  a  statement  regarding  the  relations  of  the  rocks  of 
these  areas  to  those  of  the  Marquette  sediments.  The  article  concludes 
with  petrographical  descriptions  of  the  different  rocks  met  with  in  the  Lake 
Superior  region,  among  which  are  many  from  the  Marquette  area.  The 
serpentine  of  Presque  Isle  is  found  to  be  an  altered  peridotite,  as  Wadsworth 
had  shown.  Wadsworth,  however,  believed  it  younger  than  the  overlying 
sandstones,  while  Irving  regarded  it  as  older.  The  Ishpeming  seiijentines 
were  found  to  b ,  similar  to  those  of  Presque  Isle. 

The  examination  of  thin  sections  of  the  Huronian  rocks  showed 
plainly  that — 

the  rocks  which  form  the  bulk  of  the  Huronian  in  all  areas  do  not  properly  fall  under 
the  head  of  metamorphic  rocks.  Of  the  remaining  rocks  met  with  in  these  areas,  the 
various  augitic  and  hornblendic  geeenstones,  peridotites,  and  felsitic  porphyries  I  now 
look  upon  as  in  all  probability  of  eruptive  origin.  There  remain  to  be  accounted 
for  the  various  hornblende-schists,  chlorite-schists,  mica-schists,  hydromicaschists, 


104  THE    MARQUETTE   TROX-BEARING   DISTRICT. 

jaspery  anrl  chert  rocks,  and  limestones.  As  stated  above,  the  cbloritic  and  horn- 
blendic  schists  I  regard  as  in  part  the  products  of  the  alteration  of  basic  eruptive.--, 
aud  the  hydroraica-schists  of  acid  eruptives.  The  chert  and  jasper  rocks  I  am 
inclined  to  look  upon  as  of  some  sort  of  original  chemical  origin ;  certainly  they  are 
not  the  results  of  a  metamorphism  of  sedimentary  material.  The  limestones  do  not, 
as  far  as  I  know  them,  appear  in  any  essential  respect  difl'erent  from  many  met  with  in 
the  unaltered  formations  of  later  date.  Inhere  remain  the  mica-schists  and  slates,  and 
some  of  the  hydromica-schists  and  chlorite-schist.  That  these  latter  often  contain 
much  of  the  original  fragmental  material  we  have  satisfied  ourselves,  but  how  far 
those  of  their  constituents  which  are  plainly  of  original  crystallization  were  so 
crystallized  when  the  rocks  were  iu  the  state  of  mud,  or  have  been  produced  by 
purely  pseudomorphic  change  upon  fragmental  material,  or  how  far,  finally,  they 
may  be  the  result  of  a  genuine  recrystallization  or  metamorphism,  are  questions  for 
which  I  have  no  answer.  However  they  may  be  answered,  it  seems  to  me  that  it  will 
remain  true  that  the  various  formations  here  classed  as  Huronian,  including  the 
original  type  Huronian,  are  in  the  main  not  properly  strongly  metamorphic  formations, 
as,  for  instance,  the  older  gneisses  must  be,  if  of  sedimentary  origin.     (Pp.  241-242.) 

The  general  map  of  the  Lake  Superior  region  published  with  this 
report,  so  far  as  it  relates  to  the  Marquette  district,  does  not  differ  essen- 
tially from  that  accompanying  the  earlier  ])aper  on  the  copper-l^earing 
rocks. 

At  the  close  of  the  paper  is  an  abstract  of  the  bulletin  by  Irving 
and  Van  Hise  on  the  enlargement  of  quartz  and  other  grains  in  fragmental 
rocks. 

Irving,  R.  D.  Divisibility  of  the  Arcliean  in  the  Nortliwest.  Am.  Jour.  Sci.  (3), 
Vol.  XXIX,  1885,  pages  237-249. 

In  the  paper  just  quoted  Irving  referred  to  the  gneisses  and  schists 
as  older  than  the  Huronian  series.  In  another  paper  published  in  the  same 
year  he  gives  his  reasons  for  regarding  them  as  much  older  than  the  latter 
rocks.  In  the  Penokee-Gogebic  district  he  finds  six  reasons  for  concluding 
that  the  succession  of  formations  is  as  follows:  Gneiss-granite-greeu-schist 
formation;  great  unconformity ;  iron-bearing  slate  formation ;  unconformity; 
Keweenawan  series.  In  the  Marquette  district  the  iron-bearing  rocks  are 
in  a  highly  folded  condition,  and  there  is  thus  this  difference  between  the 
Marquette  and  the  Penokee  districts.     The  author  agrees  with  Rominger 


GEOLOGICAL   EXPLORATIONS   AND    LITERATUKE— 1885.         105 

in  believing  that  the  "dioritic  group"  of  this  author  is  the  basement  upon 
which  the  rest  of  the  series  was  spread.  The  members  of  this  "group,"  if 
sedimentary,  are  in  a  highly  metamorphosed  condition. 

Where  these  greenish  schi-sts  come  into  contact  with  the,  bounding  granite  the 
latter  penetrates  them  in  the  most  intricate  manner,  so  tliat  we  can  not  resist  the 
conclusion  that  it  is  the  more  recently  formed  rock.  From  this  unmistakable  relation, 
regarding  his  Dioritic  Group  as  the  lowest  member  of  the  slaty  or  iron-bearing  series, 
Dr.  Rominger  naturally  passes  to  the  conclusion  that  the  granites  are,  in  large  meas- 
ure, subsequent  to  his  entire  series.  *  *  *  To  me,  however,  it  seems  plain  that  in 
the  greenstone- schists  at  the  base  of  the  Marquette  iron-bearing  series  we  have  the 
equivalents  of  those  *  *  *  south  of  the  PenokeeGogebic  iron -bearing  series, 
like  which  they  form,  as  I  conceive,  part,  not  of  the  higher,  but  of  the  lower  forma- 
tion. *  *  *  The  slate  series  above  the  greenish  schists,  in  the  main  composed 
of  relatively  little  altered  rocks,  was  originally  built  up  upon  a  basement  composed  of 
granite,  gneiss,  and  these  greenish  schists  themselves,  and  subsetiuently  was  i)ushed 
into  trough-like  forms  by  lateral  pressure.     (Pp.  245-246.) 

The  proofs  given  in  support  of  this  view  are  the  same  as  those  advanced 
in  the  case  of  the  Penokee  district. 

The  penetration  of  the  greenish  schists  by  the  granites  where  the  two  come  into 
contact,  as  contrasted  with  the  entire  absence  of  any  such  relation  where  the  bounding 
granite  forms  contacts,  as  it  does  at  a  number  of  places,  with  the  slates  and  (piartzites 
above  the  greenish  schist  group;  the  occurrence  in  the  lowrr  series  of  only  highly 
altered  sediments,  gneiss  and  granite,  while  the  higher  rocks  are  relatively  little 
altered;  the  occurrence  in  the  higher  series  of  fragments  from  the  lower,  "recom- 
posed"  rocks,  occurring  at  points  where  the  quartzites  of  the  upper  series  come  into 
contact  with  the  gneisses  of  the  lower — all  of  these  arguments  hold  here  as  well  as  in 
the  Penokee  region.  Here,  then,  again  it  seems  to  me  plain  that  we  have  to  deal  with 
a  lower  or  greenish-schist,  gneiss-granite  member,  and  a  higher,  uncontorniably  overlying, 
slaty,  iron-bearing  member.     (P.  24G.) 

Irving  thus  argues  that  there  are  two  portions  of  the  Azoic  or  Archean 
series  in  the  Marquette  district,  and  that  the  two  portions  are  separated  by 
an  unconformity.  To  these  two  parts  he  gives  the  names  Laurentian  and 
Huroniau,  following  Logan's  example  for  the  region  north  of  Lake  Huron. 
The  names  are  not  new,  nor  is  the  idea  new  that  the  pre-Keweenawan 
rocks  of  the  Marquette  district  may  be  divided  into  two  series.  But  we 
have  here  for  the  first  time  sufficient  reasons  given  for  their  separation  into 


106  THE   MARQUETTE   IRON-BEAKING   DISTRICT. 

two  distinct  series,  and  for  the  first  time  we  find  the  green  schists  separated 
from  the  iron-bearing  rocks  and  placed  unconformably  beneath  them  in  the 
same  series  with  the  granite. 


Irving,  R.  D.  Oiigiu  of  the  ferruginous  schists  and  iron  ores  of  tlie  Lalve 
Superior  region.     Am.  Jour.  Sci.  (3),  Vol.  XXXII,  1836,  pages  255-272. 

Irving  continued  the  discussion  in  the  following  year,  when  he  |iublished 
an  article  devoted  exclusively  to  the  origin  of  the  iron  ores  of  the  Lake  Supe- 
rior region,  but  mainly  of  those  of  the  Penokee  district.  The  conclusions 
reached  in  this  paper  had  already  been  foreshadowed  in  the  bulletin  on  the 
Enlargement  of  Quartz  Grains  in  Quartzite,  etc.  In  its  introduction  the 
author  gives  the  status  of  the  problem  at  the  time  of  the  publication  of  his 
paper.  Two  theories  had  been  proposed  to  explain  the  origin  of  the  ores 
and  jaspers  of  the  district  in  question— an  eruptive  and  a  sedimentary 
theory.  Of  the  former,  advocated  by  Wadsworth  among  later  geologists,  the 
author  states  that  the  phenomena  cited  in  its  favor  are  with  one  exception— 
mainly  trivial  matters  occurring  within  tiie  space  of  a  few  inches,  or  feet,  at  most, 
and  *  *  *  all  are  more  easily  explicable  as  irregularities  in  original  deposition, 
as  irregularities  due  to  the  crumpled  condition  of  the  strata,  or,  and  this  chiefly,  as 
due  to  infiltrations  of  iron  oxide  and  silica  into  cracks  in  the  rocks,  and  the  replace- 
ment of  rock  material  by  such  substances — on  theories  of  original  sedimentation  of 
the  iron  beds  than  on  those  of  an  eruptive  origin.  *  *  *  The  occurrence  of  frag, 
mentsof  the  banded  jasper  in  the  immediately  overlying  quartzite  deserves  more  con- 
sideration, since  it  certainly  indicates  that,  to  some  extent  at  least,  these  substances 
had  reached  their  present  condition  at  an  early  day.  But  cooling  from  a  state  of 
fusion  is  not  the  only  way  of  reaching  rapidly  the  indurated  condition,  and  a  former 
fused  condition  seems  to  be  negatived  at  once  by  the  nature  of  the  material, — quartz 
and  iron  oxide.     (P.  256.) 

He  then  dismisses  the  eruptive  tlieory  as  improbable,  and  proceeds  to 
argue  in  favor  of  a  sedimentary  origin  for  the  ores  in  question,  first  stating 
briefl}'  the  nature  of  the  sedimentary  theories  already  proposed. 

Those  who  have  maintained  the  theories  of  a  sedimentary  origin  have  relied 
chiefly  upon  the  common  intimate  interlamination  of  siliceous  and  ferruginous  mate- 
rials; upon  the  manifest  restriction  of  the  ores  and  jaspery  schists  to  definite  strati- 
graphical  horizons;  upon  their  interfolding  with  other  members  of  the  same  series, 


GEOLOGICAL    EXPLORATIONS   AND   LITERATURE— 1886.         107 

and  ui>on  tlieir  apparent  gradation  in  places  into  plainly  fragmental  deposits.  These 
conditions  being  taken  to  indicate  original  sedimentation,  ditt'erent  antbors  have 
imagined  the  unaltered  deposits  to  have  been  argillaceous  carbonates  like  those 
of  the  coal  measures;  to  have  been  brown  ores,  like  those  found  under  bogs,  or 
accumulating  in  shallow  lakes,  at  the  present  day;  or  to  have  been  magnetic  sands 
like  those  of  modern  sea  shores.  All  of  these  theories  at)pear  to  regard  the  silica  of 
the.jaspery  schists  and  ores  as  having  been  sand;  its  present  nonareuaceous,  nonfrag- 
mental  condition  being  taken  to  be  the  result  of  metamorphism.     (P.  25r).) 

The  studies  of  the  author  were  confined  largely  to  the  Penokee- 
Gogebic  district,  where  the  rocks  are  less  disturbed  than  they  are  in  the 
Marquette  district,  Ijut  tlieir  conclusions  are  made  to  cover  also  the  ores  of 
tlie  latter  area. 

While  holding  the  sedimentary  origin  of  the  ores  and  jaspers,  it 
is  sh()\vn  that  the  close  association  of  these  rocks  with  noninetamor- 
phosed  quartzites,  graywackes,  etc.,  precludes  the  notion  of  a  metamorphic 
origin  for  the  former  rocks.  Moreover,  "all  theories  of  a  formation  of 
these  ferruginous  rocks  by  metamorphism  or  recrystallization  in  situ  from 
some  sort  of  sedimentary  deposit  seem  to  regard  the  jaspery  or  cherty 
material  as  representative  of  a  fragmental  siliceous  ingredient  in  the 
original  deposit.  On  these  theories  this  substance  has  been  recrystallized 
from  a  fragmental  material"  (p.  259).  But  the  microscope  shows  that  the 
jaspers  and  cherts  are  composed  largely  of  chalcedonic  silica,  like  that 
deposited  from  solution.  It  contains  intermingled  with  it  frag'mental  grains 
of  quartz  that  hS.ve  lost  none  of  their  original  angularity,  and  which  are 
easily  distinguished  from  the  chemically  precipitated  chalcedony.  Hence, 
the  metamorphic  theory  is  abandoned  and  a  chemical  theory  advocated  in 
its  place.  According  to  this  theory  the  original  sediments  were  ferruginous 
carbonates.  The  least  altered  of  the  ferruginous  schists  still  contain  car- 
bonaceous material,  and  often  little  rhombohedra  of  siderite,  and  the  amount 
of  this  carbonate  present  varies  inversely  with  the  amount  of  disturbance  and 
alteration  the  rocks  have  suffered.  In  the  Penokee  district  a  bed  of  hematite 
was  traced  directly  into  one  of  these  carbonate-bearing  schists.  In  this  and 
the  other  unfolded  Huronian  iron  districts  there  is  excellent  proof  that  the 
origin  of  the  ores  and  jaspers  was  as  indicated.  In  the  Marquette  district 
the  complication  of  the  structure  obscures  the  evidence  to  a  considerable 


108  THE   MARQUETTE    IRO]S -BEARING    DISTRICT. 

extent,  but  from  the  similarity  between  the  rock  associations  in  this  and  in 
the  less  folded  areas  the  author  has  no  doubt  that  the  ores  and  jaspers 
here  have  the  same  origin  as  in  the  Penokee  district. 

A  good  many  of  tlie  ore  bodies,  aud  more  particularly  some  of  the  so-called 
soft  hematites,  appear  to  have  residted,  partly  at  least,  from  a  direct  oxidation  of  the 
iron  carbonate  of  some  of  the  eherty  schists.  In  other  cases  the  ore  bodies  owe  their 
origin  and  general  shape,  we  think,  to  processes  of  infiltration  and  replacement. 
(P.  207.) 

Thus  after  the  carbonates  were  precipitated  they  were  subjected  to 
metasomatic  processes. 

The  conclusion  reached  by  Irving  as  the  result  of  his  extensive 
studies  may  be  summarized  as  follows: 

(1)  The  original  form  of  the  beds  of  the  iron-bearing  horizons  was 
that  ot  a  series  of  thinly  bedded  ferriferous  carbonates  interstratified  with 
carbonaceous  slaty  layers,  like  the  carbonate-bearing  beds  of  the  Coal 
Measures. 

(2)  By  a  process  ot  silicification  these  carbonaceous  beds  were  trans- 
formed into  the  various  ferruginous  rocks.  The  silicification  varied  in 
degree,  sometimes  producing  only  a  few  seams  of  silica,  which  traverse  the 
otherwise  unchanged  rock,  at  other  times  completely  substituting  the 
original  rock,  in  which  case  cherts  were  formed. 

(3)  The  iron  thus  removed  during  silicification  passed  into  solution 
and  was  redeposited  as  it  became  further  oxidized,  making  ore  bodies  in 
one  place  and  forming  the  coloring  matter  of  the  jaspers  in  other  places. 
The  hematite  interlaminated  with  the  jasper  is  taken  to  be  mainly  the  result 
of  a  secondary  infiltration  following  the  banding  of  the  original  rock,  though 
it  may  be  imagined  to  have  been  formed  at  times  by  direct  oxidation  from 
iron  carbonate  seams. 

(4)  Sometimes,  instead  of  leaching  it  out  completely,  the  silicifying 
waters  seem  to  have  decomposed  the  iron  carbonate  in  place,  producing 
most  of  the  actinolitic  magnetite-schists. 

(5)  The  rich  ore  bodies  have  probably  had  diff'erent  origins  in  diflPerent 
cases.  Some  of  the  red  hematites  seem  to  have  resulted  from  a  direct  oxida- 
tion in  place  of  the  original  carbonate,  since  in  some  of  them  pseudomorphs 


GEOLOGICAL   EXPLOIIATIONS  AND   LITEKATURE— 18SG.        109 

of  the  carbonates  may  be  detected.     Some  of  the  niagnetite  mines  apjiear 
to  be  working  on  the  richer  portions  of  the  magnetitic  schists. 

(6)  Some  of  the  siUcifying  proces.s  went  on  before  the  folding  of  the 
formation,  and  some  of  it  afterward;  and  to  the  later  period  belong  the 
larger  bodies  of  crystalline  ore,  the  crumbling  and  shattering  of  the  layers 
affording  the  best  conditions  for  the  action  of  the  silicifying  waters. 

J'UMPELLY,  Raphael.  Report  on  the  niiniiig  industiies  of  tbe  T'nited  States 
(exclusive  of  the  precious  metals),  with  special  investigations  into  the  irou  resources 
of  the  Republic,  etc.  Department  of  the  Interior,  Census  Office,  Vol.  XV,  Washing- 
ton, 188(J,  pages  1-82.     With  maps  and  plates. 

In  the  report  of  the  Tenth  Census  on  the  iron  ores,  Pumpelly  gives  a 
brief  survey  of  the  Marquette  district  and  records  a  nvimber  of  analyses  of 
its  ores.  Very  little  that  is  new  concerning  the  geology  of  the  district  is 
communicated,  as  the  work  is  largely  a  compilation.  The  ores  are  stated 
to  be  in  the  Huronian,  which  is  regarded  as  the  upper  member  of  the 
Archean,  unconformably  reposing  upon  the  schists  of  the  Laurentian. 

While  the  irou  series  occurs  uniformly  iu  the  lower  ]>art  of  the  Hurouiau,  its 
structure  and  character  vary  iu  differeut  places.  In  the  Negaunee  district  it  consists 
of  a  lower  and  an  upper  series.  The  lower  is  made  up  of  flag  ores,  .siliceous  and  fer- 
ruginous schists,  and  some  argillaceous  and  talcose  slates  and  anthophyllitic  schists 
and  beds  of  diabase.     *     *     * 

The  upper  series,  which  is  .separated  from  the  lower  by  a  bed  of  diabase  and  a 
thinner  bed  of  chloritic  and  talcose  slate,  contaius  the  rich  ores.  It  consists  of  a  thick 
mass  of  banded  irou  and  "Jasper,"  the  iron  ore  being  pure  and  the  jasper  generally 
colored  red.  *  *  *  In  places  subsequent  chemical  action  has  removed  portions  of 
the  jasper,  while  the  space  thus  formed  has  been  filled  with  limouite  in  large  quanti- 
ties, as  at  the  Lake  Superior  mine.  The  upper  iiart  of  this  upper  series  has  generally 
a  bed  of  talcose  slate,  in  which  the  fissile  cleavage  is  wholly  independent  of  the 
bedding,  and  which  is  impregnated  with  small  octahedra  of  martite.  The  ore  in 
contact  with  this  slate  has  the  same  structure  and  is  impregnated  with  similar  crys- 
tals of  martite.  The  upper  portion  of  the  upper  series  consists  of  beds  of  rich  ore, 
often  granular  martite,  with  talcose  schists  and  very  talcose  quartzites.     (Pp.  7-8.) 

The  author  notes  that  in  the  western  part  of  the  Marquette  basin  the 
ores  are  largely  magnetic.  The  basin  itself  narrows  and  then  widens  out 
to  form  "the  broad  Huronian  area  of  the  central  part  of  the  Upper  Penin- 
sula," where  the   slates  of  the   uppermost  horizons   are  the   predominant 


110  THE   MARQUETTE   IKON-BEAEING  DISTRICT. 

country  rocks.  From  the  south  end  of  the  wider  portion  of  Lake  Michi- 
gamme  a  loop  of  Huronian  rocks  extends  southeastward,  cuhninating-  at 
RepubHc  Mountain,  where  the  entire  lower  and  upper  series  are  beautifully 
represented. 

It  Is  needless  to  add  that  Pumpelly's  Upper  and  Lower  Huronian  are 
not  coordinate  with  the  Upper  and  Lower  Marquette  as  understood  in  the 
present  volume.  Some  of  his  Upper  Huronian  rocks  are  unquestionably 
members  of  the  Lower  Marquette.  Many  of  the  "beds  of  diabase"  are 
now  known  to  be  intrusive  masses. 

Putnam,  Bayard  T.  Notes  ou  the  samples  of  iron  ore  collected  in  Michigan 
and  northern  Wisconsin.    Ibid.,  pages  421-437.     With  geological  map. 

Li  the  same  volume  Putnam,  in  the  introduction  of  his  remarks  upon 
the  analvses  of  the  Marquette  ores,  declares  that  the  Marquette  strata  "form 
a  broad  synclinal  trough,  corrugated  in  the  direction  of  its  axis  by  several 
minor  folds,  resting  on  Laurentian  rocks."  He  repeats  some  of  Pumpelly's 
statements  and  gives  an  abstract  of  portions  of  Brooks's  report.  Samples  of 
ores  were  taken  from  all  the  important  mines  of  the  district  and  were 
analyzed.  In  addition  to  the  geological  map  accompanying  the  report, 
which,  by  the  way,  is  a  reproduction  of  the  Brooks  map,  there  are  sketch 
maps  of  the  New  York,  Lake  Angeline,  and  Salisbury  mines. 

1887. 

Irving,  R.  D.  Is  there  a  Huronian  group?  Am.  Jour.  Sci.  (3),  Vol.  XXXIV, 
1887,  pages  204-216,  249-263,  365-374.  Read  before  the  Nat.  Acad,  of  Sci.,  April 
22,  1887. 

In  this  paper  Irving  returns  to  the  discussion  begun  by  him  in  1885 
The  purpose  of  his  present  article  is — 

to  inquire  if  there  can  be  carved  off  of  the  upper  portion  of  the  great  complex  which 
has  been  called  Archeau,  a  series  of  Huronian  rocks,  a  series  entitled — by  structui-al 
and  genetic  separateness,  by  clastic  origin,  by  largeness  of  volume,  and  by  being  made 
up  of  subordinate  divisions  of  the  formation  rank — to  the  rank  of  a  group,  1.  e.,  to  a 
rank  equal  in  classificatory  value  to  the  Cambrian,  Silurian,  etc.     (P.  207.) 

After  deciding  that  the  series  of  rocks  on  the  north  shore  of  Lake 
Huron,  called  Huronian  by  Logan  and  Murray,  deserves  the  title  of  group, 


GEOLOGICAL   EXPLORATIONS   AND   LITEKATUKE— 1887.         HI 

Irving  proceeds  to  discuss  the  relations  to  one  another  of  the  other  supjjosed 
Huronian  areas  in  the  Lake  Superior  region.  With  respect  to  the  Marquette 
area,  he  refers  to  the  two  views  held  as  to  the  age  of  the  granites,  etc.,  on 
the  sides  of  the  Huronian  trough,  ex^jlaining  that  Kimball,  Brooks,  and 
others  regarded  them  as  unconformabl}^  beneath  the  bedded  rocks,  while 
Wadsworth  and  Rominger,  among  the  later  geologists,  regarded  them  as 
erujjtive  into  the  l)edded  series.  Irving  himself  finds  that  the  green  schists, 
which  on  account  of  their  banding  had  always  been  placed  with  the  frag- 
mental  beds  as  part  of  the  iron-l)earing  series,  are  cut  by  granite  dikes, 
whereas,  on  the  other  hand,  the  granites  and  their  associated  schists  are 
separated  from  other  members  of  the  stratiform  series  by  unconformities  and 
by  basal  conglomerates  containing  great  bowlders  of  the  underlying  granites, 
etc.  This  is  explained  by  making  a  division  of  the  rocks  of  the  district 
into  two  series,  an  older  one  comprising'  the  green  schists,  than  whicli  the 
granite  is  younger  and  into  which  it  is  intrusive,  and  a  younger  series  com- 
posed of  the  bedded  fragmentals  and  their  associated  rocks,  into  which  the 
granite  never  sends  dikes,  but  to  whose  lower  layers  it  has  yielded  pebbles 
and  bowlders  and  large  quantities  of  finer  detritus. 

The  upper  series  is  composed  mainly  of  detrital  rocks,  of  whose  frag- 
mental  nature  there  can  usually  be  no  doubt,  though  in  many  cases  the 
rocks  are  somewhat  sheared,  and  have  had  developed  in  them  secondary 
sericite.  With  the  detrital  rocks  are  bedded  limestones,  cherts,  etc.,  that 
are  believed  to  have  been  originally  cliemical  sediments,  and  interbedded 
diabasic  eruptives.  This  series  is  a  unit  among  the  formations  of  the  Lake 
Superior  region,  and  is  so  similar  to  the  originally  described  Huronian  in 
its  lithology  and  in  its  structural  I'elations  with  other  overlying  and  under- 
lying formations,  that  it  may  safely  be  correlated  with  this  and  be  called 
Huronian.  Investigations  in  the  other  supposed  Huronian  areas  lead  to 
the  same  conclusion,  viz,  that  the  Huronian  "is  a  true  sedimentar^s'  group 
in  origin,  in  volume,  in  chronological  distinctness  from  other  groups  above 
and  below  it.  It  is  not  only  comparable,  as  to  •s'olume,  with  the  ordinarily 
recognized  rock  groups,  it  exceeds  most  of  them;  besides  which  it  is  sepa- 
rated from  the  adjacent  rocks  by  tremendous  unconformities,  re2)resentative 
of  immense  lapses  of  time."     (Pp.  370-37L) 


112  THE   MAEQUETTE   IRON  BEAEING   DISTIUCT. 

The  Huroniau  and  the  Keweenawan  series  together  represent  a  gi'eat 
interval  between  Archean  and  Cambrian  times.  They  may  be  included  in 
one  group,  comprehending  the  fragmental  series  between  the  Archean 
crystallines  and  the  Cambrian  fragmentals.  For  this  group  the  author 
proposes  the  designation  Agnotozoic,  because  there  are  here  and  there  traces 
of  life  in  some  of  the  rocks  belonging  to  it,  but  the  nature  of  this  life  is 
unknown.  For  the  Marquette  area,  then,  as  well  as  for  the  remainder  of 
the  Lake  Superior  region,  he  gives  the  following  succession: 

(1)  The  great  Basement  Complex,  of  crystalline  schists,  gneiss,  and 
granite,  as  to  whose  further  divisibility  or  nondivisibility  no  opinion  is  now 
expressed.      Unconformity. 

(2)  The  Huronian  series,  mainly  of  detrital  rocks.     Unconformity. 

(3)  The  Keweenawan  series,  of  interleaved  detrital  and  eruptive  beds. 
Unconformity.     (Absent  from  the  Marquette  range  proper.) 

(4)  The  Potsdam  or  Upper  Cambrian  sandstone. 

1888. 

Irving,  R.  D.  On  the  classification  of  tbe  early  Cambrian  and  pre-Cauibrian 
formations.  A  brief  discussion  of  i>rinciples,  illustrated  by  examples  drawn  mainly 
from  tbe  Lake  Superior  region.  Seventli  Ann.  Rept.  V.  S.  Geol.  Survey,  for  1885-86, 
Washington,  1888,  pages  305-454. 

In  the  following  year  the  same  author  published  a  pa|)er  which  has 
already  become  one  of  the  classics  of  geological  literature.  In  it  he  enun- 
ciates and  discusses  the  principles  that  should  determine  the  classification 
of  nonfossiliferous  rock  series,  and  illustrates  their  application  by  appeal  to 
the  ijre-Cambrian  formations  in  the  Lake  Superior  region. 

After  explaining  in  full  the  significance  of  unconformities  and  basal 
cong-lbmerates  and  of  lithological  diiferences  in  establishing  time  relations 
between  contiguous  formations,  the  author  cites  examples  from  the  Marquette 
district,  among  others,  to  emphasize  his  points. 

Time  gaps  are  shown  to  have  existed  between  the  deposition  of  the 
lowermost  layers  of  the  Potsdam  sandstone  and  the  formation  of  the  under- 
lying granites,  and  between  the  production  of  the  former  rocks  and  of 
the  fragmental  beds  usually  classed  as  Huronian.     A  picture  of  the  uncon- 


GEOLOGICAL   EXPLOKATIONS   AND   LITEllATUllE— 1888.         113 

forniity  Ijetweeu  the  sandstone  and  the  granite  on  tlu-  Like  shore  near 
Marqnette  is  g-jven,  and  several  sections  are  published  uhieli  ilhistrate  a 
similar  unconformity  at  Granite  Point,  and  unconfornnties  between  the 
sandstone  and  the  Hvironian  beds  at  L'Anse,  and  on  tlie  lake  shore  south  of 
Marquette  (pp.  409-411).  Several  of  these  unconformities  luul  Ijeeu  known 
as  far  back  as  Foster  -and  Whitney's  time. 

Another  time  interval  is  shown  to  have  elapsed  between  the  formation 
of  the  granitic  rocks  and  the  deposition  of  the  oldest  Huronian  beds,  pro- 
vided the  green  schists  of  the  district  are  separated  from  the  Huronian  aiid 
placed  with  the  granitic  series,  as  had  been  advocated  by  the  author  in 
1887.  The  discordance  between  the  two  series  "may  he  i)roved  on  tlie 
ground  by  the  discordant  positions  of  the  schists  of  the  two  series,  when  in 
contact  or  near  proximity,  by  the  large  development  of  l)asal  conglomerates 
wdiere  the  two  formations  come  together,  Ijy  the  indifference  in  position  of 
the  belts  of  the  upper  series  to  those  of  the  lower,  l)y  tlie  striking  contrast 
in  amounts  of  alteration  of  the  upper  and  lower  schists,  and  by  the  totally 
dissimilar  relations  of  the  two  sets  of  schistose  rocks  to  the  plainly  eruptive 
granite  masses"  (p.  433).  Three  genei-alized  sections  across  the  Marquette 
district  and  a  reproduction  of  a  photographic  view  of  a  hillside  2  miles 
south  of  Marquette  illustrate  this  portion  of  the  paper  (pp.  431-435). 

The  Marquette  iron-bearing  series  is  described  as  a  bedded  accumulation 
of  carbonaceous  slates,  ferruginous  and  jaspery  schists,  limestone,  quartzite 
and  quartzite-schists,  graywacke  and  clay-slates,  and  eruptive  greenstones, 
amounting  in  all  to  from  5,000  to  10,000  feet  in  thickness.  The  seines  is 
separated  by  unconformities  from  the  gneissic  and  schistose  beds  lielow 
it,  and  from  the  Potsdam  sandstone  above  it,  and  so  possesses  a  distinct 
individuality,  a  fact  emphasize<l  in  the  paper  of  1887. 

Keyeu,  E.  Geologie  der  auierikauisobeu  I*]iseiilagerstiitteii  (iusbesondere 
Micliigaii).  OesteiT.  Zeitsclir.  fiir  Berg-  uud  Hiittenwesen,  Vol.  XXXV,  1887,  Xos. 
10  and  11.     Abstract  in  Xenes  Jabrbucb  fiir  :Miueral.,  1888,  Vol.  I,  i)ages  248-249. 

The  original  of  this  article  has  not  been  seen,  but  from  the  abstract  of 
it  given  by  Stelzner  in  the  Neues  Jahrbuch  we  learn  that  its  author  regards 
the  iron-bearing  series  of  ]\Iichig'au  as  an  association  of  sediments  and 
MON  xxviii S 


114  THE   MAKQUETTE   IKOX-BEARING  DISTRICT. 

eruptives,  the  latter  having  been  poured  out  on  the  bottom  of  the  ocean. 
The  sihceous  sediments  are  thought  in  part  to  have  been  jiroduced  through 
the  decomposition  of  the  eruptive  masses,  in  part  to  have  come  from  the 
surrounding  land  as  erosion  detritus,  and  in  part  to  have  been  formed  by 
the  accumulation  of  the  remains  of  organisms.  The  iron  ore  associated 
Avith  these  rocks  is  thought  to  have  been  deposited  from  springs  whose  iron 
constituent  was  derived  by  solution  from  the  eruptive  rocks.  The  chemical 
processes  concerned  in  the  formation  of  the  ore  l)eds  were  supposed  to  be 
the  following:  The  basic  eruptives  contained  iron  as  oxide  and  chloride. 
Its  chloride  was  dissolved  in  the  sea  water,  and  from  this  dilute  solution 
the  iron  was  precipitated  as  an  ocherous  deposit  of  the  hydroxide.  This 
was  subsequently  dehydrated  and  changed  into  hematite.  Stelzner,  in  the 
review  of  the  article,  declares  that  the  theory  rests  upon  tlie  assumption 
that  the  diorites  associated  with  the  ores  are  eruptive,  and  this  he  states 
had  not  yet  been  proved. 

Williams,  G.  H.  Some  examples  of  the  dynamic  metamorpliism  of  the  ancient 
eruptive  rocks  on  the  south  shore  of  Lake  Superior.  Proc.  Am.  Ass.  Adv.  Sci., 
thirty-sixth  meeting,  1888,  pages  225-226. 

In  this  preliminary  notice  of  the  results  of  microscopical  work  on 
the  Marquette  banded  green  schists  Williams  states  that  this  series  of  rocks 
owes  its  schistosity  to  pressure  and  not  to  original  bedding.  The  rocks  are 
believed  to  be  squeezed  and  stretched  eruptives.  The  reasons  for  these 
conclusions  and  the  descriptions  of  the  observations  on  which  they  are 
based  were  not  published  in  full  until  1891. 

BiBKiNBiNE,  John.  Iron  ore  mining  in  1887.  Mineral  Resources  of  the  United 
States,  calendar  year  1887,  Washington,  1888,  page  34. 

This  author,  in  connection  with  statistical  tables  showing  the  ore 
production  of  the  various  American  iron-ore  districts,  incidentally  refers  to 
the  deposits  of  the  Lake  Superior  mine  as  lying  in  "a  synclinal  fold 
trending  east  and  west,  the  western  extremity  outcropping."  The  entire 
deposit  is  said  to  be  underlain  by  chlorite-schist  and  to  he  overlain  by 
hematite-jasper. 


GEOLOGICAL   EXPLOITATIONS   AND    LITERATURE— 18SS.         115 

WiNCiiELL,  N.  H.  The  iron-bearing  rocks  at  Marquette,  Michigan.  GeoL 
and  Nat.  Hist.  Snrv.  of  Minnesota,  Sixteenth  Ann.  Rept.  for  1887,  St.  Paul,  1888, 
pages  iO-54. 

A.  Wiucliell  and  N.  H.  Wiuchell  made  a,  rapid  trip  tlirougli  the  Marqviette 
district  in  the  summer  of  1887,  and  pubUshed  records  of  their  observations 
independently.  At  Negaunee,  Ishpeming,  and  Marquette,  N.  H.  Winchell 
made  a  few  observations  that  are  of  some  interest.  In  the  Negaunee  area 
the  mines  east  of  the  town  were  the  special  objects  of  study.  Here  a  gra- 
dation is  reported  between  coarse  quartzites,  through  an  impure  hematite 
banded  with  chalcedonic  siHca,  into  an  earth}',  liematitic,  jasperoid  rock. 
"This  shows  a  common  origin  for  tliem  all  rather  than  a  chemical  or 
eruptive  source  for  the  jaspilyte  and  not  for  the  others  "  (p.  42).  At  the 
Iron  Cliffs  mine  siderite  was  seen  associated  witli  the  hematite.  The  most 
important  observations  were  made  in  the  Cascade  area,  where  there  is 
described  as  imconformably  overlying  the  iron  formation  a  quartzite-con- 
glomerate.  It  contains  pebbles  of  red  jasper,  chert,  and  hematite.  This  is 
the  conglomerate  so  frequently  described  by  the  earlier  geologists.  N.  H. 
Winchell  appears  to  have  been  the  first  to  have  noticed  that  it  lay  imcon- 
formaljly  upon  the  iron  formation.  It  is  regarded  at  Cascade  as  the  southern 
rim  of  the  syncline  whose  northern  rim  is  in  the  quartzite  bluffs  of  Teal 
Lake.  From  the  character  of  the  pebbles  found  in  this  conglomerate,  "it 
is  apparent,"  writes  the  author,  "that  the  iron-ore  formation  must  have  been 
constiiidcd  In  prdtij  itcadij  its  present  state  prior  to  the  formation  of  the 
conffomerate"  (p.  44).  This  conclusion,  it  will  lie  nt)ticed,  is  the  same  as 
that  arrived  at  Ijy  Wadsworth  and  by  Irving.  A  little  farther  west,  at  the 
Saginaw  mines,  in  the  Ishpeming  district,  the  same  unconformable  relations 
were  observed  l)etween  an  overlying  conglomerate  and  the  underlying  iron 
formation.  Two  figures  are  given  illustrating  these  unconformities,  in  the 
first  of  which  the  conglomerate  is  labeled  Potsdam  (p.  45).  It  is  evident 
that  the  author  did  not  at  this  time  perceive  the  full  meaning  of  the  facts. 

North  of  Ishpeming  the  conglomeratic  gi'eenstones  of  Deer  Lake  were 
examined.  They  are  described  as  soft,  quai'tzless,  schistose  rocks,  without 
any  bedding  structure  that  can  be  attributed  to  sedimentation.  They 
appear  to  be  uuconformably  beneath  the  quartzite.     These  conglomerates 


116  THE   MAEQUETTE   IRON  BEARING   DISTRICT. 

aud  the  serpentine  of"  tlie  district  are  regarded  "as  one  group,  the  serpentin- 
ous  condition  prevaiUng  whenever,  locally,  greater  alteration  has  taken  place 
in  the  original  rock,  which  was  an  eruptive,  basic  one.  It  overflowed  and 
mingled  with  the  rocks  of  the  iron-bearing  series  unconforniably,  and  where 
it  is  now  in  contact  with  them  it  constitutes,  at  least  in  some  places,  the 
'soapstone'  and  the  'chloritic  rock'  of  the  mines.  Subsequently  the  rocks 
of  the  Huronian  were  deposited  unconforniably  on  the  iron-bearing  and 
serpentine  groups"  (p.  48).  The  iron-bearing  beds  are  thus  placed  under 
the  Huronian,  with  the  unconformity  between  this  series  and  the  Archean 
above  the  iron  formation,  rather  than  some  distance  below  it,  where  Irving 
placed  it. 

The  geology  of  the  Michigamme  mine  is  also  described,  and,  north  of 
it,  an  unconformable  contact  between  granite  and  quartzite.  At  this  contact 
is  a  conglomerate,  but  the  author  is  not  sure  which  of  the  two  rocks  is  the 
overlying  one. 

At  Marquette  the  green  schists  north  of  the  town,  which  Irving  placed 
below  the  Huronian,  Avere  examined.  These  are  regarded  as  inseparable, 
genetically  and  geographically,  from  the  serpentine  group.  They  are 
believed  to  be  eruptive,  and  to  underlie  unconforniably  the  quartzite,  and 
thus  to  constitute  a  part  of  the  "  iron  formation." 

This  greeuisli  schist  *  *  *  is  heavy,  dark  within,  free  almost,  or  entirely,  from 
original  quartz.  On  its  weathered  surface  its  structure  is  indicated  by  bauds  of 
varying  shades  of  green,  and  iu  the  seams  it  glitters  with  hydro-mica.  These  bauds 
•  of  color  are  not  continuous,  but  consist  rather  of  an  interrupted,  uarrow  striping  in 
which  the  color  lines  become  lost  bjM'unuing  to  needle-shaped  points  or  by  fading  into 
each  other.  The  single  light-colored  lines  sometimes  continue  for  only  a  few  feet  or  a 
few  inches;  and  iu  some  cases,  when  narrow,  they  rise  and  disappear  in  the  space  of 
less  than  au  inch,  bringing  the  darker  Hues  into  contact  so  as  to  present  the  aspect 
of  a  nearly  homogeneous  green  rock.  *  *  *  Iu  other  i^laces  *  *  *  the  striping, 
■which  resembles  that  of  sedimentation,  is  more  evident  and  persistent,  and  iu  some 
parts  could  be  more  correctly  denominated  a  banding,  some  of  the  bauds  being  2  or  3 
inches  iu  width;  but  even  then  they  lose  their  identity  iu  10  or  12  feet  aud  give  place 
to  a  finer  schisto-flbrous  lining.     (Pp.  51-52.) 

A  plat  of  Light-House  Point  is  given.  Here  the  schist  is  made  out  to 
be  the  oldest  rock.     This  is  cut  by  quartz-porphyry  aud  other  dikes. 


(iEOLOGICAL    EXPLORATIONS   AXD    LITERATURE— 1888.         117 

WiNCHELL,  Alexander.     The  Mar([uette  iron  region.     IhhL,  pages  171-1S5. 

In  many  points  Alexander  Winchell  (liffer.s  with  his  Ijrotlier  as  ti>  the 
significance  of  the  facts  observed.      In  other  points  the  two  are  agreed. 

Among  the  points  brought  out  bj  Alexander  Winchell  are  the  inter- 
stratification  of  the  ore  beds  with  the  associated  rocks,  and  the  occurrence 
of  a  conglomerate  above  the  ore  beds  in  the  Lake  Superior  mine. 

The  downward  succession  of  beds  in  tlie  Ishpeming  syncHne  is  thought 
to  be  as  follows:  red  slate;  black  slate  and  mixed  ore;  ore;  talcose  rock; 
diorite.  The  red  slates  are  banded  hematite  and  jasper;  the  black  slates 
are  magnetic  jaspers;  and  the  talcose  rock  is  an  argillitic  variety,  as  is  also 
the  minei''s  "  soap-i'ock." 

The  Deer  Lake  conglomerates  are  regarded  as  sedimentary  because 
"  they  contain  foreign  pebbles." 

The  contact  of  the  granite  and  quai'tzite  north  of  Micliigamme,  where 
N.  H.  Winchell  reported  the  existence  of  a  conglomerate,  is  given  a  peculiar 
interpretation  (}).  177): 

Immediately  iu  contact  with  this  [the  granite]  is  a  greenish  qnartzyte,  which 
passes  by  transition  into  the  granite.  *  *  *  It  might  also  signify  that  the 
"granite"  was  originally  a  sedimentary  rock,  but,  containing  more  feldspar-making 
elements  than  the  qiiartzyte,  metamorphism  changed  it  to  a  rock  of  the  granite  series, 
but  could  not  make  anything  but  a  qnartzyte  of  the  overlying  beds. 

Near  the  Buffalo  mine  the  author  saw  a  black  argillite,  which  was 
thought  to  be  unconformable  on  (|uartzite. 

After  describing  other  phenomena  seen  by  him,  AVinchell  sums  up  his 
study  in  these  conclusions  (p.  1 85) : 

The  Marquette  iron-bearing  rocks  are  not  of  Huronian  aye.  *  *  *  That  tUey 
are  older  than  Huronian  is  shown  by  a  fourfold  line  of  evidence,  (a)  The  rocks  are 
different.  In  the  original  Huronian  the  argillites  are  almost  exclusively  black  and 
carbonaceous  or  magnetitic,  instead  of  bluish  or  ashen  and  lucmatitic.  They  are  more 
prevalently  siliceous  or  flinty.  The  (piartzytes  attain  a  more  enormous  development, 
are  much  jiurer,  especially  the  upper,  and  hold  position  entirely  above  the  argillitic 
member,  {b)  The  Canadian  Huronian  succeeds  immediately  beneath  the  Pakeozoic  system. 
The  Marquette  strata  do  not.  The  Marquette  strata  are  succeeded  immediately 
downward  by  crystalline  schists.  Tlie  Huronian  strata  are  not.  (c)  8ome  evidences 
exist  of  an  unconformable  overlyiny  sub-Falmozoic  system  in  the  Marquette  region.     I 


118  THE  MARQUETTE  IRON-BE AEING  DISTRICT. 

refer  here  both  to  the  xniconformability  described  in  the  *  *  *  vicinity  of  the 
Buffalo  mine,  and  to  Major  Brooks's  brief  notices  of  highly  carbonaceous  blaciv  slates 
occupy  iug  a  positiou  higher  than  the  Marquette  argillytes.  {d)  Proof  is  to  be  adduced 
in  this  report  of  the  tmconformahle  suhterposition  of  the  Vermilion  iron  schists  relatively 
to  the  Animike  slates.  If  the  Marquette  aud  Vermilion  rocks  are  mutual  equivalents, 
the  former  must  hold  iiosition  beneath  the  same  system — that  is,  beneath  the  Hurouiau. 
The  Marquette  iron-hearing  rocJcs  belong  to  a  system  not  yet  defined.  If  they  underlie 
the  Huronian  they  equally  overlie  the  Laurentian.  They  are  not  separated  from 
the  Laurentian  by  a  structural  uncouformability,  but  by  the  evidences  of  a  long  inter- 
vening lapse  of  time  aud  a  most  important  change  in  the  action  of  the  geologic 
forces.  Strata  fully  crystalline  and  strata  essentially  earthy,  tliough  fou.id  in  con- 
formable juxtaposition,  must  necessarily  belong  to  two  different  ages  and  modes  of 
geological  activity. 

For  tlii.s  "older  system"  in  Minnesota  the  anther  snggests  the  name 
Marqnettian  ()).  3n5). 

1S90. 
Wadswoeth,  M.  E.    a  sketch  of  the  geology  of  the  Marquette  and  Kewee- 
nawau  districts.     Along  the  south  shore  of  Lake  Superior,     rublished  by  Duluth, 
South  Shore  and  Atlantic  R.  R.,  1890,  pages  G5-S2. 

In  1890,  after  having-  been  appointed  State  geok)gist  of  Michigan, 
"Wadsworth  pubUshed  a  very  interesting  article,  which,  thong] i  writren  for 
the  traveling  public,  gives  a  succinct  and  strictly  scientific  account  of  the 
geology  of  the  iron  and  copper  districts  of  Michigan  from  the  author's 
point  of  yie^^^ 

.  All  of  the  rocks  under  the  Potsdam  sandstone  are  placed  in  the  Azoic 
system.  They  are  stated  to  have  been  formed  in  three  ways:  (1)  By 
mechanical  means;  (2)  by  eruptive,  igneous,  or  volcanic  agencies;  and 
(3)  by  chemical  action. 

The  rocks  of  mechanical  origin  are  the  detrital  quartzites,  argillites, 
gneisses,  schists,  conglomerates,  etc 

These  rocks  were  invaded  by  eruptive  material  forcing  its  way  irregularly  through 
the  soft  sedimentary  materials,  indurating  them,  bending  their  planes  of  deposition, 
changing  their  color,  and  sending  tongues,  arms,  and  dikes  through  them  in  every 
direction.  It  has  also  been  protruded  through  the  schists  in  large  masses,  contorting 
them  and  tearing  them  across,  and  oftentimes  ending  in  small  arms  or  branches. 
This  eruptive  rock  is  now  very  greatly  metamorphosed,  and  is  termed  jaspilite.  Like 
the  siliceous  eruptive  rhyolites  and  felsites,  it  is  generally  more  or  less  banded  in  its 


GEOLOGICAL   EXPLORATIONS   AND   LITERATURE— ISOO.         119 

character,  which  banding  is  clue  to  its  having  Howed,  the  same  as  is  seen  in  the  baud- 
ingof  the  siliceous  furnace  slags.  *  *  *  It  is  this  liuidal  structure  or  baudiug 
that  is  so  often  mistaken  in  the  rliyolites,  felsites,  trachytes,  and  Jaspilites  for  the 
planes  of  sedimentation.     (P.  fiO.) 

Tlie  ore  associated  with  the  jaspihte  is  said  to  be  a  roucentvate  from 
its  mag'iua.  In  places  the  rocks  have  been  shattered  and  their  cracks  filled 
with  ore  through  the  action  of  pei'colating  water.  After  cooling,  the  jaspi- 
lites were  acted  upon  by  the  waves,  yielding  a  detritus  that  was  deposited 
ui)on  the  underlying  ore  deposits,  forming  true  sedimentary  deposits,  many 
of  which  have  been  since  worked  for  ore. 

Three  kinds  of  ores  are  distinguished,  magnetite,  hematite,  and  martite. 
The  ore  associated  with  the  jaspilite  is  usually  of  the  latter  kind. 

The  argument  in  favor  of  the  eruptive  origin  (»f  the  ores  and  jaspi- 
lites is  outlined,  and  it  is  stated  that  in  188")  Charles  E.  Wright,  at  that 
time  State  geologist  of  Michigan,  had  proved  to  his  own  satisfaction  that  the 
ores  are  eruptive.  The  author  dismisses  Irving's  argument  for  their  sedi- 
mentary origin  by  declaring  that  he  "starts  out  with  either  denying  or 
ignoring  the  occurrence  of  the  very  facts  which  the  present  writer  has 
figured,  and  which  caused  him  to  hold  the  eruptive  \iew.  A  theory  of  the 
origin  of  the  iron  ores  that  starts  out  with  denying-  the  facts  that  it  ought 
to  explain  can  hardly  be  accepted  until  it  recognizes  these  facts  and  explains 
them"  (p.  71). 

After  the  eruption  of  the  jaspilites  and  their  denudation,  other  rocks 
were  forced  through  the  strata  in  a  molten  condition.  Diabases  and 
diorites  were  the  first  rocks  introduced,  and  they  added  so  much  volume  to 
the  already  existing  rocks  that  these  were  thrown  up  into  folds.  i\Ianv  of 
these  diabases  and  other  basic  rocks  have  become  schistose,  but  they  do 
not  pass  into  sedimentary  schists,  as  has  been  supposed  by  some  observers. 
"The  two  look  closely  alike  and  are  similar  to  each  other  in  com})osition, 
but  do  not  jjass  into  one  another  any  more  than  water  and  oil  <lo,  although 
a  hasty  observer  might  not  see  the  line  of  separation  between  the  two"  (p.  72), 

The  author  mentions  that  at  the  Cleveland  mine  and  elsewhere  the 
"schistose  diabase  and  diorite  come  in  contact  with  the  sedimentarv  schists" 
(p.  72). 


120  THE    MARQUETTE   IRON  BEARING   DISTRICT. 

Granite  eruptions  succeeded  those  of  the  basic  rocks,  as  did  also  those 
of  serpentine.  The  ag-e  of  the  hitter  with  respect  to  the  g-ranite  is  not 
determined. 

Tlie  principal  deposits  of  chemical  origin  in  the  district  ai'e  the  soft  ores. 
The  jaspilites  and  their  associated  ores  and  rocks  have  been  leached  by  per- 
colating, usually  hot,  waters,  and  their  iron  oxides  dissolved  and  deposited 
elsewhere  along  the  channels  through  which  the  waters  flowed,  or  the  silica 
has  been  removed  and  the  ore  left  behind  or  locally  concentrated.  On  top 
of  the  Azoic  rocks  and  unconformable  with  them  lies  the  Potsdam  sandstone. 

The  two  important  points  t()  be  noted  are  that  the  author  had  not,  up 
to  this  time,  changed  his  views  as  to  the  age  of  the  granite,  which  he  still 
believed  to  be  younger  than  tlie  ore  l>eds,  nor  as  to  the  origin  of  the  ores 
associated  with  the  jaspilites,  both  of  which  latter  rocks  are  still  regarded 
as  eruptive. 

1891. 

Irving,  R.  D.  Explanatory  aud  liistorical  note.  The  gieeiistone-schist  areas  of 
the  Menominee  and  Marqnette  regions  of  Michigan.  Bull.  U.  S.  Geol.  Survey  Xo.  G2, 
Washington,  1891,  pages  11-24.     With  map. 

In  1891  appeared  the  paper  by  G.  H.  Williams  on  the  greenstone- 
schist  areas  of  the  Menominee  and  Marquette  regions,  in  which  it  was 
shown  that  the  banded  green  schists  (which  had  been  regarded  l)v  most 
geologists  as  sedimentary  and  had  been  placed  l)y  them  in  the  iron  series, 
and  which  Irving  had  separated  from  the  Huronian  and  placed  in  the 
Basement  Complex)  are  fragmental  ^'olcaIiic  rocks  and  lavas. 

As  an  introduction  to  tlie  discussion,  Irving  gives  an  account  of  the 
general  relations  of  the  schists  to  the  granite  and  to  the  fragmental  rocks 
associated  with  them,  and  explains  in  more  detail  than  had  been  done 
hitherto  his  reasons  for  separating  them  from  the  Huronian  series  and 
jjlacing  them  with  the  underlying  granites  and  gneisses. 

Besides  occurring  here  and  there  more  or  less  confusedly  mingled  with  masses 
of  granite  and  other  rocks,  these  greenish  schists  occur  also  in  large  continuous  areas, 
which  they  entirely  occupy,  except  for  certain  relatively  unimportant  basic  and  acid 
intrusives.  *  *  *  The  bulk  of  these  areas  is  composed  of  nondescript  fine-grained 
greenish  schists,  which  appear  to  grade  into  the  more  massive  greenstone-like  forms 


GEOLOGICAL    EXPLORATIONS   AND    LITERATURE— 1S91.         121 

on  tlio  uiie  liaiid,  and  into  the  more  distinctly  developed  cbloritic  and  bornblendic 
schists  ou  the  other.  As  a  rule  these  various  schists  present  no  parallel  structure 
other  than  that  which  seems  referable  directly  to  secondary  causes;  that  is  to  say, 
they  do  not  present  such  banded  varieties  as  would  suggest  the  action  of  sedimenta- 
tion during  their  production.  However,  such  banded  varieties  do  occur  in  subordinate 
quantity,  presenting  then  very  strikingly  regular,  rapid  alternations  of  light  and  darli 
bands.     (P.  11.) 

In  soiiiu  places  within  the  greenstone  areas  there  occur  schists  with 
a  nu))-e  or  less  obscure  fragmental  appearance,  which  is  mucli  more  pro- 
nounced on  the  weathered  surface  than  on  the  fresh  fracture.  ^Vs  a  rule 
these  schists  are  without  any  parallel  structure  except  the  slaty  cleavag'e 
which  all  the  green  schists  present.  Among  them  are  the  rocks  observed 
by  A.  Winchell  and  N.  H.  Winchell  at  Deer  Lake. 

In  his  description  of  the  map  (PI.  Ill,  fig.  2)  accoinpanying  his  pajier 
the  author  writes  (jjp.  14-1.5): 

The  line  of  demarkatio)i  between  the  schists  and  the  granites  *  *  *  is  not 
a  sharp  one,  since  the  two  seem  to  mingle  more  or  less  confusedly  on  each  side  of  the 
somewhat  arbitrary  line  indicated  upon  the  map.  Southward  of  this  greenstone- 
schist  area  »  *  *  are  belts  of  country  occupied  mainly  by  detrital  rocks,  such  as 
quartzites  and  various  fragmental  slates;  with  these,  however,  are  large  bodies  of 
crystalline  limestone  and  several  phases  of  ferruginous  schist,  all  of  which  have  in 
common  an  entire  lack  of  anything  like  a  fragmental  texture.  In  addition  to  these 
rocks  these  areas  include  also  sheets  of  diabasic  greenstone,  which  are  interbedded 
with  the  detritals  and  ferruginous  schists  alluded  to. 

The  author  contrasts  the  schists  with  the  detrital  rocks  to  the  south  of 
them.  He  agrees  with  the  earlier  workers  in  the  district  as  to  the  inferior 
position  of  the  greenstone-schists  with  respect  to  the  stratiform  series,  and 
as  to  the  intrusion  of  the  schists  by  the  granite.  But  he  disagrees  with 
previous  geologists  who  regarded  the  green  schists  as  belonging  in  the 
stratiform  series  and  the  granite  as  younger  than  the  detrital  series. 

In  otlier  words,  it  thus  far  appears  to  me  that  there  is  good  reason  to  believe 
that  these  greenstone-schists  along  with  the  granites,  gneisses,  etc.,  form  a  portion 
of  the  basement  upon  which  the  overlying  detrital  iron-bearing  series  was  once  hori- 
zontally and  unconformably  spread. 

The  granites  are  shown  to  be  unconformaljly  beneath  the  detrital 
series  and  to   be   at  the   same   time  younger  than  tlie  greenstone-schists; 


122  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

hence  these  hotter  rocks  must  be  much  okler  than  the  detrital  beds.  In  the 
conglomerates  separating  the  granites  from  the  overlying  stratified  beds 
there  are  often  fragments  of  schists,  and  frequently  there  is  a  matrix  com- 
posed of  comminuted  greenstone.  In  the  SW.  ^  of  SE.  ^  sec.  29,  T.  48  N., 
R.  25  W.,  a  conglomerate  composed  of  fragments  of  granite,  quartz,  and 
green  schist,  cemented  by  a  dark  slaty  material,  lies  immediately  on  the 
contact  of  the  greenstone-schist  area  with  that  of  the  detrital  series. 

Such  occurrences  as  these,  when  considered  in  connection  with  the  manner  in 
which  the  granite  penetrates  the  greenish  schists  and  is  involved  with  them,  seem  to 
render  necessary  the  belief  that,  while  it  is  plainly  younger  than  the  green-schists,  it 
is  nevertheless  greatly  older  than  the  overlying  detrital  rocks;  and,  more  than  this, 
that  when  the  latter  rocks  were  spread,  the  granites  and  greenstone-schists  together 
had  already  suffered  disturbance  and  deep  denudation.  It  does  not  appear  possible 
to  escape  this  conclusion  by  supposing  that,  since  granite  and  greenstone-schists  are 
eruptives,  they  may  have  furnished  fragments  to  almost  contemporaneous  sedimen- 
tary deposits;  for,  in  the  first  place,  both  the  greenstone-schists  and  the  gneissoid 
granite  must  have  received  their  schistosity  before  yielding  the  fragments.  Moreover, 
whatever  may  have  been  the  depth  at  which  the  schistose  rocks  were  first  formed, 
the  granite  masses  which  intruded  them,  according  to  all  the  later  developments  and 
doctrines  of  petrography,  must  have  been  crystallized  in  depth,  and  must  therefore 
have  had  removed  from  over  them  great  masses  of  materials  before  yielding  fragments 
to  wave  action.     (P.  23.) 

These  remarks  refer  to  the  coarse  granites  of  the  district,  and  not  to 
the  fine-grained  red  granites  which  intrude  the  coarse  granite  and  may 
be  even  as  young  as  the  iron-bearing  series,  though  none  of  its  dikes  have 
been  seen  cutting  the  detrital  rocks  in  the  Marquette  area. 

*  *  *  Accepting  Professor  Williams's  conclusions  as  to  the  surface  origin  of 
most  of  the  greenstone-schists  of  the  Marquette  region,  I  should  suppose  that,  after 
the  accumulation  of  these  rocks  to  the  thickness  of  several  thousand  feet,  they  were 
intruded  by  granite  bosses.  These  bosses  perhaps  may  have  been  merely  softened 
portions  of  the  underlying  gneissic  basement,  which,  indeed,  may  be  represented  in  an 
unaltered  condition  in  portions  of  the  granitic  areas  themselves,  for  all  that  has  yet 
been  determined  to  the  contrary.  Subsequently  mountain-making  movements  brought 
about  the  folding  and  alteration  of  these  enormous  sheets  of  eruptive  material,  now 
represented  by  the  greenstone-schists.  Following  this  was  the  great  denudation 
which  brought  to  light  the  previously  buried  granilic  masses.    This  erosion  was 


GEOLOGICAL   EXPLORATIONS   AND   LITERATUIIE— 1891.         123 

followed  ill  turn  by  the  accumulation  in  the  usual  horizontal  position  of  the  iron- 
bearing  detrital  series,  whose  folding  and  erosion  were  still  later  processes.  And  yet 
this  folding  and  erosion  all  preceded  the  deposition  of  the  horizontal  Cambrian  sand- 
stones of  theregion.     (Pp.  23-24.) 

A  confirmation  of  these  conclusions  is  furnished,  according  to  Ir^'ing, 
by  the  similarity  bet-ween  the  great  dikes  cutting  the  greenstone-schists 
and  the  sheets  of  eruptive  greenstone  in  the  iron-bearing  series. 

For  the  latter  greenstones  are  in  large  measure  interstratilied  with  the  sedi- 
mentary layers  of  the  iron-bearing  series,  following  the  bending  of  its  layers;  so  that, 
even  if  these  greenstones  are  in  the  nature  of  intruded  sheets,  it  seems  necessary  to 
believe  that  their  intrusion  took  place  before  the  folding  of  the  iron-bearing  series. 
Now,  the  corresponding  dikes  in  the  greenstone-schist  area  were  evidently  intruded 
subsequent  to  the  production  of  the  schistosity  of  the  intruded  rocks.  If,  then, 
these  are  facts,  the  time  when  the  iron-bearing  series  was  folded  was  very  iiuich 
subsequent  to  that  time  at  which  the  greenstone  schists  receivetl  their  schistosity. 
(P.  24.) 

The  map  accompanying  Irving's  paper  is  practically  Romiuger's  map 
interpreted  according  to  Irving's  view.  That  is,  the  greenstone-schists  are 
placed  with  the  granites  below  the  iron-bearing  series.  In  the  legend  of 
the  map  we  see  for  the  first  time  the  use  of  the  term  Algonkian,  by  which 
it  was  decided  by  the  United  States  Geological  Survey  to  designate  the 
fragmental  series  lying  lietween  the  Archean  crystallines  and  the  base  of 
the  Cambrian.  The  term  stands  in  the  period  place  as  e(piivalent  to  the 
Agnotozoic  group. 

Williams,  G.  II.  The  greenstone-schist  areas  of  the  JMenominee  and  Marquette 
regions  of  Michigan  Bull.  U.  S.  (ieol.  Survey  No.  G2,  Wasliington,  1891,  pages 
134-217.     With  plates  of  thin  sections. 

Williams's  paper  deals  primarily  with  the  microscopical  features  of  the 
green  schists  already  so  frequently  mentioned. 

The  author  divides  the  district  studied  into  four  areas:  (1)  The  Eastern 
area,  near  Marquette;  (2)  the  Western  area,  immediately  north  of  Teal 
Lake,  in  the  town  of  Negaunee;  (3)  the  Northern  area,  lying  nortli  of 
Dead  River;  and  (4)  the  Deer  Lake  area.  The  Ea.stern  area  is  further 
divided  into  a  northern  and  a  southern  half 


124  THE    MAEQUETTE   IRON-BEARING    DISTRICT, 

111  the  iiortli  liiilf  (if  the  Eastern  area  the  hxyers  of  the  schitsts — 
are  alternately  of  a  darker  and  lighter  shade  of  green,  which  gives  these  particular 
greenstones  their  characteristic  striped  appearance.  In  these  banded  rocks  *  *  * 
intrusions  of  comparatively  little  altered  acid  and  basic  matter  are  abundant.  These 
are  for  the  most  part  conformable  to  the  bedding  of  the  schists,  and  embrace 
granites,  gneisses,  schistose  prophyries,  diorites,  and  diabases.  Whenever,  in  these 
undoubtedly  eruptive  rocks,  a  schistose  structure  is  apparent,  this  is  conformable  to 
the  bedding  of  the  banded  greeustone-schists. 

The  southern  portion  of  the  area  «  *  *  is  occupied  by  much  more  massive 
and  homogeneous  greenstones  of  a  nearly  uniform  light-green  color  and  an  almost 
aphanitic  structure.  These  are  characterized  by  their  division  into  oval  or  lenticular 
areas  which  interlace  and  which  are  separated  by  a  finely  schistose  material  of  much 
finer  grain.  This  peculiar  parting,  *  *  *  at  first  glance,  resembles  the  spheroidal 
weathering  of  many  eruptive  rocks.  There  is,  however,  better  reason  for  regarding 
it  as  of  mechanical  origin.  *  *  *  Intrusive  rocks  are  rarer  than  in  the  banded 
greenstones  of  the  northern  portion  of  this  area.     (P.  137.) 

A  hirge  number  of  thin  sections  of  these  schists  are  carefully  described. 
Some  of  the  descriptions  will  be  referred  to  in  the  body  of  this  monograph. 
At  this  point  it  will  be  necessary  simply  to  quote  the  author's  conclusions. 

With  reference  to  the  rocks  of  the  Northern  area  he  writes  (p.  158): 

*  *  *  The  structure  of  these  greenstone  schists  is  such  as  to  necessitate  a 
belief  in  the  original  nature  of  their  stratification ;  while,  on  the  other  hand,  their 
chemical  as  well  as  their  mineralogical  composition  renders  it  impossible  to  separate 
them  from  the  massive  and  highly  altered  greenstones  (uralite,  diabases,  etc.)  with 
which  they  are  more  intimately  associated.  Their  parallel  banding  indicates  a 
fragmental,  but  their  chemical  and  their  mineral  composition  indicate  an  igneous 
origin.  The  only  satisfactory  reconciliation  of  these  opposite  sets  of  characters  is  to 
be  found  in  that  group  of  rocks  intermediate  between  sediments  and  lavas,  known  as 
volcanic  tuifs. 

In  the  opinion  of  the  writer,  then,  the  banded  greeustone-schists  of  the  Northern 
Marquette  area  are  to  be  regarded  as  consolidated  and  highly  metamorphosed  diabase 
tuffs.  These  are  intimately  associated  with  numerous  contemporaneous  flows  of 
diabase  and  quartz-porphyry,  together  with  tuffs  of  the  latter  rock;  while  all  have 
been  broken  through  by  much  younger  dikes,  both  basic  and  acidic. 

The  rocks  of  the  Southern  area  are  not  banded.  "They  are,  for  the 
most  part,  massive,  pale  green  in  color,  and  apparently  homogeneous  in 
structure." 


GEOLOGICAL   EXPLORxVTIONS   AND    LITERATURE— 1891.  125 

Of  these  the  author  says: 

The' occasioual  survival  of  the  characteristic  diabase  structure  even  in  some  of 
the  more  foliated  forms,  taken  in  connection  with  their  evident  identity  with  and 
gradual  transition  into  the  massive  varieties,  appears  to  be  sufficient  proof  that, 
with  the  exception  of  certain  unimportant  tuft"  deposits,  these  green-schists  have 
been  derived  from  basic  cruptives  through  the  agency  of  intense  mechanical  and 
chemical  action.     (P.  16.3.)     Originally  [they  were]  massive  basic  flows.     (P.  163.) 

The  gTeenstones  of  tlie  Western  or  Nej^aitnee  area,  the  second  chosen 
for  examination,  are  Hke  the  schists  of  the  Sotithern  Marquette  area,  Avhile 
those  of  the  area  north  of  Dead  RiAer  are  essentially  similar  to  the  rocks 
in  the  Northern  Marquette  district. 

Of  the  Deer  Lake  area  Williams  has  little  to  record.  A  transcriji- 
tion  of  portions  of  Irving's  field  notes  dcscrilx-s  tliis  area  as  underlain 
by  the  greenstone-conglomerates  referred  to  in  his  puljlislied  papers  and  in 
the  articles  of  Rominger,  N.  H.  Winchell,  Alexander  Winchell,  and  others. 
Williams  recognizes  these  as  composed  of  volcanic  detrittts,  ejected  by  an 
ex])losive  force  at  the  earth's  surface.  lie  calls  them  agglomerates,  which 
term  is  used  "to  designate  a  tumultuous  assemblage  of  volcanic  ejecta- 
meuta,  bombs,  foreign  blocks,  etc.,  of  all  sizes  and  shapes,  cemented  liy  a 
fine-grained  paste  of  volcanic  ash"  (p.  190). 

Van  Hise,  C.  E.  An  attempt  to  harmonize  some  apparently  conflicting  views 
of  Lake  Superior  stratigraphy.  Read  before  Wisconsin  Acad.  Sci.,  December  30, 
1890.     Am.  Jour.  Sci.  (3),  Yol.  XLI,  1891,  pages  117-137. 

In  this  article  the  author  points  out  the  significance  of  the  existence  of 
the  conglomerates  above  the  ore  horizon.  He  calls  attention  to  the  fact  that 
Foster,  Foster  and  Whitney,  Brooks,  Rominger,  Wadswortb,  Irving,  and  the 
Winchells  had  all  recognized  and  described  these  conglomerates,  and  that 
they  all  had  detected  in  them  fragments  of  the  underlying  formations,  but 
the  Winchells  only  regarded  them  as  marking-  a  break  in  the  succession 
of  beds  of  sufficient  importance  to  warrant  placing  the  rocks  above  and 
below  it  in  different  geological  ages.  N.  H.  Winchell  believed  the  break  so 
great  that  the  overlying  rocks  were  provisionally  referred  to  the  Potsdam. 

The  author  describes  the  occurrences  of  the  conglomerate  at  the  Good- 
rich, Saginaw,  Fitch,  Barron,  Winthrop,  Cascade,  Wheat,  Jackson,  Lake 


126  THE    MAKQUETTE   IRON-BBAIIING    DISTRICT. 

Superior,  Barnum,  Boston,  Spurr,  Michigamme,  and  ReJDublic  mines,  and 
shows  that  in  many  instances  its  stratification  is  unconformable  with  that 
of  the  ores  and  jaspers  in  contact  with  it  and  below  it.  He  publishes  a 
photograph  of  the  contact  at  the  Goodrich  mine,  which  exhibits  the  laj-ers 
of  the  jasper  abutting  against  the  conglomerate,  and  the  latter  unconform- 
ably  upon  them.  Since  this  conglomerate,  as  well  as  other  similar  ones, 
contains  ore,  chert,  and  jasper  fragments  in  precisely  the  same  condition  in 
which  they  occur  in  the  underlying  formation,  it  is  plain  that  the  latter 
had  reached  its  present  condition  before  the  conglomerate  was  deposited. 
There  is  thus  evidently  a  large  time  break,  represented  by  this  unconformity, 
in  the  series  of  rock  beds  lying  above  the  granites  and  schists  and  beneath 
the  Potsdam  sandstone.  In  other  words,  the  rock  series  heretofore  known 
as  the  Huronian  in  reality  includes  two  series,  one  above  and  the  other 
below  the  conglomerates  lying  upon  the  ore-bearing  formation. 

Those  [the  rocks]  upon  fclie  lower  side  of  this  break,  in  the  exceedingly  contorted 
jasper,  in  the  schistose  character  of  its  quartzites,  and  in  the  general  assumption  of  a 
semicrystalline  character,  show  the  evidence  of  profound  dynamic  action.  In  the 
upper  series,  on  the  contrary,  the  folding  has  not  been  intense;  the  fragmental  charac- 
ter of  the  slates  and  quartzites  under  the  microscope  is  evident  at  a  glance,  and  no 
indication  of  great  dynamic  action  is  seen.  While  subsequent  to  the  deposition  of 
the  upper  series  the  whole  region  has  been  subjected  to  a  new  folding,  great  enough 
in  places  to  give  the  later  series  a  dip  of  60°  or  70°,  as  at  the  Goodrich,  it  has  not 
suffered  since  tliat  time  such  intense  dynamic  movements  as  have  produced  the  more 
thoroughly  crystalline  and  folded  character  of  the  earlier  series.     (P.  122.) 

These  facts  contirm  the  correctness  of  the  view  tliat  the  two  series 
belong  to  fundamentally  different  geological  epochs. 

In  a  footnote  Van  Hise  refers  to  Wadsworth's  anticipated  objection  to 
this  theory  in  view  of  the  fact  that  the  ores  and  jaspers  are  regarded  by 
him  as  eraptive,  and  states  that  this  author's  facts  "indicate  the  eruptive 
origin  of  the  ore  and  jasper  only  if  the  schists  are  of  sedimentary  origin." 
His  own  investigations  show,  however,  that  the  Marquette  iron-bearing 
members  "  contain  many  schistose  dikes,  and  also  that  in  manj^  cases  the 
massive  greenstone  knobs  of  the  Marquette  region  vary  by  imperceptible 
stages  into  the  schists  associated  with  the  iron  ore  and  jasper.  The  schists 
are  then,  in  part  at  least,  of  eruptive  origin.     *     *     *     This  view  reverses 


GEOLOGICAL   EXrLOltATIOJfS   AND   LITERATUKE— 1S91.         127 

Dr.  Wadsworth's,  and  makes  his  sedimentary  rocks  eruptive  and  his  erup- 
tive ones  sedimentary"  Qjp.  123-124). 

The  remainder  of  the  paper  is  devoted  mainly  to  the  correLation  of  the 
several  Hurouian  areas  in  the  Lake  Superior  region,  and  to  a  discussion  of 
principles  of  nomenclature.  The  terms  Upper  Marquette  and  Lower  Mar- 
quette are  used  by  the  author  to  designate  those  portions  of  the  Huronian 
above  and  l)el()\v  the  unconformity  in  the  ■  Marquette  district,  and  these 
together  comprehend  all  the  Algonkian  series  in  this  district.  Below  the 
Lower  Marquette  is  the  Fundamental  Complex  of  schists,  granites,  etc., 
(the  Archean  rocks),  and  above  the  Upper  Huronian  lies  the  Potsdam 
sandstone. 

Of  course,  where  the  Upper  Marquette  is  in  contact  with  the  Archean 
thei'e  are  conglomerates  and  unconformities  between  the  two  series,  just  as 
there  are  between  the  Archean  rocks  and  those  of  the  Lower  ^larquette 
series.  In  each  of  the  two  series  there  is  believed  to  occur  an  iron-liearing 
formation.  The  banded  jaspers  and  ores  immediately  under  the  conglom- 
erates belong  in  the  Lower  Marquette,  and  the  cherty  ores  of  tlie  l)alil)a, 
Wetmore,  and  Beaiifort  mines,  all  of  which  are  in  black  slates,  belong 
in  the  Upper  series. 

Wadsworth,  M.  E.  a  sketcli  of  the  geology  of  tlie  Marquette  and  Keweeiiawau 
districts.  Along  the  bowstring  or  south  shore  of  Lake  Superior.  Published  by 
General  Pass.  Dept.,  Duluth,  South  Shore  and  Atlantic  Eaihoad,  1891,  pages  77-91. 

In  a  second  edition  of  the  little  handbook  of  the  Dulutli,  Soutli 
Shore  and  Atlantic  Railroad  we  find  Wadsworth's  views  on  the  origin  and 
relations  of  the  Marquette  I'ocks  rapidly  changing,  as  the  result  of  the 
studies  by  the  Michigan  survey,  of  which  he  had  been  the  chief  for  several 
years. 

Instead  of  classing  all  the  formations  of  the  district  in  the  one  "Azoic" 
group,  the  author  separates  them  into  three  divisions,  naming  them  the 
Cascade,  the  Republic,  and  the  Holvoke  formations. 

In  the  Cascade  formation  are  placed  the  hornblende-schists  which  are 
invaded  b}^  granite  and  other  eruptive  rocks  south  of  Palmer,  and  certain 
detrital  rocks  composed  of  their  debris,  besides  the  enijjtive  jaspilites  and 


128  THE   MAEQUETTE   lEOi^-BEARING   DISTEICT. 

ores  associated  with  these.  No  new  facts  are  given  with  respect  to  the 
origin  of  the  ores,  but  the  author  repeats  his  former  view  and  emphasizes 
the  notion  of  the  eruptive  origin  for  these  particuLar  ores. 

Above  the  ores,  and  forming  the  base  of  the  succeeding  RepubUc  for- 
mation, he  phxces  a  series  of  fragmental  deposits  of  ore  and  jaspihte.  These 
are  described  at  much  greater  lengtli  than  in  tlie  earher  article.  The  ores 
constitute  a  large  proportion  of  those  mined.  They  ])ass  upward  into 
quartzites.  The  contacts  between  the  frag-mental  and  eruptive  jaspilites  and 
ores  can  be  seen  at  the  Jackson,  Lake  Superior,  and  other  mines.  In  the 
Cascade  range  all  the  jaspilite  is  of  the  fragmental  kind;  its  layers  are 
interlaminated  with  those  of  quartzite. 

Above  this  fragmental  jaspilite  fonnation,  with  its  associated  quartzites 
and  schists,  both  at  Republic  and  at  Cascade,  there  is  another  fragmental 
series,  composed  of  conglomerates,  quartzites,  schists,  etc.  This  fonnation 
is  Avell  seen  at  the  Holyoke  mine,  and  hence  has  been  called  the  Holyoke 
formation. 

The  diabases  and  diorites  are  still  believed  to  have  been  erupted  after 
the  denudation  of  the  nonfragmental  jaspilites — that  is,  in  Republic  time. 
Of  the  granite,  however,  two  eruptions  are  now  recognized.  One  of  these 
"is  older  than  the  detrital  jaspilite,  if  not  older  than  any  of  the  ore-bearing 
formations,  while  another  occurred  subsequently  to  the  eruptions  of  the 
beforementioned     *     *     *     diabases  and  diorites." 

The  jaeridotite  of  Presque  Isle  is  now  regarded  as  younger  than  the 
granite  in  its  proximity,  while  that  northwest  of  Ishpeming  is  shown  to  be 
younger  than  an^'  other  of  the  large  intrusive  masses  in  the  Marquette 
district. 

The  most  important  facts  brought  out  in  the  paper,  especially  impor- 
tant because  they  are  opposed  to  the  author's  earlier  opinions,  are:  the 
divisibilit}'  of  the  Azoic  into  three  formations  of  different  geological  ages; 
the  recognition  of  an  old  granite,  older  than  most  of  the  ore  deposits;  and 
the  acknowledgment  of  the  sedimentary  origin  of  many  of  the  ores  and 
jaspilites.  The  green  schists  are  presumably  still  })laeed  above  the  oldest 
formation  of  the  district. 


GEOLOGICAL   EXPLOIiATlONS   AND    LITEKATUliE— 1891.         129 

Hunt,  T.  Sterry.  The  iron  ores  of  the  United  States.  Trans.  Am.  Inst. 
Mill.  Eng.,  Vol.  XIX,  New  York,  1891,  pages  3-17. 

In  his  Pittsburg  address  to  the  mining'  engineers  Hunt  gave  a  rapid 
review  of  the  geological  relations  of  the  iron  ores  of  the  United  States. 
The  pi-e-Paleozoie  rocks  for  the  entire  country  are  divided  by  him  into 
the  Laurentian,  Norian,  Avonian,  Huronian,  Montalljan,  and  Taconic  sys- 
tems, and  the  rocks  comprising  them  nre  l)elieved  to  have  lieen  deposited 
in  a  hot  ocean  highly  charged  with  active  chemical  agents.  The  Marquette 
ores  are  placed  in  the  last  of  the  above  divisions — the  Taconic — and  in 
that  division  of  it  which  the  author  denominates  the  Animikie.  With 
regard  to  the  ores  of  these  pre-Paleozoic  terranes  the  author  writes: 

We  moreover  reject  as  untenable  the  notion  of  the  igneous  origin  of  the  iron 
ores  themselves,  which  appear  to  be  in  all  cases  deposited  from  water,  generally  con- 
temporaneous with  the  inclosing  rocks,  but  more  rarely  by  subsequent  processes  in 
fissures,  after  the  manner  of  mineral  veins.     (P.  5.) 

GoETZ,  George  W.    Analyses  of  Lake  Superior  ores.     Ibid.,  pages  59-01. 
Goetz   gives  a  record  of  the  analyses  of  ores  from  thirty-six  mines  in 
the  Marquette  district. 

Lane,'A.  C,  Keller,  H.  F.,  and  Shabpless,  P.  F.  Notes  on  Michigan 
minerals.     Am.  Jour.  Sci.  (3),  Vol.  XLII,  1891,  pages  199-508. 

The  authors  identified  the  chloritoids  of  Humboldt,  of  the  Fitch  and 
the  Champion  mines,  referred  to  briefly  and  only  incidentally  by  the  geolo- 
gists who  liad  studied  the  district,  as  a  triclinic  chlorite  similar  to  masonite. 
It  occurs  "in  altered  arkoses  or  similar  rocks,  in  one  case  both  in  the  cement 
and  in  the  basic  and  acid  j)ebbles  of  a  conglomerate."  They  also  find  that 
the  hornblende  in  the  "actinolite"  or  "  anthophyllite "  schists,  associated 
with  the  ores,  especially  of  the  western  portion  of  the  Mai-(|uette  area,  is  a 
monoclinic  iron-amphibole  or  a  griinerite. 

Van  Hise,  C.  E.  The  pre-Cambrian  rocks  of  North  America.  Gompte-rendu 
5th  sess.  Internat,  Cong.  Geologists,  Washington,  1891,  pages  109-150. 

This  article  is  a  condensation  of  the  eighth  chajjter  of  the  correlation 
bidletin  on  the  Archean  and  Algonkian  of  North  America  which  is  referred 
to  on  pages  133-135. 

MON  XXTIII 9 


130  THE   MAEQUETTE   lEON-BEAKING  DISTRICT. 

Van  Hise,  C.  R.  Pre-Cambriau  geology  of  tbe  Lake  Superior  region.  Ibid., 
pages  489-512.     With  maps. 

In  this  paper,  which  is  explanatory  of  a  trip  made  through  the  iron 
districts  of  Lake  Superior  with  some  members  of  the  International  Con- 
gress of  Geologists,  the  author  gives  descriptions  of  the  general  character  of 
the  Basement  Complex  and  of  the  Upper  Huronian  and  Lower  Huronian 
series.  Very  little  is  contained  in  these  descriptions  that  had  not  been 
given  in  earlier  jDublicatioiis,  of  which  it  is  largely  an  abstract.  The  maps 
are  reproductions  of  those  published  by  Brooks,  Irving,  Gr.  H.  Williams, 
Pumpelly,  and  Van  Hise. 


Van  Hise,  C.  R.  The  iron  ores  of  the  Marquette  district  of  Michigan.  Am. 
Jour.  Sci.  (3),  Vol.  XLIII,  1892,  pages  116-132. 

In  this  paper  Van  Hise  gives  a  resumti  of  the  results  of  a  preliminary 
examination  of  the  Marquette  district. 

Two  ore-bearing  formations  are  recognized  in  the  district — one  belong- 
ing in  the  Lower  Marquette  and  the  other  in  the  Upper  Marquette  series,  as 
defined  by  the  author. 

The  ore  deposits  of  the  Lower  Marquette  series  consist  of  the  banded 
jasper  and  ore  so  many  times  described.  The  jasper  is  not  a  fragmental 
rock.  It  is  composed  of  individualized  silica,  forming  bands  which,  when 
white,  are  known  as  chert.  When  the  grains  are  stained  with  red  oxide  of 
u-on  they  give  rise  to  jasper.  The  siliceous  bands  are  not  regular  in  thick- 
ness. They  sometimes  extend  for  long  distances  with  a  nearly  uniform 
width.  Sometimes  they  wedge  out  abruptly.  Often  they  occur  as  rows  of 
lenticiilar  masses.  Near  the  top  of  the  formation  the  silica  is  nearly  all 
jasperized.  Farther  down,  as  a  rule,  it  is  more  cherty,  until  in  the  lower 
portions  of  the  formation  the  siliceous  bands  are  white. 

Actinolite-magnetite-schists  are  also  associated  with  the  ores,  especially 
where  these  are  magnetites. 

Another  and  important  component  of  the  ore-bearing-  formation  is  a 
cherty  carbonate  of  iron.  This  has  been  found  most  frequently  where  the 
formation  dips  under  a  greenstone.      Cutting  these  rocks  are  bosses  and 


GEOLOGICAL   EXPLOltATIOXS   AND   LITEEATUEE— 1892.         131 

dikes  of  "  g-reenstone,"  which  were  originally,  in  most  cases,  diabase.  In 
their  present  condition  tlipy  consist  of  diorites,  chlorite-schists,  talc-schists, 
and  other  similar  rocks  which  are  known  locally  aw  "  Soapstones,"  "paint 
rock,"  etc. 

The  rocks  overlying-  the  ores  are  the  conglomerates  and  quartzites  at 
the  base  of  the  Upper  series.  The  rocks  occupy  a  great  synclinal  fold, 
which  in  some  places  is  corrugated  into  minor  folds. 

After  thus  outlining  the  general  features  of  the  lower  iron  formation, 
the  author  proceeds  to  discuss  the  mode  of  origin  of  the  ores.  He  clas- 
sifies the  deposits  as  follows:  (1)  Those  at  the  contact  of  the  quartzite- 
conglomerate  and  the  ore-beax'ing  formation;  (2)  those  resting  upon  soap- 
rock,  which  grades  into  massive  diorite;  (3)  those  resting  upon  soap-rock 
dikes  cutting  the  formation ;  (4)  those  interbedded  in  the  jaspers  and  cherts. 

Deposits  of  the  first  kind  consist  usually  of  specular  or  magnetic  ore. 
They  occur  either  within  the  underlying  jasper-ore  formation  or  within  the 
basal  layers  of  the  conglomerate.  In  the  latter  case  they  are  genuine 
detrital  concentrations.  Those  deposits  within  the  jasper,  but  at  the  contact 
of  ihis  with  the  conglomerate,  are  found  usually  wdiere  the  former  rock  is 
shattered,  or  sharply  folded,  or  where  cut  by  dikes.  The  jasper  often 
passes  into  the  ore  gradually. 

lu  following  a  ja.sijer  baud  toward  tlie  ore  it  was  fouDd  that  instead  of  remaining 
solid  it  becomes  porous  and  frequently  contains  considerable  cavities.  These  spaces 
ill  the  transition  zone  are  lined  with  crystalline  ore.  In  passing  on  toward  tlie  ore 
dei^osit  more  and  more  of  the  silica  is  found  to  have  been  removed,  and  the  ore  has 
.eplaced  it  to  a  corresponding  degree.  An  examination  at  many  localities  led  to  the 
conclusion  that  the  transition  from  the  banded  ore  and  jasper  to  the  ore  takes  jilace 
as  a  consequence  of  the  lemoval  of  silica  and  the  substitution  of  iron  oxide.  Often 
in  these  cases  the  finegrained  part  of  the  ore  is  that  of  the  original  rock,  while  the 
coarser  material  is  the  secondary  infiltration.     (P.  121.) 

The  deposits  of  the  second  class  comprise  many  of  the  soft  ores  and 
some  of  the  hard  ores.  In  either  case  the  ore  body  follows  along  the  con- 
tact plane  between  the  impervious  "soapstone"  and  the  unchanged  jasper. 

In  deposits  of  the  third  class  the  ore,  which  is  usually  soft,  may  lie 
upon  both  sides  of  a  vertical  dike.  When  the  dike  is  inclined  at  a  high 
angle  the  ore  lies  on  its  upjier  side  only.      In  the  troughs  formed  by  inter- 


132  THE   MAEQrETTE    IRON  BEARING   DISTRICT. 

penetrating-  dikes  ov  by  offshoots  from  large  masses  of  "diorite"  the  ore 
deposits  are  the  thickest. 

Large  ore  bodies  of  the  fourth  cdass  are  unknown. 

All  the  ore  bodies  of  the  second,  third,  and  fourth  classes  lie  wholly 
within  the  lower  iron  formation,  and  at  any  horizon  within  it,  provided  in 
the  second  and  third  cases  there  be  present  a  "soapstone."  Thus  the  distri- 
bution of  the  ores  is  similar  to  their  distribution  in  the  Penokee  district, 
where  they  have  been  shown  to  be  dependent  upon  the  presence  near 
them  of  some  rock  impervious  to  water. 

All  the  facts  bear  toward  the  conclusion  that  the  ore  is  a  secoudary  concen- 
tration produced  by  the  action  of  downward  percolating  water.  *  *  *  The  ore 
deposits  occur  at  places  where  circulating  wiiters  are  sure  to  be  concentrated.  The 
soap  rock  accommodates  itself  to  folding  without  fracture,  and  while  probably  allowing 
more  or  less  water  to  pass  through,  acts  as  a  practically  impervious  stratum,  along 
which  water  is  deflected  when  it  once  comes  in  contact  with  it.  *  *  *  On  the  other 
hand,  the  brittle,  siliceous,  ore-bearing  formation  has  been  fractured  by  the  folding  to 
which  it  has  been  subjected,  so  that  where  these  processes  have  been  extreme  water 
passes  through  it  like  a  sieve.  That  the  tilted  bodies  of  diorite  or  soap  rock,  especially 
when  in  a  pitching  synclinal  or  forming  a  pitching  trough  by  the  union  of  a  dike  and 
a  mass  of  diorite,  must  have  guided  downward-flowing  waters  is  self-evident.  *  *  * 
It  is  also  plain  that  the  contact  plane  between  the  quartzite-conglomerate  and  the 
ore-bearing  formation,  that  is,  the  plane  of  unconformity  between  the  Upper  and 
the  Lower  Marquette  series,  must  have  been  a  great  horizon  for  downward-flowing 
waters.     (P.  125.) 

Alono'  these  channels  silica  was  removed  and  iron  oxide  concentrated. 

After  arguing  the  question  the  author  concludes  that  the  concentration 
of  the  ores  occurred,  in  all  probability,  "during  and  later  than  the  folding 
and  erosion  subsequent  to  Upper  Marquette  time"  (p.  126),  through  the  long 
continued  action  of  percolating  waters,  flowing  downward  along  channels 
whose  courses  were  determined  by  the  existence  of  impervious  rock  masses 
cutting  through  less  impervious  ones,  or  were  directed  l)y  the  contact  plane 
between  the  Upper  and  the  Lower  series.  The  iron  probably  came  from  the 
formation  in  which  the  ore  now  occurs,  and  the  diabases  in  their  alteration 
to  "soapstones"  provided  some  of  the  alkalis  necessary  to  dissolve  the  silica. 

The  difference  between  the  soft  and  the  hard  ores  is  supposed  to  be 
due  to  the  fact  that  the  latter  were  first  deposited  in  a  crystalhne  condition 


GEOLOCICAL    EXPLOKATIONS    AND    LITEIJATUEE— 1892.         133 

and  were  afterward  slieared,  and  that  the  schistosity  of  the  ore  at  the  con- 
tact of  the  Upper  and  Lower  series,  as  well  as  that  interbanded  with  the 
jaspers,  is  the  result  of  this  sheanng.  The  magnetic  ore  in  the  contact 
deposit  is  thought  to  have  been  directly  deposited  in  that  form,  while  that 
in  the  actinolite-schists  may  have  been  formed  l)y  direct  oxidation  of  an 
original  iron  carbonate. 

The  Upper  Marquette  ores  have  in  general  the  same  origin  as  the  Lower 
ones.  The  impervious  strata  here  are  often  beds  of  black  slate.  Unaltered 
carbonate  is  often  found  associated  with  the  ores,  and  there  is  little  difficulty 
in  recognizing  all  the  stages  of  alteration  between  this  and  the  oxide  ores. 

Sometimes  a  single  ore  body  may  belong  in  part  to  the  Lower  Huro- 
iiian  and  in  part  to  the  Upper  Huronian,  the  replaced  ore  of  the  former  and 
the  mechanical  ore  of  the  latter,  at  the  contact  of  the  two  formations,  being 
welded  together  by  secondary  infiltrations. 

Van  Hise,  C.  E.  The  iron  ores  of  the  Lake  Superior  region.  Trans.  Wisconsin 
Acad.  Sci.,  Arts,  and  Letters,  Vol.  VIII,  1892,  pages  219-228. 

The  author  treats  the  same  subject  in  this  article  as  in  the  last.  The 
discussion  in  the  present  paper,  however,  is  more  comprehensive  and  less 
technical  than  that  in  the  jjreceding  one. 

Van  Hise,  C.  E.  Correlation  papers — Archean  and  Algoukian.  Bnll.  U.  S. 
Geol.  Survey  No.  80,  Washington,  1882,  pages  52-208,  and  Chapter  VIII.  With  map, 
page  48. 

This  is  the  correlation  essay  to  which  reference  has  already  been 
made.  In  it  the  author  reviews  the  literature  on  the  Lake  Superior  region, 
summarizes  it,  and  interprets  it  with  the  aid  of  his  own  experience.  The 
conclusions  as  to  the  succession  in  the  Marquette  district  are  in  accord  with 
the  author's  views  as  published  in  earlier  papers. 

Some  of  the  general  conclusions  reached  in  the  study  have  a  direct 
bearing  on  the  geology  of  the  Marquette  district,  since  this  is  one  of  the 
areas  that  have  yielded  premises  for  the  conclusions. 

It  is  shown  that  the  schistose  crystallines  under  the  Huronian  rocks  in 
this  district  are  older  than  the  latter,  fVn-ming  a  basement  on  which  the 
fragmental  rocks  were  deposited. 


134  THE   MAEQUETTE    lEONBEAEING    DISTEICT. 

The  basement  series — 
consists  of  a  most  intricate  mixture  of  nearly  massive  rocks,  among  which  granite 
and  granite  gneiss  are  jiredominant;  of  gneissic  and  schistose  rocks,  all  of  which  are 
completely  crystalline,  and  so  folded  and  contorted  that  nowhere  has  any  certain 
structure  ever  been  made  out  over  considerable  areas.  *  *  *  The  minerals  in  the 
rocks  generally  show  evidence  of  dynamic  action ;  they  do  not  have  the  clear-cut, 
definite  relations  characteristic  of  the  later  plutonic  rocks.  *  *  *  Further,  the 
basal  complex  is  not  only  recognized  by  its  positive  but  by  its  negative  characters. 
Nowhere  in  it  is  a  persistent  thick  formation  of  quartz-schist  (although  vein-quartz 
is  abundant),  of  limestone  or  marble,  of  a  graphitic  schist,  or  of  a  conglomerate. 
(P.  470.) 

This  complex,  in  its  positive  as  v^^ell  as  in  its  negative  characteristics,  is 
unique  among  the  geological  formations;  hence,  observes  the  author,  it 
deserves  a  descriptive  name  (Archean)  to  distinguish  it  from  the  clastic 
formations  above  it. 

Concerning  the  origin  and  stratigraphy  of  this  complex  little  is  known. 

The  only  division  generally  applicable  to  the  Archean  warranted  by  present 
knowledge  is  its  separation  into  (1)  line  grained  mica-schists,  feldspathic  mica-schists 
(teclinicallygneisses),hornl)lende  schists,  hornblende-gneisses,  etc.,  and  (2)  the  granites 
and  granitoid  gneisses,  with  their  associates.     (P.  488). 

The  term  Laurentian  is  proposed  for  the  lighter-colored,  gneiss-granite 

portion  of  the  complex,  and  Mareniscan  for  the  darker,  schist  portion. 

The  classification  of  the  Marquette  rocks,  then,  in  its  general  features, 

is  as  follows: 

Paleozoic -  -  -  -  Cambrian Potsdam. 

f  Keweenawan. 
Affnotozoic  or  Proterozoic .  .  Algonkiau  —  I  _^ 

Tj         •  Upper. 

[  Huroman I     ^  "^ 

[  Lower. 
Archean Archean Mareniscan  and  Laurentian. 

In  the  Lake  Superior  region,  between  the  Archean  and  the  Potsdam  sandstone, 
the  great  Algonkiau  system  is  subdivided  into  three  series,  which  are  separated  by 
very  considerable  unconformities.  The  lowest  series  is  closely  folded,  semicrystalline, 
and  consists  of  limestones,  quartzites,  mica-slates,  mica-schists,  schist-conglomerates, 
and  ferruginous  and  jaspery  beds,  intersected  by  basic  dykes,  and  in  certain  areas 
also  by  acid  eruptives.     It  includes  volcanic  elastics,  often  agglomeratic,  and  a  green 


GEOLOGICAL   EXPLOEATIONS   AND   LITEEATURE— 1892.         135 

chloritic,  finely  lamiDated  scliist.  The  thickness  of  tbis  series  lias  not  been  worked 
out  witli  accuracy,  but  at  its  maximum  it  is  probably  more  than  5,000  feet.  As  the 
term  Huronian  has  been  for  many  years  applied  uot  only  to  the  Upper  Huronian 
series,  but  to  this  inferior  series  about  Lake  Superior,  it  is  called  Lower  Huronian. 

Above  this  series  is  a  more  gently  folded  one  of  conglomerates,  quartzites,  slates, 
shales,  mica-schists,  ferruginous  beds,  interbedded  and  cut  by  greenstones,  the  whole 
having  a  maximum  thickness  of  at  least  12,000  feet.  *  *  *  In  its  volume,  degree 
of  folding,  and  little  altered  character  the  Upper  Huronian  is  in  all  respects  like 
the  upper  series  of  the  original  Huronian,  and  can  be  correlated  with  it  with  a  con- 
siderable degree  of  certainty.  Above  the  Upper  Huronian  is  the  great  Keweenawan 
series.     *     *     * 

The  unconformity  which  separates  the  Lower  Huronian  from  the  Upper  Huro- 
Diau  and  that  which  separates  the  latter  from  the  Keweenawan  each  represents  an 
interval  of  time  sufficiently  long  to  raise  the  land  above  the  sea,  to  fold  the  rocks,  to 
carry  away  thousands  of  feet  of  sediments,  and  to  depress  the  land  again  below  the 
sea.  That  is,  each  represents  an  amount  of  time  which  perhaps  is  as  long  as  any 
of  the  periods  of  deposition  themselves.  In  jiarts  of  the  region  the  lowest  clastic 
series  rests  unconformably  upon  the  Fundamental  Complex,  but  in  certain  areas  the 
relations  have  not  been  ascertained.  The  upper  of  the  three  clastic  series,  the 
Keweenawan,  rests  unconformably  below  the  Cambrian.     (P]i.  499-500.) 

VVadswokth,  M.  E.  a  sketch  of  the  geologj^  of  the  iron,  gold,  and  copper 
districts  of  Michigan.     Nature,  December  1,  1892,  page  117. 

In  this  same  year,  at  a  meeting  of  the  Greological  Society  of  London, 
Wadsworth  gave  an  advance  abstract  of  his  report  for  1891-92  as  State 
geologist  of  Michigan.  In  this  paper  all  the  rocks  below  the  Cambrian  are 
still  called  Azoic,  but  the}'  are  separated  into  three  formations,  beginning 
w^ith  the  Cascade  as  the  oldest,  as  in  the  brochures  of  the  Duluth,  South 
Shore  and  Atlantic  Railway.  The  Cascade  formation  embraces  gneissoid 
granites,  basic  eruptives  and  schists,  jaspilites,  and  the  associated  iron 
ores  and  granites.  Next  follows  the  Republic  formation.  Beginning  Avith 
the  oldest  beds,  this  embraces  conglomerates,  breccias  and  conglomeratic 
schists,  quartzite,  dolomite,  jaspilite  and  associated  iron  ores,  argillite,  schists* 
granite,  felsite,  diabase,  diorite,  peridotite,  and  porphyrite.  In  view  of  dis- 
coveries made  by  the  State  survey  of  Michigan  and  by  the  United  States 
Geological  Survey  regarding  the  jaspilites,  the  author  is  inclined  to  give 
up  his  former  view  of  the  eruptive  origin  of  these  rocks  and  their  associated 
ores.     The  newest  Azoic  formation  is  the  Holyoke  (the  Keweenawan  is 


13G 


THE   MARQUETTE   IRON-BEARTXG   DISTRICT. 


regarded  as  Cambrian).  This  comprehends  conglomerates,  breccias,  con- 
glomeratic schists,  quartzites,  dolomite,  argillite,  graywacke  and  graywacke- 
schist,  granite,  felsite  (?),  diabase,  diorite,  peridotite,  porodite,  serpentine, 
and  melaphyr  or  picrite. 

Wadsworth,  M.  E.  Subdivisions  of  the  Azoic  Arcbaan  in  Northern  Michigan. 
Science,  December  23,  1802,  page  355. 

In  this  article  the  author  further  subdivides  the  "Azoic,"  adding  two 
new  formations  to  those  recognized  in  the  preceding  paper.  The  table  he 
publishes  shows  the  supposed  equivalency  between  his  own  formations  and 
those  proposed  by  Van  Hise. 


U.MTED  STATES  GEOLOGICAL  SITRVET. 


Azoic  or  Archean  system :  i 

Laureutian  (?)  period Cascade  formation. . 

.    ,  (■  Republic  formation . 

period J I 

I   Mesnard  formation  . 

J   Holyoke  formation  . 

I  Negaimee  formation 


Michigan  period. 


Fundamental  Complex. 
Lower  Marquette  series. 

•Upper  Marquette  series. 


Gkesley,  W.  S.  a  hitherto  undescribed  phenomenon  in  hiematite.  Am. 
Geologist,  Vol.  IX,  1802,  pages  219-223. 

Grresley  describes  a  specimen  of  fibrous  red  hematite  from  the  Lake 
Superior  region  that  seems  to  consist  of  fragments  of  botryoidal  masses. 
In  the  midst  of  the  masses  are  cavities,  around  which  the  fibers  sometimes 
curve.  No  theory  is  proposed  to  account  for  these.  It  is  suggested,  hoAv- 
ever,  that  hematite,  in  "growing,"  encountered  obstructions  that  have  since 
been  removed. 

1893. 

Wadsworth,  M.  B.  Report  of  the  State  Geologist  for  1891-92.  State  Board 
of  Geol.  Surv.  for  the  years  1801  and  1892,  Lansing,  1893,  pages  61-73.  Signed 
October  17,  1802. 

In  this  paper  the  author  gives  an  account  of  the  work  done  under  his 
direction  by  the  Michigan  survey  during  the  years  1891  and  1892.  The 
discovery  of  the  conglomeratic  base  of  the  Republic  formation,  resting 


GEOLOGICAL   EXPLORATIONS    AND    LITERATURE— 1893.         137 

upon  the  old  gneisses  and  gi-anites  of  Cascade  age,  south  of  the  Wintlirop 
mine,  is  i-eported.  The  quartzites  of  Teal  Lake  are  described  as  probaljly 
belonging  with  the  Holyoke  formation,  because  their  basal  portion  is  n  tliin 
conglomerate  resting  upon  chloritic  and  sericitic  schists  that  resemble  cer- 
tain of  the  schists  in  the  Republic  formation.  This  quartzite  is  unconform- 
ably  above  the  marbles  or  dolomites  of  the  eastern  portion  of  the  Mar- 
quette district,  which  in  turn  are  conformably  above  the  Mount  Mesnard 
quartzite.  There  are  thus  two  quartzites  here,  one  above  and  the  other 
below  the  marble. 

The  marbles  and  the  underlying  quartzites  appear  to  l)c  unconform- 
ably  above  the  Republic  formation,  which,  according  to  the  author,  includes 
sericite-schists  and  green  schists,  and  at  tlie  same  time  to  be  below  the 
Holyoki'  formation.  This  series  of  rocks  is  known  jirovisionally  as  the 
Mesnard  formation.  One  of  the  difficulties  in  determining  the  exact  rela- 
tions of  the  Mesnard  rocks  to  the  neighboring  series  is  due  to  the  fact  that 
there  is  believed  to  exist  above  the  Holyoke  series,  and  unconformably 
above  it,  another  series  of  graywackes,  quartzites,  etc.,  which  is  designated 
the  Negauuee  formation.  The  existence  of  the  Mesnard  and  the  Negaunee 
formations  is  not  proved  beyond  doubt,  but  it  is  thought  to  be  probable. 

The  sequence  in  the  Marquette  district  is  thought  to  be  that  indicated 
in  the  scheme  published  in  Science  the  preceding  year. 

Wadsworth,  M.  E.  A  sketch  of  the  geology  of  the  iron,  gold,  and  copper 
districts  of  Michigan.     Ibid.,  pages  75-155.     Dated  March  26,  1892. 

The  details  upon  which  the  conclusions  of  the  preceding  article  are 
based  are  given  in  this  article  in  the  same  report. 

The  author  begins  this  paper  with  a  discussion  of  the  means  l)y  which 
the  various  Marquette  rocks  were  produced,  and  gives  a  table  of  rock 
classification.  In  these  preliminary  remarks  he  refers  to  the  green  schists 
north  of  Teal  Lake  and  those  near  Marquette  as  altered  or  metamorphosed 
detrital  deposits,  in  which  class  he  also  places  many  of  tlie  chlorite-schists 
of  the  iron  mines,  the  griinerite-schists  and  mica-schists  in  the  western 
portion  of  the  area,  and  certain  ottrelite-schists  near  Palmer,  on  the  Cascade 


138  THE   MARQUETTE   IRON  BEARING   DISTRICT. 

The  Cascade  formation  shows  best  its  relations  to  the  yonnger  forma- 
tions near  the  Volunteer  mine,  on  the  Cascade  range. 

Here  one  finds  an  old  horiibleudic  scliist  tliat  Las  been  invaded  by  eruptive 
granite  and  otber  volcanic  rocks.  *  *  *  It  is  not  impossible  that  tbe  borublendic 
schist  may  be  an  altered  eruptive  or  volcanic  rock,  instead  of  being  a  sedimentary 
one,  although  the  evidence  thus  far  obtained  points  to  the  latter  origin. 

Near  the  mine  the  Republic  formation,  with  a  basal  conglomerate,  may 
be  seen  reposing  unconformably  upon  the  Cascade  rocks,  and  imconform- 
ably  above  these  may  be  seen  the  Holyoke  rocks. 

The  rocks  comprising  the  Cascade  formation  have  already  been  men- 
tioned. Some  of  them  are  detrital  accumulations  derived  either  from  an 
older  formation  or  from  some  volcanic  source.  It  may  be  that  the  Cascade 
series  should  be  divided  into  two  parts,  since  it  appears  that  in  a  few  areas 
some  of  the  gneisses  are  eruptive  in  origin  instead  of  fragmental,  as  is 
the  case  with  most  of  the  Cascade  gneisses,  which  are  younger  than  the 
-  eruptive  gneisses  and  contain  fragments  of  them.  All  these  gneisses  are 
cut  by  granites  and  by  basic  dikes. 

The  composition  of  the  Republic  formation  has  already  been  outlined. 
In  it  are  most  of  the  jaspilites  of  the  district.  These,  it  is  stated,  may  still  be 
regarded  as  eruptive  at  Islipeming  and  Negaunee,  though  elsewhere  they 
seem  to  be  sedimentary.  The  phenomena  formerly  supposed  to  prove  the 
eruptive  origin  of  the  jaspilites  and  ores  are  shown  to  be  explained  by 
the  eruptive  nature  of  the  green  schists  with  which  they  are  in  contact. 

The  sedimentary  origin  of  the  jaspilites  is  plainly  shown  in  the  Cascade 
range,  south  of  Palmer,  where  these  rocks  are  interlaminated  with  quai-tz- 
ites,  often  in  thin  beds.  Some  of  the  beds  contain  fragments  of  banded 
jaspilite,  which  would  indicate  that  there  is  an  occurrence  of  this  rock  some- 
where beneath  the  Republic  formation.  Moreover,  most  of  the  jaspilites 
interlaminated  with  the  quartzites  "appear  to  be  composed  of  a  fine 
jaspilite  mud  derived  from  the  jaspilitic  debris." 

The  belief  in  the  existence  of  jaspilite  in  the  Cascade  formation  rests 
upon  the  evidence  just  given;  but  the  author  thinks  it  possible  that  the 
jaspilite  fragments  in  tlie  conglomerates  may  have  come  from  veins  in 
the  older  rocks,  althougli  these  are  rarely  observed. 


GEOLOGICAL    EXPLOEATIONS   AXD    LITERATURE— 180;3.         139 

The  Republic  formation  as  a  whole  is  separated  from  the  Cascade 
formation  below  and  from  the  Holyoke  formation  above  Ija'  cono'lomerates 
and  unconformities.  The  conglomerates  are  well  known  at  ;i  iiumber 
of  places.  Objections  are  raised  to  the  acceptance  of  Irviui^'s  tlicory  of 
the  orig'in  of  the  ores  from  nn  oriti'inal  fcn-u<i'inous  carljonatc,  since  it  is 
believed  that  the  carbonate  is  itself  secondar^'  and  that  the  ])henomc-na  of 
alteration  reported  are  "due  to  surface  action  on  tlie  iron-l)earing-  schists." 

ilore  detailed  accoun.ts  of  the  erupti^'e  rocks  cutting-  the  Republic 
formation  are  given  in  tliis  re})ort  than  in  the  al)stracts  already  referred  to. 
Granites,  felsites,  diabases,  diorites,  and  jiorphyrites  are  all  recognized 
as  intrusive  in  the  Re23ublic  rocks.  Many  of  the  green  schists  of  the 
formation  are  thought  to  be  eruptive,  though  it  appears  that  some  of  them 
are  still  regai-ded  as  sedimentary. 

The  jjorodites,  or  volcanic  ashes,  are  common  in  both  the  Republic  and 
the  Holyoke  formations,  though  "more  apt  to  be  seen  in  tlie  former." 

In  treating  of  the  Holyoke  formation  Wadsworth  refers  to  his  fonner 
belief  that  Brooks's  quartzite  tongue  in  the  iron  formation  at  Republic  is 
an  eruptive  greisen.  He  now  corrects  this  earlier  statement  and  acknowl- 
edges the  rock  to  be  quartzite,  but  supposes  the  iron-schists  below  and  aVx  )ve 
this  tongue  to  be  of  diiferent  ages.  The  former  he  regards  as  Republic  and 
the  latter  as  Holyoke.  At  this  place  and  at  several  other  localities  there 
were  oliserved  in  the  Repid)lic  rocks  fissures  filled  with  Holyoke  sediments. 
These  the  author  proposes  to  call  clasolites. 

Very  little  information  is  given  with  respect  to  the  rocks  of  the  Holyoke 
formation  other  than  that  contained  in  previous  j)apers. 

The  Marquette  and  Ishpeming  sei'pentines  are  believed  to  be  i)roved 
the  youngest  of  the  large  intrusive  masses  in  the  district,  and  to  be 
younger  than  any  of  the  dike  .rocks  except  the  yovingest  diabases  and  the 
melaphyrs. 

Under  the  heading  of  chemical  deposits  the  author  jdaces  the  soft  iron 
ores  of  the  district  and  the  quartz  veins,  some  of  which  carry  auriferous 
pyrite  and  native  gold. 

Chapter  IX  of  the  report  contains  a  description  and  an  analysis 
of  a  new  fibrous   mineral  from  the  Chamjjion   mine,  which  A.  C.    Lane 


140  THE    MARQUETTE    IRON-BEAKINa   DISTRICT. 

calls  l)eaconite,  and  also  analyses  of  a  cliloritoid,  an  amphil)()le,  and  a 
clay.  A  list  of  minerals  occurring-  in  the  Up})er  Peninsula  concludes  the 
chapter. 

Lane,  A.  C.  Microscopic  characters  of  rocks  and  minerals  of  Michigan. 
Rept.  of  State  Board  of  Geol.  Surv.  for  1891-92,  Lansing,  1893,  pages  176-183. 

In  an  a])pendix  to  Wadsworth's  report  Lane  describes  very  briefly  the 
microscopic  characters  of  some  of  the  most  interesting  rocks  of  the  Marquette 
district. 

Among  these  the  class  of  the  quartz-diabases  is  mentioned  as  well 
characterized  by  the  presence  of  quartz,  sometimes,  indeed,  only  in  small 
quantity.  The  rocks  are  the  youngest  eniptives  of  the  district.  They 
occupy  well-defined  dike  fissures,  and  have  altered  the  slates  adjacent  to 
them.  Their  quartzose  component  is  iisually  in  the  interstices  between  the 
diabasic  constituents,  and  is  frequently  intergi-own  with  plagioclase.  It  is 
regarded  as  original.  The  rocks  are  present  in  some  of  the  mines,  and  in 
these  cases  they  seem  to  have  aided  in  guiding  the  aqueous  currents  active 
in  producing  the  ore,  but  they  are  different  from  the  miners'  "diorites,"  and 
are  younger  than  these. 

The  amphibolites  and  hornblende-schists  of  the  author  include  the 
"diorites"  referred  to.  They  are  all  secondary  rocks,  having  been  derived, 
in  all  probability,  from  diabases.  In  the  alteration  of  the  latter  rocks  into 
the  former  ones  iron  oxides  were  removed  and  were  condensed  into  ore 
bodies.  Transitions  from  chlorite-schists,  which  are  often  but  marginal 
forms  of  the  amphibolites,  into  ore  bodies  can  be  seen  at  the  Champion 
mine,  the  magnetite  replacing  the  eruptive  rock  in  places.  The  author 
ascribes  to  some  of  the  ores  of  the  district  this  origin,  but  excludes  the  soft 
ores  from  the  category. 

Patton,  H.  B.  Microscopic  study  of  some  Michigan  rocks.  Ibid.,  pages 
184-180. 

In  another  appendix  Patton  deals  with  the  microscopic  features  of 
some  of  the  rocks  referred  to  in  Wadsworth's  report.  Aplitic  diorites  are 
mentioned  as  occurring  in  the  SE.  \  sec.  14,  T.  48  N.,  R.  25  W.,  and  on  the 
Middle  Picnic  Island. 


GEOLOGICAL    EXPLORATIONS    AND    LITERATURE— 1S!)3.         141 

BiRKiNUiNE,  John.  Iron  ores.  Mineral  Resources  of  the  United  States,  cal- 
endar year  1891,  Washington,  1893,  pages  10-46. 

We  find  in  this  paper  an  abstract  <,)!"  the  reports  of  C.  D.  Lawton, 
commissioner  of  mineral  statistics  for  Michigan,  exphiining  the  occurrence 
of  the  ores  in  the  Marquette  district.  The  rocks  associated  witli  the  ores 
are  stated  to  l)e  buik  up  of  the  detritus  of  tlie  Laurentian  rocks.  The  ore 
formation  is  fohled,  the  ore  bodies  l)eing-  found  in  the  trouf^hs  of  the  folds, 
with  a  hanging  wall  of  quartzite,  which  is  often  sejjarated  from  'the  ore  l)y 
"soap  rock"  (p.  17). 

Van  Hise,  C.  R.  The  succession  in  the  Marquette  iron  district  of  Michigan. 
Bull.  Geol.  Soc.  Am..  Vol.  V,  1893,  pages  5-6. 


Tlie  first  announcement  of  results  reached  by  tlic  Lake  Superior 
division  of  the  United  States  Geological  Survey  in  the  detailed  examina- 
tion of  the  Marquette  area  was  made  in  18!>3  by  Prof  C.  R.  Van  Hise,  in 
a  slioi't  paper  read  at  the  Madison  meeting  of  the  Geological  Society  of 
America. 

The  oldest  group  of  the  district  is  called  the  Basement  Complex,  follow- 
ing Irving's  suggestion.  It  consists  of  granite,  gneisses,  hornblende-schists 
and  mica-schists,  green  schists,  and  greenstone-conglomerates,  which  appear 
to  be  surface  volcanics,  in  part  lavas  and  in  part  tuffs.  The  schists  are 
intruded  by  granite  and  gneissoid  granite.  No  sedimentary  rocks  are  known 
in  the  group. 

Unconformably  on  this  group  is  the  Lower  Marquette,  consisting,  in 
ascending  order,  of  conglomerates  and  quartzites,  and  the  iron  formation, 
including  actinolite-magnetite-schists,  ferruginous  cherts,  jaspers,  etc. 

The  Upper  Marquette  rests  upon  any  one  of  the  hjwer  formations. 
Broadly  speaking,  it  is  a  shale,  mica-slate,  and  nnca-schist  formation, 
although  at  its  base  are  often  found  quartzites  and  conglomerates.  Where 
the  underlying  rock  is  the  iron-bearing-  formation  of  the  Lower  ^larquette 
the  basal  member  of  the  u])per  series  consists  of  a  recomposed  iron-bearing 
formation,  which  may,  in  consequence  of  secondary  concentration,  afford 
Avorkable  ore  bodies.  Another  iron-ore  horizon  occurs  at  from  several  hun- 
dred tn  1,0()0  feet  above  the  base  of  the  series.  This  formation  is  not  much 
unlike  the  corresponding  formation  in  the  loAver  series. 


142  THE   MARQUETTE    IKOX-BEAllING   DISTRICT. 

In  the  eastern  portion  of  the  district  is  the  Mesnard  formation  of  the 
Michigan  survey.  In  this  the  succession  comprises:  (1)  A  lower  quartzite 
and  conglomerate;  (2)  a  dolomite  interstratified  with  slates  and  cherty 
quartzites;  and  (3)  an  upper  quartzite.  The  true  position  of  this  forma- 
tion is  not  yet  determined. 

Grreat  intrusive  dikes  and  bosses  of  altered  diabase  liave  broken  through 
the  Marquette  series  and  have  been  the  causes  of  minor  folding  in  them. 
In  the  upper  series  is  also  an  extensive  area  of  contemjDoraneous  volcanics, 
largely  tuffs,  running  from  north  of  the  Saginaw  mine  to  Champion.  The 
locus  of  the  ancient  volcano  was  southeast  of  Clarksburg.  In  passing  east 
and  west  from  this  center  more  and  more  waterwom  sediment  is  found 
intermingled  with  the  volcanic  debris,  until  finally  the  rocks  pass  into  the 
ordinary  sediments  of  the  district. 

WiNCHELL,  N.  H.  Field  observations  of  N.  H.  Wincbell  in  1892.  Twenty-first 
Ann.  Rept.  Geol.  and  Nat.  Hist.  Svirv.  of  Minnesota  for  1892,  Minneapolis,  1893, 
pages  80-99. 

A  few  hastily  taken  field  notes  and  a  statement  claiu^ing  priority  in  the 
discovery  of  the  existence  of  two  Huronian  formations  in  the  Marquette 
district  comprise  the  only  portions  of  N.  H.  Winchell's  Twenty-first  Annual 
Report  that  deal  with  the  area  under  discussion. 

The  Republic  mine  is  placed  by  the  author  in  the  Keewatin  formation. 
With  respect  to  the  second  part  of  the  article,  it  will  be  sufiicient  to  relate 
that  Winchell  appeals  to  his  discovery  of  the  conglomerate  in  the  Saginaw 
mine  in  1888  as  evidence  that  he  recognized  the  existence  of  two  ore- 
bearing  members  of  the  Huronian  in  the  Michigan  iron  district.  He  omits 
reference  to  the  essential  fact  that  he  placed  the  rocks  above  the  con- 
glomerate in  the  Potsdam.  The  Saginaw  deposits  are  now  placed  in  the 
Taconic.  There  accompany  the  article  several  sketches  of  the  contacts 
between  the  conglomerate  and  the  underlying  iron  formation  as  the  author 
saw  them  in  the  Saginaw  and  Goodrich  mines. 

Winchell,  Horace  V.    Historieal  sketch  of  the  discovery  of  inineial  deposits 
in  the  Lake  Superior  region.     Proceedings  of  the  Lake  Su]ieiior  Mining  Institute, 
19-22. 


In  this  article  Horace  V.  Winchell  recounts,  among  other  things,  the 
incidents  leading  to  the  discovery  of  the  ore  deposits  of  the  Jackson,  Cleve- 
land, Republic,  and  other  mines. 


GEOLOGICAL   EXPLORATIONS   AND   LITEEATIJIIE— 189.!.         143 

Smyth,  II.  L.  A  contact  between  the  Lower  Huroniaii  and  tlie  underlying 
granite  in  the  Republic  trough,  near  Republic,  Michigan.  Jour,  of  Geol.,  Vol.  T, 
1893,  pages  208-274. 

Smyth  describes  as  existing  south  of  the  Rei)ubhc  mine,  in  tlic  Ix-nd 
of  the  horseshoe  made  by  the  outcroj^pings  of  tlie  iron-bearing  rocks, 
the  fii-st  proved  unconformity  between  what  is  regarded  as  the  h)west 
meml)er  of  the  Lower  Huronian  series  in  the  Marquette  district  and  the 
underlying  Basement  Complex.  The  existence  of  this  unconformity  was 
inferred  by  Brooks  from  the  fact  that  the  strike  of  the  (piartzites  and  actin- 
olite-schists  near  this  place  runs  directly  across  the  foliation  in  the  neigh- 
boring granite.  Smyth  discovered  the  actual  contact  of  the  two  series, 
and  found  the  basal  member  of  the  overlying  one  to  be  a  coarse  conglom- 
erate containing  large  bowlders  of  the  same  granite  as  that  below  the 
contact.  Between  the  conglomerate  and  the  granite  is  a  thin  liand  of  a 
schistose  rock,  in  all  j^i'obability  representing  a  sheared  portion  of  the 
granite  or  of  the  conglomerate. 

Van  Hise,  O.  R.  An  historical  sketch  of  the  Lake  Superior  region  to  Cambrian 
time.     Jour,  of  Geol.,  Vol.  I,  1893,  pages  113-128.     With  map. 

,  Much  of  the  information  imparted  in  this  summary  statement  of  the 
knowledge  concerning  the  relations  of  the  pre-Cambrian  formations  to  one 
another  in  the  Lake  Superior  region  is  contained  also  in  the  correlation 
essay  on  the  Archean  and  Algonkian,  already  referred  to.  There  are 
described,  however,  a  few  additional  facts  of  detail  that  are  of  interest. 

The  Lower  Huronian  is  now  said  to  be  composed  of  three  members, 
instead  of  the  two  recognized  in  earlier  papers.  In  ascending  order  they 
are  as  follows:  (1)  Conglomerates  and  quartzites;  (2)  limestone  and  chert; 
(3)  the  iron-bearing  formation.  These  tlii'ee  members  are  not  often  seen 
in  a  single  section,  in  consequence,  in  some  cases  at  least,  of  the  entire 
absence  of  one  or  the  other  of  them.  Basic  eruptive  rocks  are  also 
abundant  in  the  Lower  Huronian,  and  acid  eruptives  occur  not  infrequently. 

At  the  end  of  Lower  Huronian  time  the  Lake  Superior  region  was 
raised  above  the  sea,  folded,  and  subjected  to  erosion,  and  the  Upper 
Huronian  sediments  were  deposited  upon  the  Lower  Huronian  ones.  Like 
the  Lower  series,  the  Upper  series  consists  also  of  three  formations,  which  are 


144  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

all  less  crystalline  than  the  three  formations  of  the  Lower  series.  These  are: 
(1)  A  lower  slate,  passing-  locally  into  a  quartzite  or  conglomerate,  (2)  an 
iron-bearing  formation,  and  (3)  an  upper  slate  member.  Where  the  lowest 
member  of  this  series  rests' immediately  upon  the  Lower  Huroniau,  the 
underlying  member  may  be  any  one  of  the  tlu-ee  older  formations,  and 
the  character  of  the  overlying  conglomerate  varies  accordingly. 

Volcanics,  as  distinguished  from  intrusive  eruptive  rocks,  occur  inter- 
laminated  with  the  Upper  Huronian  sediments.  "In  the  Michigamme 
iron  district  is  an  extensive  area  of  greenstones,  greenstone-conglomerates, 
agglomerates,  and  surface  lava-ilows,  many  of  which  are  amygdaloidal" 
(p.  121). 

At  the  end  of  Upper  Huronian  deposition  the  land  was  again  raised 
above  the  sea,  and  after  the  rocks  had  been  folded,  gently  as  a  rule,  but 
intensely  locally,  the  atmospheric  agents  once  more  began  their  work  of 
cutting  them  down.  The  land  was  then  again  submerged,  and  after  some 
time  (during  which  elsewhere  the  Keweenawan  rocks  were  formed)  the 
Potsdam  sandstone  was  laid  down  upon  them. 


Smyth,  H.  L.  The.  quartzite  tonj^ue  at  Republic,  Michigan.  Jour,  of  Geol., 
Vol.  II,  1894,  pases  G80-691. 

The  subject  of  discussion  in  this  paper  is  the  origin  of  the  quartzite 
tongue  mapped  by  Brooks  as  penetrating  the  iron-bearing  formation  at 
Republic,  on  the  western  side  of  the  eastern  heel  of  the  horseshoe.  It  is 
this  quartzite  that  was  stated  by  Wadsworth  in  1880  to  be  an  eruptive 
greisen,  and  was  later  (in  1892)  determined  to  be  a  quartzite  whose  position 
between  two  portions  of  the  iron-bearing  formation  was  explained,  by 
supposing  that  the  rocks  on  the  different  sides  of  the  "tongue"  were  of 
different  ages — that  on  the  western  side  belonging  with  the  Holyoke  forma- 
tion, and  the  larger  eastern  mass  of  the  same  rocks  to  the  Republic  series. 
An  unconformity  was  shown  to  exist  by  Wadsworth  between  the  quartzite 
and  the  eastern  jaspilites  and  ores. 

Smyth  explains  the  phenomenon  as  due  to  a  fault  along  the  contact 
plane  between  the  quartzite  and  the  iron  formation,  which  is  also  a  plane 


GEOLOdlCAL   EXPLORATIONS   AND    LITERATURE— 1895.         J  45 

of  unconformity.  The  iron-bearing  rocks  on  both  sides  of  tlie  quartzite 
are  of  the  same  age,  and,  indeed,  are  portions  of  the  same  formation; 
consequently  there  are  not  two  ore  horizons  in  the  Reijubhc  area,  as 
Wadsworth  supposed. 

The  Repubhc  structure  is  described  as  a  syneUne  some  7  miles  long. 
Its  axis  runs  about  northwest,  and  is  nearly  horizontal,  exce})t  at  its  south- 
eastern end,  south  of  Smith  Bay,  where  it  dips  about  45°  to  the  nortliwest. 
The  rocks  are  thus  exposed  in  a  horseshoe-shaped  curve.  They  have  been 
squeezed  nearl}'  into  parallelism  on  the  two  sides  of  the  axial  plane,  the 
Lower  Marquette  rocks  dipping  a  little  more  steeply  than  those  belonging 
in  the  Upper  Marquette  series.  The  radius  of  the  curve  at  the  toe  of  the 
horseshoe,  measured  from  the  base  of  the  upper  quartzite,  can  be  very 
little  greater  than  the  thickness  of  that  formation.  The  pressure  caused 
by  this  sharp  folding  has  not  only  crushed  some  portions  of  the  more 
brittle  rocks  affected  by  it,  but  has  also  produced  three  synclines  and  two 
anticlines  subordinate  to  the  main  svncline. 


WiNCHELL,  N.  H.  The  origin  of  the  Arcbeau  greeustones.  Twenty-tliird 
Ann.  Rept.  Geol.  and  Nat.  Hist.  Surv.  of  Minnesota,  1895,  pages  4-35. 

N.  H.  Winchell  criticises  Williams's  work  in  the  greenstone-schist  areas 
of  the  Marquette  and  Menominee  districts,  but  adds  nothing  to  our  knowl- 
edge concerning  them.  The  author  seems  to  believe  that  Williams  had 
concluded  that  the  greenstone-schists  of  the  Marquette  district  are  mainly 
squeezed  irruptive  rocks,  whereas  the  strong  point  of  his  paper  is  the  doc- 
trine that  they  Avere  originally  basic  tuffs  and  surface  lava  flows.  After 
discussing  the  jiroblem  somewhat  at  length,  Winchell  reaches  very  nearly 
the  same  conclusion  as  does  Williams,  i.  e.,  he  concludes  that  the  greenstone- 
schists  are  mainly  altered  tuffs  of  basic  rocks.  He  does  not  believe,  how- 
ever, that  they  readied  their  present  condition  through  the  action  of 
dynamic  metamorphism,  but  the  processes  by  Avhich  they  have  beco,me 
schists  are  not  clearly  set  forth. 

The  author  furthermore  ascribes  to  the  schists  a  definite  horizon  at 
the  close  of  the  Archean,  and  places  under  them  an  iron-bearing  formation, 
MON  xxviii 10 


146  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

chlorite-schists,  clay-slates,  graywackes,  conglomerates,  and  quartzites.  He 
also  states  that  the  greenstone  knobs  in  the  neighborhood  of  Negaunee  are 
outliers  of  the  greenstone-schists,  older  than  the  iron-formation  rocks  by 
which  they  are  surrounded;  but  he  cites  no  proofs  of  the  correctness  of  this 
statement. 

Dana,  J.  D.  Manual  of  geology.  Fourth  edition.  American  Book  Company, 
1895. 

Professor  Dana,  in  the  last  edition  of  his  Manual,  declines  to  recog- 
nize the  Algonkian  system  as  distinct  from  Archean.  He  places  the  Mar- 
quette ores  in  the  Huronian  as  recognized  by  Logan  and  Murray,  and 
accepts  the  conclusions  of  Irving  and  Van  Hise  as  to  their  origin  from  a 
carbonate  by  metamorj^hism  (pp.  449-450). 

RoMiNGER,  C.  Geological  report  on  the  Upper  Peninsula  of  Michigan,  exhib- 
iting the  progress  of  work  from  1881  to  1884.  Iron  and  copper  regions.  Geol.  Surv, 
of  Michigan,  Vol.  V,  Lansing,  1895,  pages  1-91. 

This  is  the  long-delayed  report  by  Rominger  on  the  iron  and  copper 
districts  of  Michigan,  to  which  reference  has  already  been  made  several 
times.  In  justice  to  its  author  it  should  be  stated  that  the  work  upon  which 
it  is  based  was  done  in  the  years  1881  and  1882,  and  the  manuscript  was 
completed  in  May,  1885,  but  the  publication  was  deferred  for  the  reason, 
given  in  Rominger's  own  words,  that  "  the  description  of  all  the  results 
obtained  comprises  the  space  of  about  50  or  60  printed  pages,  too  small  for 
a  separate  publication  in  book  form."  It  was  not  until  the  fall  of  1893  that 
enough  additional  material  had  been  accumulated  by  the  Michigan  survey 
in  proper  shape  to  be  incorporated  with  Rominger's  report  and  make  a 
volume  of  sufficient  size  to  warrant  its  publication  by  the  State  board  of 
geological  survey. 

The  report  is  a  continuation  of  that  of  1881.  Some  of  the  conclusions 
reached  in  it  are  different  from  those  reached  in  the  previous  report,  but  on 
the  whole  the  author  maintains  his  original  position  with  resj^ect  to  most 
of  the  disputed  points.  He  still  declines  to  regard  the  Marquette  rocks 
as  comprising  portions  of  two  distinct  series,  as  ' '  no  tenable  line  of 
demarcation  between  an  older  Laurentian  and  a  younger  Huronian  group 


GEOLOGICAL   EXPLORATIONS   AND   LITEKATUEE— 1895.         147 

unconformably  deposited  on  the  first  could  be  observed"  (p.  1).  The 
entire  series  of  rocks  observed  in  the  district  is  classed  as  "  Huroniau." 

The  granites  and  gneisses  of  the  lowermost  portion  of  the  series  are 
now  thought  to  have  the  characters  of  eruptive  rather  than  of  sedimentary- 
rocks.  A  solid  crust  of  granite  served  as  the  substratum  on  which  the 
Huronian  beds  were  laid  down,  as  is  proved  by  the  occurrence  of  belts  of 
granitic  conglomerate  and  breccia  in  different  horizons  of  the  series. 

Two  occurrences  of  these  conglomerates  associated  with  the  granites 
are  particularly  described,  ^nz,  that  in  the  SE.  J  sec.  22,  T.  47  N.,  R.  26  W., 
and  that  in  the  N.  |  sec.  29,  T.  48  N.,  R.  25  W.  The  first  locality  furnishes 
excellent  proof  of  the  correctness  of  the  author's  conclusion.  Here  are 
several  knobs  with  nuclei  of  massive  granite  surrounded  by  mantles  of 
coarse  breccia,  made  of  fragments  of  granite  in  a  cement  composed  of  well- 
laminated  quartzose  material,  and  around  these  are  hydi'omica-slates  inter- 
bedded  with  heavy  belts  of  compact  quartzite,  which  are  conformable 
with  the  breccia.  At  the  second  locality  the  conglomerate  is  interlami- 
nated  with  diorite-schists,  and  is  at  some  distance  from  any  known  outcrops 
of  granite. 

In  the  former  report,  the  author  described  a  gradation  of  the  granite 
into  quartzite.  The  outcrops  thought  to  show  this  gradation  were  again 
visited.  The  gradation  rocks  are  now  regarded  as  a  mixture  of  gi-anite 
fragments  and  of  sand  material. 

Granite  was  upheaved,  according  to  the  author,  and  was  intruded  into 
the  overlying  strata  near  the  close  of  Huronian  time,  since  it  is  found  in 
contact  with  all  the  Huronian  strata  up  to  the  youngest,  though  intrusive 
belts  of  it  are  rarely  found  higher  than  the  iron-bearing  formation.  More- 
over, the  beds  in  contact  with  the  granite  are  always  dislocated.  The 
dislocation,  however,  is  not  always  due  to  the  upheaval  and  intimsion  of 
the  granite.  It  has  been  caused  in  part  by  diorites  and  diabases,  which 
intersect  the  granite  as  well  as  the  incumbent  beds. 

The  greenstones  intersecting  the  granite  are  identical  with  those 
intersecting  the  schists  of  the  Huronian  series,  and  with  those  interlami- 
nated  with  these  rocks.  Their  massive  forms  gi-ade  into  greenstone-schists. 
This  fact  led  the  author  in  his  previous  report  to  regard  the  diorites  as 


148  THE   MARQUETTE    IKON  BEARING   DISTRICT. 

fused  sediments,  representing  the  lowermost  beds  in  the  sedimentary  suc- 
cession. In  the  second  report  he  does  not  look  upon  them  as  belonging 
with  the  sediments,  but  believes  them  always  to  be  later  erujitives.  Of 
the  two  kinds  of  greenstone  the  diabases  are  regarded  as  the  younger. 
They  are  often  found  cutting  the  diorites.  The  latter  rocks,  on  the  con- 
trary, are  old  rocks.  Although  in  a  few  cases  they  were  observed  to 
contain  the  remnants  of  decomposed  augite,  the  greater  portion  of  them 
are  thought  to  be  original  rather  than  secondary  rocks  (uralitic  diabases) 
derived  from  diabases  by  alteration.  In  this  respect  the  author  differs 
from  those  geologists  who  are  inclined  to  look  upon  all  the  diorites  of  the 
district  as  altered  diabases. 

As  the  result  of  closer  investigation,  an  iron-bearing  horizon  was 
discovered  in  the  arenaceous  slate  group,  so  that  the  author  now  recognizes 
two  ore  horizons  in  the  Marquette  district  instead  of  one  only,  as  originally 
was  the  case. 

HoBBS,  W.  H.  Mineralogical  notes.  Am.  Jour.  Sci.  (3),  Vol.  L,  1895,  pages 
121-128. 

2.  Barite  .and  nianganite  from  the  Lucy  mine,  Negaunee,  Michigan,  pages 
123-125. 

3.  Chloritoid  from  blocks  on  the  south  shore  of  Michigamme  Lake,  Michigamme, 
Michigan,  pages  12.5-127. 

The  first  of  these  two  notes  is  purely  mineralogical.  The  two  min- 
erals described  are  from  the  Lucy  mine,  in  the  SW.  5  sec.  6,  T.  47  N., 
R.  26  W. 

In  the  second  note  the  author  calls  attention  to  the  existence  of  a 
chloritoid  with  the  composition  of  masonite  in  specimens  of  a  phyllite- 
schist  picked  up  on  tlie  south  shore  of  Lake  Michigamme.  The  rock  is  not 
known  in  situ.  The  chloritoid  is  the  same  as  that  described  by  Lane, 
Keller,  and  Sharpless  in  189L 


CHAPTEK   II. 


By  W.  S.  Bayley. 


THE  BASP:MENT  CH)MPLKX. 

Relow  tli(^  Alyoukiaii  dejjofsits  of  the  Manjuette  area  are  schistose  and 
massive  ])liases  of  crvstalliiie  and  pvroelastic  roeks,  so  diflferent  from  the 
Algonkian  sediments  that  thcn-e  is  rareK'  am'  diftieulty  in  distinguishing 
between  tliem  and  the  chistic  roeks  al)ove  tliem.  These  inferior  rocks  are 
unconformald\'  below  the  hiwest  members  of  the  Manjnette  series.  It  is 
probable  that  they  embrace  members  of  widely  different  ages,  but  up  to 
the  present  time  no  separation  of  the  schists  into  sharply  defined  subgroups 
upon  the  basis  of  age  has  been  attempted,  because  of  the  complexity  of  the 
relations  existing  between  the  various  rock  types,  due  largely  to  the  many 
vicissitudes  through  which  they  have  passed  and  the  cons^cpient  alterations 
to  which  they  have  been  subjected. 

Divisions  corresponding  to  the  Laurentian  and  Grrenville  series  of 
Canada,  as  defined  by  Adams, ^  and  even  the  lithological  ones,  Laurentian 
and  Mareniscan,  proposed  by  Van  Hise,"  are  not  clearly  distinguishable  in 
all  of  the  district  that  has  been  studied.  But  the  work  on  the  jjre-Cambrian 
areas  is  as  yet  far  from  complete.  A  more  careful  investigation  of  large 
areas  will  probabU'  show  that,  in  a  general  sense,  such  broad  distinctions 
as  those  recognized  in  the  terms  "Laurentian"  and  "Mareniscan"  do  exist 
in  the  Michigan  "Archean."  The  j^resent  study  of  the  Marquette  district 
was   primarily  directed   to   the  Algonkian   series.     In   the    pre- Algonkian 

'A  further  contribution  to  our  knowleilge  of  tlii^  Laurentian,  by  F.  P.  Adams:  Aiu.  Jour.  Sci. 
3a  teries,  Vol.  L,  1895,  p.  58. 

-'Correlation  papers— Archuan  an<l  Algoukian  :   Bull.  V.  S.  Geol.  Survey  No.  86, 1892,  pp.  488-4;:0. 


150  THE   MAEQUETTE   lEON-BEARINCx    DISTRICT. 

part  of  the  district  it  has  been  prosecuted  only  carefully  enough  to  insure 
accurate  mapping  of  the  main  areas. 

In  consequence  of  the  widespread  occurrence  of  the  g-ranites  and 
schists  beneath  the  sedimentary  formations,  and  the  complexity  of  their 
structural  relations,  the  term  "Basement  Complex,"  first  proposed  by 
Ir^^ng,  has  been  used  to  designate  them,  as  indicating  that  they  constitute 
the  basement  upon  which  the  Marquette  sediments  were  laid  down. 

This  Basement  Complex  in  the  Mai-quette  district,  as  elsewhere  in  the 
Lake  Superior  region,  consists  of  a  series  of  acid  and  basic  schists,  cut  by 
veins  and  dikes  of  granite  and  other  acid  rocks,  dikes  of  basic  eruptives, 
and  bosses  of  acid,  intermediate,  and  basic  materials.  The  whole  comprises 
a  confusing  mass  of  crystalline  rocks,  the  relations  between  which  may 
sometimes  be  discovered,  but  which  more  frequently  are  not  decipherable. 
So  different  are  these  rocks  from  the  members  of  the  Marquette  series  in 
appearance,  structure,  and  composition  that  even  where  there  is  no  apparent 
structural  break  between  the  two  series,  on  lithological  grounds  alone  they 
would  naturally  be  regarded  as  of  different  ages,  or  at  least  as  having-  been 
prodviced  under  very  different  conditions.  It  is  partly  for  the  jiurpose  of 
contrasting  their  characteristics  with  those  of  the  Algonkian  rocks  that  the 
members  of  the  Basement  Complex  are  here  described. 

The  Marquette  rocks  are  bounded  both  to  the  north  and  to  the  south 
by  ai-eas  of  the  Basement  Complex.  The  northern  area  differs  somewhat 
from  the  southern  one  in  the  nature  of  its  rocks,  and  therefore  the  two  are 
discussed  separately.  In  addition  to  these  two  large  areas,  there  are  smaller 
ones  tliat  are  entirely  surrounded  by  the  Algonkian  beds,  like  islands  in  a 
sea  of  rocks.  In  these  areas  the  rocks  are  not  materially  different  from 
those  of  the  larger  ai'eas,  but  for  the  sake  of  clearness  in  })icturing-  the 
structure  of  the  Marquette  range  they  will  be  referred  to  separately  and 
described  briefly. 

SECTION   I.     THE   :NrORTHEE>r  COMPLEX. 

Throughout  nearly  its  whole  extent,  from  Lake  Superior  as  far  west  as 
beyond  Lake  Michigamme,  the  Marquette  series  is  limited  on  the  north  by 
a  belt  of  crystalline  and  pvroclastic  rocks,  cut  by  basic,  interiiiediate,  and 


THE   NORTHERN   COMPLEX.  151 

acid  dikes  and  bosses  and  by  granite  and  quartz  veins.  Near  Lake  Superior 
the  two  series  are  separated  by  a  small  area  without  outcrops,  except  occa- 
sional ledg-es  of  Potsdam  sandstone.  Whatever  rocks  may  underlie  this 
area,  they  are  buried  deep  beneath  Pleistocene  sands  and  gravels.  Else- 
where the  exposures  of  the  two  series  are  close  together,  and  at  many 
places  actual  contacts  may  be  seen. 

The  rocks  comprising  the  Northern  Complex  are  gneissoid  granites, 
syenites,  greenstone -tuffs,  greenstone -conglomerates,  greenstone  -  schists, 
peridotites,  aplites,  vein  granites,  diabases,  diorites,  and  epidiorites.  The 
first  five  are  at  many  places  highly  foliated,  while  the  last  six  are  massive, 
or  but  slightly  schistose.  The  former  occupy  the  greater  portion  of  the 
belt,  so  far  as  it  is  within  the  limit  of  the  map  (Atlas  Sheet  IV).  They 
are  older  than  the  latter,  which  cut  them  in  the  form  of  dikes  and  bosses. 

The  foliated  rocks  occupy  areas  whose  boundaries  are  not  so  Avell 
defined  as  is  the  case  with  the  Marquette  fragmentals,  since  the  schists 
and  the  granitoid  gneisses  and  syenites  gradually  pass  into  one  another, 
through  the  intrusion  of  the  basic  rocks  by  dikes  and  veins  of  the  acid 
ones.  Nevertheless,  an  attempt  has  been  made  to  map  these  areas.  In 
their  interiors  the  different  phases  of  schists,  granites,  and  syenites  are  well 
characterized,  but  on  their  peripheries  there  is  always  a  complex  mixture 
of  the  various  schists  with  one  another  or  with  the  granitic  rocks.  The 
respective  colors  on  the  map  are  believed  to  cover  the  areas  within  which 
the  corresponding  rocks  predominate  largely  over  other  rocks.  The  bound- 
ary lines  separating  the  different  areas  are  drawn  at  about  the  places  where 
the  different  varieties  are  found  in  approximately  equal  quantities. 

The  greenstone-schists  include  two  classes.  The  rocks  of  the  first  class 
are  nonconglomeratic  green  schists.  These  are  called  the  Mona  schists, 
because  good  exposures  are  found  on  hills  of  this  name  southwest  of 
Marquette.  Those  of  the  second  class  contain  pebble-like  bodies,  and 
these  are  discussed  separately.  Their  best  development  is  on  the  Kitclii 
Hills,  in  the  neighborhood  of  Deer  Lake,  northwest  of  Ishpeming,  and 
hence  they  are  called  the  Kitchi  schists.  The}^  have  frequently  been 
referred  to  in  the  literature  on  the  district  as  the  Deer  Lake  conglomerates 
or  agglomerates. 


152  THE   MARQUETTE    IKON-BEARING    DISTRICT. 


THE    MONA    SCHISTS. 

The  Mona  schists  embrace  green  and  gray  fibrous  rocks  with  a  well- 
characterized  schistosity,  dense  greenish-gray  ones  that  are  not  pronouncedly 
schistose,  and  highly  foliated  dark-green  ones  with  the  aspect  of  hornblende- 
schists.  These  are  interbanded  with  one  another,  and  with  certain  light- 
pink,  yellow,  or  white  talcose  and  sericite-schists,  described  later  under  the 
heading  "acid  schists."  The  green  schists  are  all  composed  of  the  altered 
constituents  of  diabases.  They  are  probably  all  derived  from  the  lava  or 
the  tuff  form  of  this  rock.  They  are  referred  to  as  greenstone-schists  to 
distinguisli  them  from  the  true  amphibole-schists,  which  are  composed 
essentially  of  amphibole  and  quartz,  and  which  are  without  the  distinctive 
features  of  an  altered  eruptive,  whatever  their  origin  may  have  been. 

DISTRIBUTION    AND    TOPOGRAPHY. 

The  Mona  schists,  with  the  associated  acid  schists,  occupy  the  eastern 
portion  of  tlie  Northern  Comjilex,  extending  westward  from  Lake  Superior 
on  the  east  to  about  the  west  line  of  R.  26  W.,  where  they  are  replaced 
by  the  Kitchi  schists.  In  its  eastern  p(jrtion  the  area  stretches  northward 
a  mile  beyond  the  northern  limit  of  the  accompanying  map  (Atlas  Sheet  IV) 
to  a  great  area  of  gneissoid  granite,  which  is  similar  in  its  characteristics  to 
the  granites  farther  west.  A  mile  west  of  the  east  line  of  R.  26  W.  the 
belt  narrows  and  has  a  width  of  only  IJ  or  2  miles.  Here  it  is  bounded 
on  the  north  by  a  narrow  belt  of  coarse  red  syenite,  lying  partly  within 
the  limits  of  the  map.  On  the  south  the  schists  are  in  contact  with  the 
transgression  quartzite  of  the  Lower  Marquette  series  throughout  its  entire 
extent,  except  toward  the  east,  where  they  pass  beneath  the  Pleistocene 
deposits  bordering  the  lake. 

The  topography  of  the  area  is,  in  a  minor  way,  rugged  in  the  extreme. 
Large  and  small  hills  of  the  schists  rise  with  rough,  precipitous  faces  above 
the  level  of  the  surrounding  country,  and  lift  their  smooth,  glaciated  heads 
from  200  to  900  feet  above  the  level  of  the  waters  of  Lake  Superior.  When 
the  hills  are  low  their  tops  only  project  as  smooth,  round  knobs  above  the 
drift   deposits    surrounding    them  (PI.  IV,  fig.  1).     The    higher    hills    are 


■I&.   1. -GREENSTONE-SCHIST   KNOB,   DEAD  RIVER. 


Fig.  2.— river  COURSE  THROUGH   MONA  SCHIST, 


THE   MONA   SCHISTS.  153 

composed  of  groups  of  these  knobs,  raised  high  above  the  valleys  between 
them.  Their  sides  are  ragged  and  rough,  or  smooth  and  vertical,  and  their 
tops  are  rounded  knolls 

The  streams  flowing  over  the  schists  have  not  yet  succeeded  in  trenching 
their  channels  to  any  considerable  depth.  Their  cotn-ses  are  marked  by 
rapids  and  cascades,  over  which  the  waters  tumble  in  a  senes  of  low  falls. 
The  rocks  in  the  beds  of  the  streams  and  along  their  sides  are  usually  rough 
surfaced,  in  consequence  of  their  highly  developed  schistosity.  As  a  result, 
we  find  the  drainage  chaimels  through  the  greenstone-schists  presenting  an 
entirely  different  aspect  from  those  through  the  granite  areas  or  tlu'ough 
the  areas  underlain  by  the  Algonkian  rocks.  The  view  shown  in  PI.  IV, 
fig-.  2,  is  typical  for  the  larger  streams  through  this  district. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

It  has  already  been  stated  that  the  green  schists  pass  gradually  into 
the  gneissoid  granites  to  the  north  through  the  intrusion  of  the  former  Ijy 
apophyses  of  the  latter.      In  referring  to  this  contact  Williams  writes:^ 

There  is  no  such  shaiji  line  of  contact  as  is  represented  on  Roniiuger's  map,  but, 
on  the  contrary,  as  Rominger  liiniself  explains,  there  is  a  complete  interpenetration 
of  the  two  rock  masses.  The  granite  has  intruded  itself  into  the  schistose  greenstones, 
for  the  mo.st  part  following  their  bedding  and  forcing  apart  their  strata.  The  amount 
of  the  acid  rock  gradually  diminishes  as  we  go  southward. 

At  a  greater  or  less  distance  from  the  contact  the  granite  is  completely 
absent,  and  the  green  schists  occur  alone,  except  for  the  dikes  of  aplite  and 
diabase  that  everywhere  cut  through  them,  as  well  as  through  all  other 
members  of  the  Basement  Complex,  ai\d  the  narrow  Ijands  of  acid  schists 
with  which  they  are  interlaminated. 

The  stratigraphic  separation  of  the  Mona  schists  from  the  Kitchi 
formation  to  the  west  has  not  yet  been  possible.  In  passing  from  the  former 
area  into  the  latter,  beds  of  the  conglomeratic  schists  are  found  more  and 
more  frequently  between  those  of  the  nonconglomeratic  kinds.  As  we 
shall  see  later,  many  of  the  Mona  schists  are  probably  altered  tuffs,  while 

'  The  greenstoue-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  G.  H. 
Williams:  Bull.  U.  S.  Geol.  Survey  No.  62,  1890,  p.  146. 


154  THE   MARQUETTE   IRON-BE AEING   DISTRICT. 

others  are  squeezed  basic  lavas.  A  sharp  line  of  demarcation  between 
these  rocks  and  the  tuffaceous  greenstones  of  the  Kitchi  formation  to  the 
west  is  therefore  not  to  be  expected,  for  the  former  are  probably  only  much 
metamorphosed  phases  of  rocks  like  the  latter. 

From  the  Algonkian  beds  to  the  south  the  schists  are  separated  by 
conglomerates  and  great  unconformities.  In  the  conglomerates  large 
bowlders  of  tlie  schists  are  often  found;  consequently  there  can  be  no 
question  but  that  the  latter  rocks  were  consolidated  and  had  been  made 
schistose  before  the  basement  beds  of  the  overlying  clastic  series  were  laid 
down  upon  them. 

The  Mona  schists  are  therefore  jjre-Algonkian.  They  are  older  than 
the  granites  of  the  Basement  Complex,  and  are  of  about  the  same  age 
as  the  rocks  of  the  Kitchi  formation,  which  are  probably  their  Avestem 
equivalents. 

PETKOGBAPHICAL    CHARACTER. 

The  structure  of  the  schists  varies  within  wide  limits.  In  some  places 
the  rocks  are  very  fine  grained  and  as  fissile  as  slates;  at  others  they  are 
coarser-grained,  fibrous,  and  distinctly  foliated;  again  they  may  be  A-ery 
coarse  grained  and  fibrous  btit  possessed  of  only  indistinct  foliation ;  and, 
finally,  they  may  be  dense  and  apparently  quite  massive.  In  the  latter 
case  they  always  yield  to  fracture  much  more  readily  in  one  direction 
than  in  others,  and  in  thin  section  under  the  microscope  they  are  seen  to 
have  a  schistose  structure.  The  schistosity  of  all  the  well-foliated  varieties 
dips  at  high  angles,  and  strikes  nearly  east  and  west,  approximately  parallel 
to  the  trend  of  the  Marquette  trough. 

Dr.  Williams,  who  has  studied  the  rocks  of  this  area  in  detail, 
divides  the  eastern  portion  of  the  area  into  a  northern  and  a  southern 
half,  in  the  former  of  which  banded  schists  prcA'ail;  in  tlie  latter,  apha- 
nitic  varieties.  In  neither  half,  howe\'er,  are  the  rocks  of  either  A^ariety 
excluded  by  those  of  the  other.  Farther  Avest  the  dense  and  the  banded 
fibrous  schists  are  associated  in  the  most  intimate  manner. 

BASIC   SCHISTS. 

The  dense  varieties. — The  apliauitic  scliists  as  a  rule  have  a  light-green 
color,  sometimes  shading  to  grayish  or  pinkish  green,  and  a  uniformly  fine 


THE  MONA   SCHISTS.  155 

grain.  Occasionally  their  texture  is  so  fine  that  hand  specimens  resemble 
greenish  cherts  in  appearance,  or  massive  graywacke-like  sediments.  In 
the  ledge  the  rocks  present  a  i-udely  schistose  structure,  which  is  lost  in  the 
specimens.  In  some  exposures,  as  in  the  knob  in  the  NE.  ^  NW.  \  sec. 
28,  T.  48  N.,  R.  26  W.  (Atlas  Sheet  XXX),  the  rock  is  divided  Int..  ..val 
or  lenticular  masses,  separated  from  one  another  by  schistose  material  of 
the  same  nature  as  that  composing  the  oval  masses,  but  of  much  finer  grain. 
This  structure,  as  has  been  pointed  out  by  Williams,  is  neither  concretionary 
nor  agglomeratic.  It  is  similar  to  the  structure  of  certain  Saxon  schists 
which  Rothpletz  has  shown  to  be  mechanical  in  origin. 

In  thin  section  the  aphanitic  schists  are  found  to  be  nearh'  as  uniform 
in  composition  as  they  are  in  appearance.  They  consist  of  granular 
epidote,  small  flakes  and  needles  of  chlorite  and  hornblende,  and  altered 
plagioclase,  with  the  addition  usually  of  calcite,  leucoxene,  a  little  quartz, 
and  mosaic  areas  of  albite  and  quartz.  The  plagioclase  may  sometimes  be 
detected  in  small  lath-shaped  crystals,  lying  in  all  azimuths  amidst  the 
other  components,  but  more  frequently  the  mineral  is  so  much  decomposed 
that  its  original  form  can  no  longer  be  recognized.  The  epidote  grains  are 
usually  scattered  through  the  slide.  Not  infrequently,  however,  thev  are 
aggregated  into  little  groups  with  the  cross-sections  of  feldspars.  The 
plates  and  needles  of  chlorite  and  the  needles  of  hornblende,  whicli  are 
rather  abundant  in  some  sections  of  the  rocks,  are  quite  small.  They 
are  intermingled  with  a  few  sericite  flakes,  a  little  calcite,  and  small  areas 
of  the  clear  mosiac  already  referred  to.  Usually  these  constituents  inclose 
the  leucoxene  and  the  altered  plagioclase  crystals  in  the  same  way  as 
glass  incloses  the  crystal  components  in  a  glassy  basalt.  In  other  cases  the 
use  of  crossed  nicols  brings  out  an  arrangement  of  the  various  constituents 
in  such  a  way  as  to  resemble  the  structure  of  fine-grained  diabases,  and 
even  of  gabbros.  In  still  other  instances,  in  the  apparently  heterogeneous 
aggregate  of  components,  under  crossed  nicols  a  structure  resembling  that 
of  tuff  is  discerned..  Broken  pieces  of  altered  plagioclase  are  discovered 
in  a  fine-grained  matrix  with  no  well-defined  structure.  In  composition 
the  greenstones  are  altered  diabases  or  basalts,  and  their  structure,  when 
discernible,  is  either  that  of  basic  lavas  or  that  of  tuff's. 


156  THE   MARQUETTE    IRON-BEARIXC    DISTRICT. 

The  scliistosity  of  these  greenstones,  which  is  seen  both  in  the  ledge 
and  to  a  Hmited  degree  in  thin  section,  where  the  chlorite  spicules  are  found 
to  lie  with  their  long  axes  in  a  uniform  direction,  is  explained  best  as  a 
result  of  movement,  as  Williams  has  ali-eady  observed.  This  geologist 
declares  that  the  study  of  these  pale-green  aphanitic  greenstones  seems  to 
indicate  that  they  were  not  originally  to  any  great  extent  tuif  deposits,  but 
that  they  were  massive  flows  of  diabase,  which  have  since  suffered  profound 
cheraical  and  structural  changes,  in  consequence  of  having  been  subjected  to 
intense  dynamic  action.  Perhaps  the  greater  portions  of  these  dense  green- 
stones were  originally  lava  flows,  as  suggested  by  Williams.  A  large  portion 
of  them,  however,  were  tuff  deposits.  The  significant  fact  in  connection 
with  them  is  that  the}'  were  all  surface  materials. 

Many  of  the  dense  schists  have  been  weathered  until  they  now  consist 
largely  of  calcite  and  epidote,  so  that  no  evidence  of  their  original  character 
remains. 

The  banded  varieties. — Tlic  baudcd  sclilsts,  bcst  cxposcd  lu  tlie  northern 
porti(.)n  of  the  Mona  schist  area,  are  composed  of  alternate  laj^ers  of 
darker  and  lighter  shades  of  green,  giving  them  a  striped  appearance. 
Their  texture  is  much  coarser  than  that  of  the  aphanitic  greenstones 
described  above,  and  their  structure  is  chai-acteristically  schistose.  They 
all  contain  an  abundance  of  secondary  amphibole,  and  consequent]}'  the}^ 
are  all  more  or  less  fibrous.  Where  their  fibrosity  is  pronounced  and  their 
schistosity  marked  they  form  very  fissile  schists.  Where  the  scliistosity  is 
less  marked  the  rocks  may  still  be  fibrous,  but  the  fibers  are  grouped  around 
centers  scattered  through  the  specimen,  and  tlie  rock  has  the  aspect  of  a 
uralitized  diabase  or  gabbro. 

On  account  of  their  banded  character  these  schists  have  been  regarded 
as  sedimentary  by  nearly  all  geologists  who  have  studied  them.  "The 
alternation  \n  the  color  and  composition  of  the  layers  is  so  frequent  and  so 
constant,  and  their  parallelism  to  the  east  and  west  strike  of  all  the  rocks 
of  this  neighborhood  is  so  exact,"  writes  Williams,  "that  ]io  hypothesis  of 
their  originally  massive  character  will  satisfactorily  account  for  the  observed 
facts.  On  the  other  hand,  the  chemical  and  the  microscopical  characters  of 
these  schists  agree  closely  with  those  of    associated  massive  greenstones 


THE    MONA    SCHISTS.  157 

which  are  known  to  have  been  derived  by  the  alteration  of  basic  erujitive 
rocks." ' 

Normally  these  schists  show  in  thin  section  large  or  small  sheaf-like 
bundles  of  bluish-green  hornblende  scattered  through  the  slide  indiscrim- 
inately or  aggregated  intt)  groups  with  irregular  outlines  and  frayed  edges, 
and  embedded  in  a  groundmass  consisting  of  much  decomposed  plagio- 
clase  and  a  mosaic  of  colorless  grains  of  albite  and  (juartz.  With  the 
hornblende,  chlorite  is  frequently  associated,  the  areas  occupied  by  the 
two  minerals  sometimes  having  the  outlines  of  an  amijhibole  or  a  pyroxene 
crystal.  Much  leucoxene  is  observed  in  most  sections,  and  not  infrequently 
gramdar  epidote  is  intermingled  with  the  components  of  the  mosaic.  In 
addition  to  these  substances,  which  are  well  defined,  there  are  certain 
obscurely  outlined  plagioclases,  which  present  between  crossed  nicols  the 
shapes  of  sharp-edged  fragments.  In  none  of  the  slides  of  these  rocks  has 
anything  been  detected  that  may  be  regarded  as  a  waterworn  sand  grain. 

The  plagioclase,  whether  in  the  fragments  or  in  the  indefinite  areas 
that  serve  as  a  sort  of  groundmass  to  much  of  the  hornblende,  is  altered 
to  epidote,  sericite,  chlorite,  and  calcite.  The  mosaic  filling  the  interstices 
between  everything  else  is  in  all  probability  secondary,  as  it  not  infre- 
quently fills  little  veins  cutting  through  the  hornblende,  chlorite,  and  altered 
plagioclase.  This  mosaic  is  like  that  described  by  Lossen-  in  the  schistose 
diabases  of  the  eastern  Hurz,  which  have  been  shown  to  owe  their  foliation 
to  mashing. 

The  Marquette  banded  schists  show  the  structure  sometimes  of  massive 
rocks  and  sometimes  of  pyroclastic  ones,  but  more  frequently  they  exhibit 
no  structure  from  which  their  origin  can  be  inferred.  Their  composition, 
however,  is  that  of  diabases.  Their  field  aspects  are  very  diff"erent  from 
those  of  most  schistose  diabases.  The  rocks  are  banded,  like  sedimentary 
ones.  A  possible  explanation  of  these  opposite  sets  of  characters  is  that 
the  rocks  exhibiting  them  are  water-deposited  elastics  of  volcanic  origin, 

■The  greenstone-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by 
G.  H.  Williams :  Bull.  U.  S.  Geol.  Survey  No.  62, 1890,  p.  1.54. 

-Zeitschr.  Deutsch.  geol.  Gesell.,  Vol.  XXIV,  1872,  p.  730;  Jahrbuch  K.  preuss.  geol.  Lande.san- 
stalt,  188.3,  p.  640;  1884,  p.  528. 


158  THE   MARQUETTE   IliON-BEAKING   DISTRICT. 

like  the  tuffs  of  modern  volcanoes,  which  have  been  tilted  from  their 
original  position  and  have  been  rendered  schistose  by  mashing,  as  have 
also  many  of  the  dike  masses  that  intrude  them.  They  possess  many  of  the 
characteristics  of  dynamically  metamorphosed  tuffs,  and  others  due  to 
weathering  processes. 

other  varieties. — 111  additiou  to  thc  apliaiiitic  schists  and  the  banded  schists 
of  this  area,  there  are  three  other  phases  that  should  be  mentioned.  The 
first  phase  strongly  resembles  schistose  varieties  of  the  dike  rocks  to  which 
the  name  "  epidiorite "  is  often  given.  These  rocks  probably  represent  the 
coarser  lavas  that  were  associated  with  the  glassy  and  fine-grained  lavas 
and  the  tuffs  that  gave  rise  to  the  more  common  types  of  schist  in  the 
district. 

In  the  second  phase  the  structure  is  plainly  diabasic,  but  the  quan- 
tity of  hornblende  is  so  great  that  the  rocks  might  well  be  called 
amphibolites.  In  a  rock  from  about  200  paces  east  of  the  NW.  corner  of 
sec.  35,  T.  48  N.,  R.  27  W.  (Atlas  Sheet  XXVII),  for  instance,  the  hornblende 
is  very  abundant.  It  is  a  fibrous  variety,  consisting  of  long,  almost  color- 
less needles  or  thin  prisms  scattered  through  a  felt  of  greener  fibers,  the 
mass  forming  pseudomorphs  after  diabasic  augite.  Feldspar  is  not  abun- 
dant in  the  rock.  That  which  is  present  is  penetrated  by  needles  of  horn- 
blende and  spicules  of  chlorite  to  such  an  extent  that  its  characteristic 
features  are  often  almost  completely  obscured. 

The  third  phase  is  anore  nearly  like  the  true  crystalline  schists  than 
are  any  others  of  the  greenstone-schists.  This  is  the  least  common  type 
in  the  Mona  schist  area.  It  appears  to  be  confined  to  its  northeastern 
portion.  In  the  hand  specimen  the  rocks  of  this  type  are  dark-green  in 
color,  very  fine  in  grain,  and  extremely  schistose.  Under  the  microscope 
they  appear  to  be  much  fresher  than  the  other  green  schists.  They  are 
composed  almost  wholly  of  long,  narrow  prisms  and  needles  of  light-green 
hornblende,  lying  in  a  mass  of  tiny,  clear  grains  of  plagioclase,  which 
interlock  in  the  manner  of  the  grains  of  a  crystalline  schist.  Intermingled 
with  these  clear  feldspars  are  a  few  larger  grains  of  reddish,  altered  ones, 
clouded  by  inclusions  of  epidote,  kaolin,  and  sericite.  On  their  edges 
some  of  these  seem  to  be  passing  into  the  clearer,  fresher-looking  feldspar. 


ACID   SCHISTS    IN   THE   MONA    SCHIST   AREA.  159 

which  is  no  doubt  a  new  product,  derived  from  the  plagioclase  of  an  older 
rock.  Epidote  in  small,  almost  colorless  grains  is  quite  common  in  and 
between  the  new  plagioclase  particles,  sometimes  as  single  individuals, 
sometimes  as  clusters  of  grains  that  are  so  thickly  crowded  as  to  be 
almost  opaque.  Plates  of  the  common  yellow-green  epidote  are  occasion- 
ally met  with,  and  crystals  of  zoisite  are  common  in  the  altered  plagioclases 
of  some  sections.  As  a  rule,  ilmenite  and  leucoxene  are  not  so  widely 
spread  in  these  rocks  as  they  are  in  some  of  the  other  schists.  In  the 
schists  derived  from  dike  material  and  from  the  compact  and  coarse-grained 
lavas  leucoxene  is  abundantly  present,  whereas  in  the  banded  schists, 
supposed  to  be  altered  tuffs,  and  in  the  amphibole-schists,  it  is  uncommon. 
In  a  rock  from  1100  steps^  N.,  100  steps^  W.,  SE.  corner  of  sec.  2,  T.  48  N., 
R.  26  W.,  however,  the  section  is  sprinkled  with  little  black  particles  of 
ilmenite,  each  one  of  which  is  surrounded  by  a  rim  of  colorless  leucoxene. 
These  schists  are  like  those  described  by  Williams'  from  the  "Brook 
section"  west  of  Marquette.  Thus  far  they  have  been  found  only  in  the 
southern  halves  of  T.  48  N.,  R.  25  W.,  and  T.  48  N.,  R.  26  W.,  although 
they  no  doubt  exist  in  other  portions  of  the  northern  greenstone  area.  It 
is  impossible  at  present  to  decide  whether  these  schists  are  squeezed  tuffs 
or  squeezed  lavas,  but  they  are  no  doubt  mashed  rocks  derived  from  basic 
volcanic  material  of  some  kind. 

ACID   SCHISTS. 

In  a  number  of  places  within  the  area  of  the  Mona  schists  the  green 
schists  are  associated  with  light-colored  rocks  that  are  very  like  certain 
schistose  acid  dikes  that  cut  across  the  greenstones.  There  is  great  diffi- 
culty in  determining  whether  these  light  schists  were  derived  from  eruptive 
porphyries  or  from  their  tuffs.  In  many  instances  the  latter  is  supposed  to 
be  the  case.  The  larger  decomposed  fragments  that  lie  in  the  fine-grained 
groundmass  of  these  rocks  are  so  badly  shattered,  and  the  different  pieces 
near  together  fit  into  one  another  so  imperfectly,  that  it  would  seem  hardly 


'  In  this  volume  locations  will  freiiuently  be  given  from  the  southeast  corners  of  sections,  in  steps 
at  the  rate  of  2,000  per  mile. 

-The  ■rreenstone-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  G.  H. 
Williams :  Bull.  U.  S.  Geol.  Survey  No.  62,  1890,  pp.  156-157. 


160  THE    MAKQUETTE   IKON-BEARING   DISTRICT. 

possible  that  they  could  be  fragments  produced  by  the  crushing  of  crystals. 
Besides,  these  white  or  pink  schists  and  the  green  ones  occur  side  by  side 
in  the  same  ledge,  and  the  two  apparently  grade  into  each  other. 

In  many  of  the  sections  cut  from  the  acid  rocks  only  quartz  and  sericite, 
with  perhaps  a  little  epidote,  can  be  discovered.  The  three  minerals  form 
a  very  fine  grained  aggregate,  resembling  strongly  the  mosaic  of  many 
devitrified  rhyolites.  The  tiny  quartz  grains  are  separated  from  one  another 
by  flakes  of  sericite,  arranged  with  their  longer  axes  in  a  single  direction. 
At  present  the  rocks  are  sericite-schists.  In  a  few  of  them  obscure  traces 
of  feldspathic  fragments  may  be  detected  when  their  sections  are  examined 
with  low  powers  between  crossed  nicols,  but  from  most  of  them  every  trace 
of  fragmental  material  has  disappeared  and  the  rocks  are  now  thoroughly 
crystalline. 

Schists  like  these  have  been  described  by  Williams,^  who  regards  them 
as  metamorphosed  acid  tuft's.  They  may  possibly  have  been  acid  sheets 
interstratified  with  the  basic  lavas  and  tuffs  that  formed  the  greenstones, 
but  when  the  fact  is  considered  that  they  grade  imperceptibly  into  the  green 
schists  and  that  in  some  of  them  traces  of  fragments  may  be  recognized,  it 
seems  more  probable  that  they  were,  as  Williams  supposes,  originally  acid 
tuffs  which  have  been  altered  and  made  schistose  by  processes  similar  to 
those  that  changed  the  diabasic  lavas  and  tuffs  into  the  greenstone-schists. 

THE    KITCHI    SCHISTS. 

Many  of  the  green  schists  of  the  Northern  Complex  are  noticeable  for 
the  pebble-like  and  bowlder-like  bodies  scattered  through  them.  These 
fragments  stand  out  so  plainly  on  the  weathered  surfaces  of  the  exposures 
on  the  Kitchi  Hills  in  the  vicinity  of  Deer  Lake  (Atlas  Sheet  XXVII)  that 
they  may  be  seen  from  long  distances.  They  are  usually  so  well  rounded 
that  the  rock  containing  them  looks  very  much  like  a  sedimentary  con- 
glomerate. Indeed,  so  conglomeratic  are  their  features  that  they  have 
frequently  been  called  the  Deer  Lake  conglomerates.  (See  fig.  4.)  The 
rocks  are,   however,    j^lainly  basic  tuffs,   but  they  have   preserved  then- 

'The  greenstone-schist  ureas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  G.  H. 
Williaius:  Bull.  U.  S.  Geol.  Survey  No.  62,  1890,  p.  151 


THE    KITCei    SCHISTS. 


161 


tuffaceous  character  so  much  more  i)erfectly  than  liave  the  banded  varieties 
of  the  Mona  schists,  from  which  they  differ  also  in  composition,  that  they 
have  been  desio-uated  bv  the  distinctive  name  Kitchi  schists. 


DISTRIBUTION    AND    TOPOGRAPHY. 


The  Kitchi  scliists  occur  just  west  of  the  Moua  schist  area,  stretchino- 
from  the  east  hue  of  R.  27  W.  to  the  west  hue  of  sees.  25  and  36  in  T.  48  N., 
R.  28  W.,  with  a  width  varying-  from  2  miles  to  3^  miles  (Atlas  Sheet  IV). 
At  the  west  end  the  schists  are  in  contact  with  a  coarse  "-neissoid  "-ranite. 


L'^**:'  V 


% 


■S 


-v?JwW 


t,."^ 


Fig.  4.— Cliff  of  Kitclii  scliists,  in  sec.  33,  T.  48,  R.  27. 

Both  to  the  north  and  to  the  south  the  "  cong-lomerate "  area  is  bounded 
by  Algonkiau  deposits,  on  the  north  l)y  those  belong-ing-  in  the  Dead  River 
or  Silver  Lake  area,  and  on  the  south  b}-  those  of  the  Marquette  district. 
It  is  not  to  l)e  understood  that  these  conglomeratic  schists  occupy  this  area 
to  the  exclusion  of  all  other  rocks.  There  are  associated  with  the  conglom- 
eratic phases  many  greenstone-schists,  similar  to  those  farther  east,  in  which 
MON  xxviii 11 


162  THE  MARQUETTE   lllON-BEARIXG   DISTRICT. 

no  traces  of  a  fragmental  structure  can  be  detected  until  their  thin  sections 
ar,e  examined  microscopically,  and  also  some  light-colored  acid  schists, 
identical  in  all  their  features  with  those  among  the  Mona  schists.  All 
these  schists  are  cut  by  large  and  small  dikes  of  altered  diabase,  and  by 
a  few  acid  dikes.  But  the  conglomeratic  schists  are  the  predominant  ones, 
and  are  those  that  give  character  to  the  western  portion  of  the  green-schist 
area. 

The  topography  of  the  country  covered  by  these  rocks  is  not  very 
different  from  that  of  the  country  underlain  by  the  Mona  schists.  Isolated 
rounded  knobs  are  not  so  frequent  in  the  area  of  the  "conglomerates"  as  in 
that  of  the  Mona  schists,  but  the  larger  hills  have  the  same  character  in  both 
areas.  Drift  is  less  thick  in  this  district  than  in  that  of  the  Mona  schists,  the 
larger  hills  being  oftener  separated  from  one  another  by  swamp  lands  than 
by  drift  deposits. 

RELATIONS    TO    ADJACENT    KOCKS. 

The  relations  of  the  Kitehi  schists  to  the  altered  tuifs  of  the  Mona 
schists  have  already  been  described.  The  relations  of  the  Kitehi  schists  to 
the  granite  on  the  west  are  observalile  north  of  the  west  end  of  a  pond 
in  the  SW.  1  sec.  26,  T.  48  N.,  R.  28  W.  (Atlas  Sheet  XXI),  where  they 
seem  to  be  the  same  as  the  relations  existing  between  the  Mona  schists  and 
the  granite  north  of  these  rocks.  In  passing  from  the  schist  to  the  granite, 
dikes  of  the  latter  rock  first  appear  in  the  fonner;  then  the  granite  gradually 
becomes  predominant,  schist  layers  being  interlaminated  with  the  dikes  or 
anastomosing  through  the  granite  in  an  irregular  manner.  Passing  into  the 
granite  the  schists  are  fonnd  included  in  it  as  angular  blocks,  and  finally 
the  massive  rock  appears  completely  free  from  the  schistose  one.  The 
granite  is  therefore  clearly  intrusive  in  the  Kitehi  schists.  The  sedi- 
mentary rocks  north  and  south  of  the  schists  repose  unconformably  upon 
the  latter,  from  which  they  are  separated  by  true  conglomerates. 

PETROGRAPHICAL    CHARACTER. 
BASIC   SCHISTS. 

Macroscopicai.— Irving,  in  his  introduction  to  Williams's  article,  already 
repeatedly  referred  to,  describes  the  Kitehi  schists  as  greenish  schists,  v.dth 


THE   KITOIU   SCUISTS.  163 

a  tendency  to  schistose  structure  that  is  never  very  pronounced,  but  which 
varies  considerably  in  its  deg-ree  of  development.  The  rocks  contain 
pebble-like  bodies  varying-  in  size  from  2  feet  in  diameter  down  to  minute 
fragments. 

Occasionally  tliese  appear  t;»  b.*  well  roundel,  but  more  coinmonlv 
they  are  subangular  and  flattened  in  a  direction  parallel  to  the  schistosity 
planes  in  the  inclosing  rock.  On  exposed  surfaces  the  "  pebbles "  stand 
out  by  virtue  of  their  whiter  weathering.  (See  hg.  4,  p  l(il.)  On  a  fresh 
fracture  they  are  not  nearly  so  apparent,  but  seem  to  differ  from  the  rest 
of  the  rock  by  their  finer  grain  and  their  pinkish  or  greenish  color,  the 
body  of  the  rock  having  usually  a  dark  greenish-gra}'  tint. 

This  description  applies  well  to  the  exposures  along  the  Deer  Lake 
road.  Elsewhere  the  "pebbles"  are  more  connnonly  rounded  than  angu- 
lar. Many  of  them  are  as  rounded  as  the  waterworn  pebbles  of  a  modern 
beach.  (See  PI.  V.)  In  many  places  they  may  be  seen  disposed  in 
bands  of  diffei-ent  widths  that  run  parallel  to  the  schistosity  of  the  rock, 
whose  strike  is  about  east  and  west  and  whose  dij)  is  at  some  high  angle 
to  the  south.  Between  these  bands  are  others  from  which  the  "pebbles" 
are  absent,  or  in  which  they  are  very  scarce.  These  noncouglomeratic 
beds  are  like  the  matrix  of  the  conglomeratic  ones,  except  that  there  are 
scattered  through  the  rock  small,  light-colored  grains  of  feldspar.  These 
are  often  mashed  into  lenses,  or  even  into  thin,  sheet-like  layers,  running 
parallel  to  the  planes  of  schistosity,  when  the  rock  presents  somewhat  of  a 
gneissic  aspect.  In  these  rocks  sharp-edged  fragments  of  plagioclase  may 
not  infrequently  be  detected  in  the  midst  of  a  fine,  satiny  groundmass  of 
chlorite  plates  and  calcite  grains,  entirely  different  in  character  from  the 
matrix  of  any  schistose  sedimentary  rock  met  with  in  the  district.  The 
rocks  are  evidently  basic  tuffs. 

Another  variety  of  the  schistose  tuff  occurs  most  commonly  near  the 
edges  of  the  area,  especially  at  the  contacts  with  the  Marquette  beds.  In 
the  field  notes  the  rock  is  called  a  sericite-schist.  It  is  a  pink  to  white, 
platy,  and  schistose  rock,  with  a  very  pronounced  soapy  feel.  On  surfaces 
of  the  hand  specimens  that  are  at  right  angles  to  the  schistosity  sharp 
jiarticles  of  different  minerals  are  to  be  seen,  but  on  the  surfaces  parallel 


164  THE   MAKQUETTE   lliOi^  BEAEINU  DISTKIOT. 

to  the  foliation  the  rock  appears  to  be  a  typical  sericite-schist  or  talcose 
schist. 

Except  for  the  arrangement  of  the  pebble-like  masses  in  bands,  there 
is  little  in  the  macroscopic  appearance  of  these  rocks  that  resembles  the 
bedding-  of  water-deposited  materials.  In  two  or  three  hand  specimens  a 
fine  banding  was  detected,  a  slight  difference  in  tint  between  the  alternate 
layers  leading  to  their  recognition,  bnt  this  is  all. 

Microscopical. — Tlie  conglomcratic  green  schists  are  so  much  decomposed 
that  it  is  difficult  to  learn  from  their  thin  sections  much  more  concerning 
their  original  character  than  can  be  learned  from  their  study  in  the  field. 

The  pebble-like  masses  scattered  through  the  schists  are  fragments  of 
a  purplish-pink,  fine-grained  rock,  speckled  with  tiny  green  dots  of  chlorite 
and  red  or  white  ones  of  altered  plagioclase.  Enoug-h  of  the  feldspar 
remains  to  exhibit  traces  of  twinning  bars,  although  most  of  it  has  been 
replaced  by  sericite,  calcite,  and  quartz.  These  plagioclases  were  origi- 
nally well-outlined  crystals.  They  lie  in  a  groundmass  composed  of  small 
feldspar  laths,  grains  of  epidote,  and  a  weakly  polarizing  felsitic  substance 
that  is  probably  a  devitrified  glass.  The  rock  of  which  the  pebbles  Avere 
a  part  was  probably  a  porphyrite,  unlike  anything  that  has  yet  been  found 
in  place  within  the  limits  of  the  district. 

The  schistose  groundmass  in  which  the  pebbles  are  embedded  consists 
of  sharply  angular  fragments  and  complete  crystals  of  altered  plagioclase 
in  a  matrix  composed  of  much  chlorite  and  sericite,  small  fragments  and 
crystals  of  plagioclase,  always  some  calcite,  and  a  fine-grained  mosaic  of 
secondary  quartz.  To  these  is  sometimes  added  epidote  in  grains  and 
plates.  It  is  noteworthy  that  in  these  rocks  chlorite  has  replaced  the 
original  iron-bearing  silicate,  while  in  the  tuffs  of  the  Mona  schists  to 
:the  east  these  silicates  are  now  represented  by  hornblende.  Whether  this 
■difference  in  composition  is  due  to  differences  in  the  nature  of  the  alteration 
processes  to  which  the  different  rocks  have  been  subjected,  or  to  the  fact 
that  the  rocks  of  the  greenstone-conglomerate  area  have  suffered  the  effects 
of  weathering  to  a  greater  extent  than  the  Mona  schists,  is  not  certain.  It 
seems  most  probable,  however,  that  both  causes  are  resj)onsible  for  the 
differences. 


THE   KITOIII   SCHISTS. 


165 


The  larger  feldspar  pieces  in  the  groundiuass  have  heen  referred  to  as 
crystals  and  fragments  of  crystals.  In  most  cases  tlie}'  are  iniquestionably 
fragments.  In  many  instances  tlie  pieces  lying  close  together  are  dissevered 
portions  of  the  same  crystal,  fractured  across  at  riglit  angles  to  the  i)lanes 
of  schistosity  in  the  rock.  Tlui  fragments  tlnis  formed  have  heen  moved 
apart  iu  the  planes  of  foliation.  The  fracturing  and  the  mo\ements  of  the 
fragments  are  believed  to  be  an  effect  of  mashing,  -which  is  shown  also  in 
the  rock's  foliation.     Many  fragments  are  ragged  in  outline.      They  do  not 


Fig.  5 — Outlines  of  iilagioclasi>  firaiiis  in  noniongloiiiiTiitii-  l.auil  <if  Kit.-lii  siliist. 

correspond  with  other  fragments  in  their  vicinity-,  nor  Iv.we  the}'  the  straight- 
edged  contours  of  the  fractured  crystals.  These  are  like  the  fraginents  in 
modem  tuffs.  They  are  in  all  probability  pieces  of  crystals  blown  from 
volcanic  vents  (fig.  5). 

The  chlorite  in  the  groundmass  is  in  small  prisms  and  needle.s,  arranged 
usually  in  approximately  parallel  directions,  forming  narrow  bands,  which 
may  be  laminae  in  some  places  and  in  others  may  expand  into  comparatively 
large  lenticular  masses.      Chlorite  occurs  also  in  oval  and  irregular  areas,. 


166  THE   MARQUETTE   lEON-BEARING   DISTKICT. 

formed  by  the  agg-reg-atiou  of  its  plates  intermingled  with  grains  of  epidote, 
of  calcite,  and  of  great  numbers  of  rutile  crystals  that  are  usually  included 
within  the  chlorite.  As  Williams  has  suggested,  these  areas  may  represent 
the  basic  fragments  of  the  tuffs  that  have  been  altered  beyond  recognition. 
The  sericite  is  as  abundant  as  the  chlorite  in  some  specimens,  \vhile  in 
others  it  is  present  in  small  quantity  only.  It  appears  as  colorless  or  light- 
green  flakes  that  lie  scattered  among  the  other  constituents,  especially 
between  the  chlorite  prisms,  and  as  tiny  spicules  that  penetrate  the  grains 
of  the  mosaic  matrix.  Of  the  calcite  nothing  need  be  said,  except  that  it 
occurs  as  nests  or  as  grains  between  the  other  minerals.  In  some  of  the 
more  compact  tuffs  a  second  carbonate  is  sometimes  met  with  in  the  form 
of  small  rhombohedra.  The  substance  is  more  opaque  than  the  calcite, 
and  is  of  a  yellow  or  vellowish-bro\'\ai  color.  It  is  probably  aidcerite  or 
ferruginous  dolomite. 

The  matrix  in  which  all  the  components  of  the  groundmass  lie  resem- 
bles very  closely  the  silicified  background  of  many  aporhyolites.  Under 
very  high  powers  of  the  microscope  it  is  seen  to  be  made  up  of  intricately 
interlocking,  colorless  grains.  Their  aggregate  polarizes  like  a  fine-grained 
quartz  mosaic.  No  twinning  bars  were  seen  in  an}-  of  the  grains  examined, 
but  many  grains  exhibit  an  undulatory  extinction.  This  groundmass  is 
therefore  regarded  as  quartzose.  Some  of  the  quartz  may  have  been  derived 
from  the  original  constituents  of  the  tuff  by  alteration,  but  most  of  it,  in 
all  probability,  has  been  introduced  from  without.  The  rocks  have  evidently 
been  silicified,  for  Ave  find  ledges  cut  through  and  through  by  quartz  veins, 
and  in  the  microscopic  section  these  veins  can  be  followed  as  they  break 
up  into  smaller  and  smaller  ones,  until  their  ramifications  are  finally  lost 
in  the  mosaic  above  mentioned. 

The  evidences  of  pressure  resulting  in  mashing  are  not  so  abundant  in 
these  schists  as  thcA'  are  in  some  of  the  other  rocks  of  the  district,  it  niay 
be  for  the  reason  that  many  of  its  effects  are  obscured  b}'  the  secondary 
substances  produced  subsequent  to  its  action.  Many  of  the  larger  feld- 
spar particles  are  broken  and  their  fragments  are  displaced  laterally;  the 
longer  axes  of  all  the  fragments  are  often  approximatel}'  in  the  same  2)lane; 


THE    KITCIII    SCHISTS.  167 

the  little  cliloritr  rods  are  arranged  likewise  -witli  tlieii-  loiiii-cr  axes  in  the 
same  direction,  and  the  lenticular  ai-eas  of  the  mineral  arc  ('lonfrat('(l  in 
the  same  wav.  Around  tlie  fragments,  large  and  small,  the  lamin;e  of 
chlorite  and  sericite  hend,  and  occasionallv  one  of  the  hn-ger  gi-ains  of  the 
mosaic  matrix  exhibits  undulatory  extinction.  These  ai-e  the  only  evidences 
of  the  action  of  pressure  within  the  schists,  l)ut  tliey  arc  sufKcient  to  show 
that  the  rocks  luive  suffered  mashing.  AVhere  the  schistosity  is  greatest 
the  quantitv  of  chlorite  and  sericite  present  in  the  rock  is  greatest;  where 
the  foliation  is  scarceh'  discernible  there  may  be  j)resent  nnich  chlorite, 
but  little  sericite. 

In  addition  to  the  tuffaceous  schists  in  the  Kitchi  formation,  there  are 
others  in  which  no  fragmental  material  can  be  detected.  In  the  held  these 
rocks  were  taken  for  the  more  massive  phases  of  the  tuffaceous  schists. 
Under  the  microscope,  however,  a  diffei'ence  in  the  structure  of  the  rocks 
is  detected.  The  feldspars  of  the  nontuffaceous  bands  are  in  crystals,  and 
not  in  fragments  of  crystals,  and  the  association  of  feldspar  and  chlorite 
approaches  the  ophitic  association  of  the  plagioclase  and  augite  in  dial)ase, 
or  the  association  of  feldspar  laths  and  glass  in  certain  feldspatliic  liasalts. 
In  composition  the  fragmental  and  the  nonfragmental  schists  are  identical. 
In  all  probabilitv  these  bands  represent  the  lava  flows  or  sheets  that  accom- 
panied the  extravasation  and  dei)osition  of  the  tufts.  Whetlier  they  were 
siu-face  flows  or  intrusive  sheets  can  not  now  be  told,  for  all  the  hner  detail 
of  their  structure  has  disappeared. 


The  sericite-schists — most  abundantly  developed  on  the  south  side  of 
the  Kitchi  area,  in  contact  with  the  Marquette  beds,  l)ut  occurring  also 
elsewhere,  interbedded  with  the  basic  schists — in  thin  section  are  essentially 
similar  to  the  green  schistose  rocks  interbedded  with  the  conglomerates. 
They  differ  principally  in  having  the  sericite,  ratlier  than  the  chlorite,  as 
their  principal  micaceous  component.  Many  of  these  light-colored  schists 
are  composed  almost  exclusively  of  sericite  and  (piartz,  but  most  of  them 
contain  chlorite  also.  By  increase  in  the  ]iroportit)n  of  chlorite  in  them 
they  pass  gradually  into  the  green  tuff's. 


168 


THE   MARQUETTE    IROI^BEARING   DISTRICT. 


ORIGIN    OF    THE    KITCHI    SCHISTS. 

Tlie  Kitclii  schists  are  evidently  frag-mental  deposits.  Their  compo- 
sition, ho^^^ever,  is  so  different  from  that  of  water-made  sediments  that  ^\e 
must  ascribe  some  other  origin  to  them.  The  banding-  of  certain  of  the 
conglomerates  and  the  alternation  of  layers  in  some  of  the  finer-grained 
varieties  '\\-(iuld  indicate  that  tliere  was  a  partial  sorting  of  the  fragments, 
but  the  nature  of  the  fragments  themselves,  and  the  composition  of  the 
matrix  in  which  they  lie,  would  seem  to  preclude  the  notion  that  the 
materials  were  furnished  by  the  wasting  of  preexisting  rocks. 

In  one  or  two  of  the  several  hundred  sections  examined  roundish 
quartz  grains  were  observed,  but  these  are  so  very  rare  that  they  can  afford 
no  basis  for  a  theory  of  origin  of  the  rocks  containing-  tliem. 

The  composition  of  one  of  the  green  schists  (I)  and  of  one  of  the  sericite- 
schists  (II)  associated  Avith  them  is  given  below.  The  first  is  composed 
largely  of  plagioclase,  chlorite,  and  quartz;  the  second  consists  principally 
of  sericite  and  quartz.  Both  analyses  were  made  by  George  Steiger  in  the 
Surve}^  laboratory. 

Analyses  of  Kitchi  schistx. 


I.' 

n.-2 

61.35 
.26 
16.45 
.94 
4.20 
3.46 
3.12 
1.05 
5.24 
.18 

10 
2  51 

100.  84 

70.76 
.33 

14.83 
1.46 
3.09 

1.99 

3.50 
.47 
.26 

Al.Q., 

Fe.Oi 

FeO 

C'aO 

MgO 

K,0 

Na-iO 

P.O., 

H.OatlOO° 

2.70 
99  84 

'  Green  schistose  rocks  speckled  with  red-weatheriug  feldspars.  No.  22062.  From  near  center 
of  sec.  34,  T.  48  N.,  R.  27  W. 

=  Light  grayish-green  sericitic  schist  from  between  two  well-characterized  conglomeratic  l>ed8 
of  the  green  schist.     No.  22085.     From  about  200  paces  south  of  northwest  corner  of  same  section. 


THE   GNEISSOID    GKANITRS.  169 

From  their  composition  and  structure  it  is  evident  that  the  acid  schists 
associated  with  the  greenstone-conglomerates,  as  well  as  these  latter  rocks 
themselves,  are  tuflfaceous  deposits  which  suffered  dynamic  metamorj)liisra 
and  weathering  until  their  original  composition  was  entirely  changed.  The 
darker-colored  schists  have  now  the  characters  of  "  schalsteins ; "  the  lighter- 
colored  ones  are  sericite-schists.  The  former  were  originally  Ijasic  tuffs, 
and  the  latter,  in  all  probability,  acid  ones  interstratified  with  the  former. 
The  basic  rocks  are  much  the  more  abundant.  The  pebbles  occurring  in 
the  conglomerates  are  all  of  the  same  general  character.  They  are  very 
similar  to  the  schistose  matrix  in  which  they  are  embedded,  but  are  less 
schistose.  They  must  be  looked  upoii  as  volcanic  bombs  or  as  large  frag- 
ments of  the  lavas  whose  ashes  produced  the  matrix.  If  fragments,  they 
have  become  rounded  by  the  mashing  that  caused  the  foliation  of  the  finer 
particles. 

Since  the  green  schists  are  surface  materials,  they  must  have  been 
deposited  upon  some  previously  existing  basement.  This  basement  has  not 
yet  been  identified.  It  can  not  be  the  gneissoid  granite,  for  the  granite  is 
intrusive  in  the  schists. 

THE   GNEISSOID   GRANITES. 

The  granites  and  gneisses  of  the  Northern  Complex  are  closely  related 
genetically.  Both  are  coarse-grained,  both  vary  in  color  from  dark  greenish- 
gray  to  bright-red,  both  are  usually  granular,  and  occasionally  porphyritic, 
with  large  red  phenocrysts  lying  in  a  coarse  red  groundmass,  and  both 
have  suffered  more  or  less  severely  the  effects  of  mashing.  The  gneisses 
differ  from  the  granites  only  in  the  perfection  of  the  foliation  that  has  been 
imparted  to  them  and  in  the  amount  of  dynamo-clastic  material  discover- 
able in  them.  The  gneisses  are  indisputably  foliated  phases  of  the  granite, 
which  is  always  more  or  less  schistose.  Since  the  origin  of  these  gneisses 
is  known,  it  seems  better  to  designate  them  by  a  name  that  will  indicate 
their  origin,  leaving  the  term  "gneiss"  to  cover  those  foliated  rocks  of  the 
composition  of  granite  whose  origin  is  problematic. 

DISTRIBUTION   AND   TOPOGRAPHY. 

The  gneissoid  granites  occupy  two  distinct  areas  in  the  Northern 
Complex.     Although  widely  separated,  the  rocks   occurring  within  them 


170  THE   MARQUETTE   lEONBEAEING   DISTRICT. 

may  be  treated  together,  since  they  are  aHke  both  in  macroscopic  and  in 
microscopic  characters,  and  so  far  as  can  be  learned  thej^  bear  exactly  the 
same  relations  to  the  surrounding-  sedimentaries  and  crystallines.  The 
easternmost  of  the  two  areas  is  north  of  the  Mona  schists  and  beyond  the 
limits  of  the  map  (Atlas  Sheet  IV).  It  extends  northward  nearly  to  the 
lake  shore,  and  westward  until  it  connects  outside  the  limits  of  our  work 
with  the  eastern  limb  of  the  second  area.  This  second  area  begins  at 
the  western  side  of  the  Kitchi  form;ition  and  extends  westward  beyond  the 
district  treated  in  this  paper.  (Jii  the  soutli  it  is  bordered  by  the  Algoukian 
beds  of  the  Marquette  area,  and  on  the  north  by  the  slates  and  quartzites 
of  the  Arvon  district. 

The  granites,  whether  massive  or  gneissoid,  form  knobs  with  rounded 
and  smooth  surfaces,  where  they  have  been  exposed  clearly  to  view  by  the 
removal  of  their  forest  covering.  At  many  localities  these  are  isolated  from 
one  another  by  stretches  of  glacial  or  lake  sands.  At  others  a  number  of 
knobs  together  form  large,  rugged,  boss-like  masses,  ha^dng  as  many  inde- 
pendent eminences  as  there  are  individual  knobs  comprising  the  main  one. 
The  hills  never  assume  the  dignity  of  mountain  peaks.  The  surface  fea- 
tures of  the  area  underlain  by  the  granite  are  thus  essentially  similar  to 
those  of  the  green-schist  area.  There  is  a  difference,  hoAvever,  that  is 
■usually  recognizable  in  those  portions  of  the  district  where  ledges  are 
abundant.  In  the  areas  of  green  schist  the  sm-faces  of  the  ledges  are 
usually  rough  and  broken;  in  the  areas  of  granite  the  surfaces  are  smooth 
as  a  result  of  glacial  action,  so  that,  whereas  the  bare  tops  of  hills  and  the 
bottoms  of  stream  channels  in  the  greenstone-schist  are  rough  and  uneven, 
in  the  granite  they  are  comparatively  even  and  smooth.     (See  PI.  VI.) 

RELATIONS    TO    AD.IACENT    BOCKS. 

The  relations  of  the  granites  to  the  green  schists  with  which  they  are 
in  contact  have  already  been  mentioned.  The  granites  and  their  accom- 
panying gneisses  are  younger  than  the  schists.  They  are,  however,  older 
than  the  fragmental  beds  above  the  schists,  since  none  of  their  dikes  intersect 
these,  even  Avhen  the  granitic  rocks  are  in  contact  with  the  sedimentary 


THE    r.IOTlTE-(;UANITES.  171 

ones.     On  the  other  haiul,  Ixmlders  of  the  former  are  often  found  in  the 
L)wer  beds  of  the  latter. 

Since  the  granites  and  their  accompanying  gneisses  can  not  be  the 
foiindation  upon  the  surface  of  which  the  materials  of  the  green  schists  were 
spread,  and  since  these  latter  are  all  fragmental  volcanic  I'ocks  and  surface 
lavas,  it  follows  necessarily  that  there  must  have  1)een  a,  basement  Ijeneath 
the  green  schists  wliich  is  older  than  these  and  tlie  gneissoid  granite  that 
intrudes  them.  This  basement,  however,  has  not  yet  been  identified  in 
the  Northern  Complex.  Occasionally  a  small  mica-schist  ledge  is  met  with 
in  the  midst  of  granite  ledges,  and  this  may  represent  a  series  of  rocks 
underlying  the  green  schist  and  older  than  they;  but  no  evidence  either  in 
favor  of  this  view  or  in  opposition  to  it  has  vet  been  collected. 

THE    inOTITE-GRANITES. 


Macroscopicai. — As  lias  already  been  stated,  the  more  massive  and  the  more 
schistose  phases  of  the  granites — the  gneissoid  granites  and  the  granitoid 
gneisses — are  believed  to  be  portions  of  the  same  rock  mass,  and  therefore 
they  are  discussed  together.  Further  investigation  may  show  that  some  of 
the  gneisses  are  older  than  some  of  the  granites,  but  up  to  this  time  no 
discrimination  lietween  the  massive  and  the  schistose  granites  has  been 
attempted  in  mapping. 

The  rocks  vary  in  color  from  grayish-green  to  bright-red,  the  color  of 
the  former  varieties  being  due  to  the  abundance  of  chlorite  in  them.  Their 
feldspar  is  rarely  white.  It  is  usually  of  a  light-red  or  })ink  color.  When 
bright-red  it  gives  the  entire  rock  of  Avhich  it  is  a  part  a  red  tint,  which 
varies  in  brilliancy  according  to  the  quantity  of  feldspar  in  it.  In  a  few 
instances  bright-red  orthoclases  are  scattered  through  a  groundmass  of  gray 
granite,  but  this  variety  is  usually  found  only  near  the  contacts  of  the  rock 
with  the  greenstones  or  in  its  apophyses  that  intrude  the  latter. 

Microscopical. — Tlic  gTanites  and  their  gneissoid  varieties  are  ali  composed 
of  clouded  orthoclase,  microeline,  plagioclase  (the  first-named  mineral 
predominating),   quartz,   and  brownish-green  biotite   or   its  decomposition 


172  THE   MARQUETTE   lEOX  BEARING   DISTRICT. 

products.  Occasionally  chlorite  is  jiresent.  This  appears  from  its  shape  to 
have  been  derived  from  hornblende,  but  no  undoubted  amphiboles  have 
been  detected  in  the  northern  granites.  With  few  exceptions  the  rocks 
are  all  biotite-granites  or  granitites.  The  accessories  are  small  crystals  of 
sphene,  some  leucoxeue  and  magnetite,  and  an  occasional  zircon. 

The  original  constituents  of  the  granites  require  no  special  description. 
The  orthoclase  and  plagioclase  are  altered  to  kaolin,  sericite,  and  calcite. 
These  products,  together  with  a  red  earthy  dust,  probably  an  ocher,  are  so 
thickly  clustered  that  they  very  nearly  obscure  the  twinning  bars  of  the 
plagioclase  and  cause  it  to  be  confounded  with  the  orthoclase.  The  biotite 
was  originally  a  brownish-green  variety.  At  present  but  few  remnants  of 
the  mineral  remain.  It  has  been  changed  to  single  plates  and  aggregates 
of  flakes  of  a  pale  to  bright  green  chlorite,  polarizing  with  blue  tints.  This 
chloi-ite  is  sometimes  intergrown  with  muscovite,  but  only  in  those  cases 
where  the  latter  is  evidently  a  ])roduct  of  dynamic  action. 

The  quartz  appears  in  two  forms,  either  as  irregular  grains  of  the  usual 
character  of  granitic  quartz  or  as  little  masses  filling  triangular  areas 
between  the  other  components  and  sending  arm-like  projections  into  them. 
Some  of  it  is  in  all  probability  original;  much  of  it  is  unquestionably  sec- 
ondary. All  of  it  is  marked  by  the  undulatory  extinction,  and  a  part  of  it 
is  completely  shattered. 

No  specimen  of  the  granites  examined  is  free  from  the  effects  of  mash- 
ing. In  every  slide  placed  under  the  microscope  more  or  less  distinct  traces 
of  dynamo  -  metamorphism  are  recognized.  The  feldspar  is  granulated 
peripherally  and  the  quartz  is  more  or  less  completely  shattered.  The 
fragments  thus  derived  are  mingled  with  chlorite  flakes,  epidote  grains,  and 
occasionally  a  little  muscovite,  and  the  whole  is  cemented  by  newly  formed 
feldspars,  among  the  most  prominent  of  which  is  raicrocline.  This  mineral 
was  evidently  formed  in  large  quantity  after  the  crushing  of  the  original 
minerals  of  the  granite.  It  inserted  itself  into  every  crevice  and  space 
between  these;  in  some  cases  it  has  even  formed  tiny  veins  cutting  across 
quartz  grains. 

Not  only  is  microcline  present  in  this  fragmental  aggregate,  but  it 
occurs  also  as  colorless  rims  around  the  cloudy  orthoclase,  and  also  often 


THE   BIOTITE  GltANITE8.  173 

replacing  the  material  of  the  latter.  A  large,  cloudy  orthoclase  may  in 
many  cases  be  found  completely  saturated  with  clear,  colorless  microcline 
substance.  There  is  no  sharp  line  of  contact  between  the  two  feldspars, 
but  they  seem  to  grade  into  each  other.  As  the  microcline  replaces  the 
orthoclase  it  absorbs  the  alteration  products  of  this  mineral,  the  resulting 
new  feldspar  thus  being  free  from  inclusions,  while  the  original  feldspar  is 
full  (if  them. 

Dr.  Williams,  in  his  report  on  the  Marquette  greenstones,  referred  to 
the  microcline  in  the  granites  as  more  probably  the  effect  of  pressure  twin- 
ning in  orthoclase  than  the  product  of  chemical  alteration.  To  the  writer  it 
appears  more  probable  that  the  microcline  is  all,  or  nearly  all,  new  material, 
produced  by  chemical  agencies.  In  evidence  of  this  view,  and  against  that 
which  regards  the  mineral  simply  as  a  pressure-twinned  orthoclase,  we  would 
cite  the  freshness  of  the  latter  mineral  as  compared  with  the  orthoclase,  its 
freedom  from  inclusions,  its  irregular  occurrence  within  the  orthoclase 
grains,  and  its  existence  in  large  quantity  in  veins  and  as  the  cement  of  the 
crushed  mosaic. 

Fresh  plagioclase  is  also  a  common  new  product  in  some  sections.  It 
occurs  as  grains  among  the  crushed  materials,  and  sometimes  it  suiTounds 
cloudy  feldspar  as  a  clear,  colorless  zone.  Its  twinning  bars  are  commonly 
much  bent,  and  nearly  always  they  present  a  few  or  more  of  the  usual 
features  due  to  movement. 

The  epidote  grains  in  the  mosaic  need  no  description.  They  are  very 
light  in  color,  and  therefore  show  no  pleochroism.  The  muscovite  that  is 
in  some  cases  associated  with  the  biotite  or  chlorite  is  found  with  these 
minerals  only  where  they  are  in  the  mosaic  aggregate,  and  then  only 
where  in  contact  with  orthoclase,  a  large  mass  of  chlorite  in  some  cases 
being  separated  from  the  orthoclase  by  a  rim  of  muscovite.  This  mineral 
is  also  present  in  laminar  aggregates  of  flakes,  which  in  some  slides  pene- 
trate the  mosaic,  but  which  in  most  slides  separate  it  from  the  unfractured 
original  granitic  components. 

The  mosaic  of  fractured  minerals  and  new  products  is  always  more  or 
less  schistose.  This  structure  is  produced  by  the  lengthening  of  the  frag- 
ments in  a  common  direction,  and  by  the  development  of  the  chlorite  and 


174  THE   3IA1IQUETTE   lliON-BEAKING  DISTRICT. 

muscovite  in  large,  naiTow  flakes  and  groupings  of  flakes.  The  mosaic  is 
also  traversed  by  bands  in  which  the  fragments  are  very  much  smaller 
than  elsewhere,  as  though  the  rock  had  slipped  along  certain  planes  and 
bad  ground  into  powder  the  neighboring  fragments.  These  bands  run  in 
the  same  direction  as  do  the  stringers  of  chlorite  and  nmscovite,  and  so  help 
to  impress  schistosit}'  on  the  mosaic.     They  are  microscopic  shear  zones. 

The  structure  of  all  these  granites  is  that  described  by  Tornebohm 
under  the  name  of  "mortar-structure."  Williams  has  already  referred  to 
it  as  characteristic  of  the  granites  of  this  region,  and  has  cited  its  existence 
as  evidence  that  the  rocks  in  which  it  is  found  have  been  subjected  to 
severe  dvnamo-metamorphism. 

The  gneissoid  granites  difter  from  the  more  massive  phases  of  the 
rocks  simply  in  the  possession  of  more  marked  foliation.  The  mortar- 
structure  is  most  beautifully  exhibited  in  all  the  sections.  The  larger  rem- 
nants of  the  crushed  original  components  are  embedded  in  the  mosaic, 
which  suiTOunds  them  as  the  crystalline  matrix  surrounds  the  eyes  of  an 
"  augen-gneiss,"  the  combination  of  fragments  and  mosaic  producing  lentic- 
ular areas,  separated  from  other  like  areas  by  narrow  bands  of  very  flne 
mosaic. 

It  is  not  uncommon  to  see  in  a  slide  of  the  gneissoid  granite  a  grain  of 
orthoclase  or  of  plagioclase  broken  into  three  or  four  pieces  and  the  j^ieces 
separated  from  one  another  by  distances  of  a  quarter  millimeter.  The 
fissures  between  the  fragments  are  filled  with  an  aggregate  of  crystallized 
quartz  and  microcline,  or  with  a  portion  of  the  fragmental  mosaic. 

The  quartz  grains  have  suff'ered  crushing,  but  their  parts  have  not  been 
separated.  Quartz  areas  now  consist  of  nuclei  peripherally  granulated,  or 
of  several  grains  differently  orientated,  the  whole  forming  a  lenticule.  Each 
component  of  the  lenticule  exhibits  undulatory  extinction. 

THE    MUSCOVITE-GRANITES. 

Nearly  all  of  the  granites  of  the  Northern  Complex  are  biotite-gi'anites. 
A  very  few  of  a  different  character  are  found  whoso  relations  to  the  com- 
mon granite  have  not  been  determined.  In  the  SW.  J  sec.  29,  T.  48  N., 
R.  28  W.  (Atlas  Sheet  XVIII),  for  instance,  is  a  mediumly  fine  grained, 


THE   MUSCOVITE-GRANITES.  175 

light  grayish-pink  rock,  forniiiig  a  small  ledge  between  the  coarser  biotite- 
gi-anites  south  and  the  quartzite  north  of  it.  The  rock  may  be  a  dike  in 
the  coarser  granite,  and  probably  is,  though  no  observations  on  this  i)oint 
are  recorded  in  the  note-books.  The  rock  is  so  badly  shattered  that  it  is 
difficult  to  determine  its  original  composition.  The  thin  section  shows  no 
more  evidence  of  schistosity  than  does  the  hand  specimen.  It  does  exhibit, 
however,  a  crushed  mass  of  plagioclase,  orthoclase,  (piartz,  and  muscovite, 
cemented  by  iiner-grained  debris  of  the  same  minei-als  and  microcline. 
Between  the  finer  grains  there  is  sometimes  quartz  and  sometiujes  musco- 
vite, but  usually  the  grains  interlock  Avith  one  another.  All  of  the  large 
grains  are  sharply  angular,  and  many  of  them  are  cracked  across  in  various 
directions.  Others  that  were  single  fragiuents  have  been  broken  into  nianv 
small  ones  that  are  now  separated  from  one  another.  Enlargements  of 
quartz  and  plagioclase  fragments  are  noted.  The  muscovite  is  in  large 
colorless  ilakes  that,  like  the  other  components,  are  shattered.  The  cracks 
are  filled  with  fine  shreds  of  the  same  mineral,  and  the  edges  of  the  plates 
are  frayed  out  into  smaller  shreds,  which  form  a  matted  mass  of  tiny  mus- 
covite fibers,  in  which  the  larger  plates  lie.  These  fibers  ajjpear  to  l)e  the 
broken  portions  of  the  larger  plates  that  have  been  split  \)y  the  force  that 
crushed  the  quartz  and  feldspar.  The  matted  aggregate  of  fibers  is  thicker 
where  a  large  plate  has  been  fractured  into  four  or  five  fragments  than 
where  it  is  halved.  ^luscovite  is  also  in  minute  lamina?  between  the  crushed 
portions  of  the  other  constituents,  where  it  is  no  doubt  a  product  of  the 
alteration  of  orthoclase.  The  rock  is  evidently  a  muscovite-granite  that 
has  been  crushed  but  has  not  been  rendered  schistose. 

ORIGIN    OF    THE    GRANITES. 

As  to  the  origin  of  the  granites  and  their  gneissoid  phases  there  can 
be  little  question.  The  rocks  appear  like  eruptives  hi  the  field.  The  clastic 
grains  discoverable  in  their  thin  sections  are  evidently  of  dynamic  origin. 
All  are  sharply  angular.  None  have  the  rounded  outlines  of  waterworn 
grains.  The  structure  of  the  rocks  is  very  similar  to  that  of  schists  else- 
where that  have  been  shown  to  be  mashed  eruptives;  hence  there  is  no 
reason  to  believe  the  granites  and  gneisses  of  the  Northern  Complex  to  be 


17<3  THE    MAKQUETTE   IKON-BEAlilNG   D18TiUGT. 

anything  but  altered  igneous  rocks.  It  is  impossible  to  trace  them  back 
to  an  earlier  source  than  a  molten  magma;  therefore,  whatever  may  have 
been  the  origin  of  this  magma,  we  are  justified  in  calling  the  rocks  igneous. 
There  is  no  evidence  of  any  kind  to  support  the  belief  that  the  gneissoid 
granites  in  this  portion  of  the  Marquette  district  were  ever  water-deposited 
sediments  that  have  been  crystallized  bv  metamorphic  processes. 

THE   HORNBLENDE-SYENITE. 
DISTRIBUTION    AND    TOPOGRAPHY. 

The  syenite,  with  its  gneissoid  phases,  so  far  as  has  been  observed,  is 
found  only  in  a  naiTOw  belt,  from  a  quarter  of  a  mile  to  a  mile  in  width, 
lying  between  the  green  schists  on  the  south  and  the  fragmental  beds  of 
the  Silver  Lake  Algonkian  area  on  the  north.  The  belt  is  about  5  miles 
long,  and  it  lies  almost  entirely  within  T.  48  N.,  R.  26  W.  (Atlas  Sheets 
XXX  and  XXXIII).  The  syenite  is  so  like  the  granite  in  its  nature  that 
but  little  remains  to  be  said  concerning  it,  except  to  describe  its  microscop- 
ical features.  The  topography  of  the  area  occupied  by  it  is  exactly  like 
that  of  the  granitic  country. 

RELATIONS   TO   ADJACENT   ROCKS. 

The  relations  of  the  syenite  to  the  surrounding  rocks  are  also  like 
those  of  the  granite.  Its  apophyses  cut  the  green  schists,  and  its  main 
mass  is  unconformably  beneath  the  Algonkian  sediments.  As  to  the  rela- 
tions existing  between  the  syenite  and  the  granite  nothing  is  yet  known 
positively.  A  very  few  ledges  of  the  gneissoid  granite  have  been  found 
within  the  limits  of  the  syenite  area,  and  these,  when  examined  with  refer- 
ence to  the  latter  rock,  appear  to  have  been  intruded  by  it.  The  appear- 
ances, however,  are  not  decided  enough  to  warrant  an  expression  of  opinion 
as  to  their  meaning. 

PETROGRAPHICAL   CHARACTER. 

The  primary  constituents  of  the  syenite  are  orthoclase,  plagioclase, 
hornblende,  spheue,  magnetite,  and,  very  rarely,  liiotite.  Its  secondary 
components  are  plagioclase,  microcline,  chlorite,  quartz,  epidote,  muscovite, 
and  leucoxeue. 


THE    HOUNBLENDP^-SYENITE.  177 

The  primary  feldspars  are  clouded  with  alteration  products,  while  the 
secondary  ones  are  clear.  The  primary  and  the  secondary  minerals  bear 
the  same  relations  to  one  another  as  they  do  in  the  granites.  The  horn- 
blende is  in  dark  brownish-green  crystals  that  are  idiomorphic  in  the  pris- 
matic zone,  but  badly  terminated  at  their  extremities.  It  is  nearly  always 
more  or  less  completely  altered  to  chlorite.  The  sphene  is  also  rarely  fresh. 
It  is  usually  changed  into  a  cloudy,  light-coloz'ed  substance  that  looks  yellow 
in  reflected  light.  In  general  appearance  it  resembles  the  leucoxene  so 
often  seen  surrounding  ilmenite  or  titaniferous  magnetite  in  greenstones, 
and  hence  it  is  regarded  as  this  substance. 

A  similar  alteration^  of  sphene  into  leucoxene  has  been  described  by 
Werveke,  ^•on  Kuch,  Velani,  and  Grroth,  the  latter  author  regarding  it  as  a 
product  of  weathering.  In  the  Michigan  rock  the  leucoxene  forms  perfect 
pseudomorphs,  which  retain  the  diamond-shaped  cross-section  of  the  origi- 
nal sphene.  Whether  it  is  a  product  of  weathering  or  a  result  of  dynamic 
metamorphism  can  not  ha  told. 

The  quartz,  which  is  always  present  to  some  extent,  but  never  so 
abundantly  as  in  the  granites,  occurs  sometimes  as  small  grains  with  an 
undulatory  extinction,  sometimes  as  larger  ones  broken  up  into  an  aggregate 
of  diiferently  orientated  particles  Most  of  the  mineral,  however,  is  in  the 
angular  s^jaces  between  the  feldspars  or  in  the  cracks  traversing  the  older 
constituents.  A  small  portion  of  the  quartz  may  be  original,  but  the  greater 
portion  is  thought  to  be  secondary. 

The  structure  of  the  gneissoid  syenites  is  identical  with  that  of  the 
granites;  so  it  needs  no  discussion  in  this  place.  The  syenite,  as  well  as 
the  granite,  is  an  igneous  rock  that  has  suffered  dynamic  metamorphism. 
The  latter  is  a  quartz-biotite-orthoclase  rock,  and  the  syenite  an  aggregate 
of  hornblende  and  orthoclase.  Even  were  the  two  rocks  not  distinguished 
by  the  abundance  of  quartz  in  the  one  and  its  rarity  in  the  other,  they 
would  be  distinguished  by  the  presence  of  the  biotite  in  the  granite  and 
of  the  hornblende  in  the  syenite. 

'  I-ehrbnrh  der  Petrographie,  by  F.  Zirkel,  Vol.  I,  1893,  p.  410. 
MON  XXVIIl ll! 


178  THE    MARQUETTE    IRON-BEARING   DISTRICT. 

THE  INTRUSIVES  IN   THE   NORTHERN   COMPLEX. 

The  g-ranites,  gneisses,  and  schists  of  the  Northern  Complex  are  cut  by 
numerous  dikes  of  basic  and  acid  material  and  certain  boss-like  masses  of 
peridotite,  or  of  its  altered  form,  serpentine.  Of  the  dikes  the  basic  ones 
are  much  more  common  than  the  acid  ones,  if  we  exclude  from  the  latter 
those  that  are  l)ut  apophyses  of  the  coarse  granite. 

THE    BASIC    DIKES. 

The  basic  dikes  cut  the  gneissoid  granite,  the  syenite,  and  the  schists 
indisci'iminately,  though  they  may  be  most  abundant  in  the  greenstone- 
schist  areas.  They  vary  in  width  from  an  inch  or  two  to  75  feet  or  more, 
and  some  of  them  have  been  followed  2  or  3  miles. 

These  dikes  have  been  so  well  described  by  Williams'  that  there  is 
little  left  to  be  said  in  this  place  concerning  them.  Diabases,  epidiorites, 
and  diorites  were  distinguished  by  this  author.  The  diabases  are  of  the 
usual  types.  The  epidiorites  are  thought  to  be  uralitized  and  epidotized 
diabases,  since  their  structure  is  plainly  ophitic,  the  feldspar  occuiTing 
in  lath-shaped  crystals,  and  the  amphibole  forming  fibrous  wedge-shaped 
masses  between  the  plagioclase  laths.  The  diorites  differ  from  the  epidiorites 
mainly  in  structure  and  in  the  nature  of  their  hornblendic  component.  The 
amphibole  in  the  diorites  is  compact  and  idiomorphic,  and  hence  it  was 
considered  by  Williams  as  original.  In  some  slides  of  these  rocks,  how- 
ever, are  cross-sections  of  a  compact  brownish-green  hornblende  that  is 
perfectly  idiomorphic,  while  at  the  same  time  nests  of  light-colored  augite 
may  be  seen  included  within  its  mass.  If  this  hornblende  is  secondar}-, 
as  it  seems  to  be,  then  it  is  proljable  that  many  of  the  supposed  diorites  of 
this  district  are  altered  diabases,  just  as  are  the  epidiorites,  which  contain 
fibrous  amphibole. 

The  freshest  diabases,  those  still  containing  large  quantities  of  pyro.xene, 
are  quite  massive,  even  when  the  rocks  through  which  they  cut  are  com- 
pletely schistose.     These,  then,  must  have  been  intruded  in  the  schists  after 

'  The  greeii.stone-schi8t  areas  of  the  Menominee  anrl  Marquette  regions  of  Michigan,  by  G.  H. 
Williams:  Bull.  U.  S.  Geol.  Survey  No. 62,  1890,  pp.  138-146,168-175,  180-184,  189-190. 


BASIC    DIKES    IN   THE   NORTHERN    COMPLEX.  179 

the  latter  had  become  fohated,  and  must  be  younger  than  the  diorites  and 
e^iidiorites,  all  of  which  are  schistose. 

In  texture  the  diabases  may  be  very  coarse  grained,  very  fine  grained, 
or  they  inav  contain  some  glass.  Mineralogically  they  present  no  special 
features.  Most  of  them  are  nonolivinitie ;  a  few  contain  pseudomorphs  of 
chlorite  and  limonite  after  olivine. 

An  example  of  one  of  tlie  freshest  of  the  coarse  dialjases  is  found  in  a 
dike  75  feet  in  width  cutting  granite  at  1,230  steps  N.,  450  stejjs  W.  of  the 
8E.  corner  of  sec.  23,  T.  48  N.,  R.  28  W.  (Atlas  Sheet  XXI).  So  coarsely 
granular  is  it  that  the  rock  approaches  a  gabbro  in  structure,  though  the 
augite  is  younger  than  the  plagioclase  and  fills  the  interstices  between  the 
laths  of  this  mineral.  The  components  of  the  rock  are  magnetite  or  ihnen- 
ite,  apatite,  labradorite,  and  pyi'oxene,  besides  various  alteration  products 
of  the  two  last-named  minerals.  The  fresh  pyroxene  has  the  pink  tint  so 
common  to  the  monoclinic  pyroxene  of  Lake  Superior  rocks.  On  its  edges 
it  is  altered  to  fibers  of  light-green  hornblende,  with  which  are  interspersed 
a  few  grains  of  magnetite.  The  plagioclase  is  mainly  well  preserved.  Its 
twinning  lamellae  are  broad  and  clearly  defined,  and  the  symtiietrical  extinc- 
tion on  each  side  of  the  twinning  line  is  about  21°.  In  the  small  areas 
between  the  most  altered  pyroxene  grains  the  feldspar  is  decomposed.  It 
is  reddened  by  cloudy  secondary  substance,  and  is  filled  with  cldorite  and 
amphibole  flakes  and  needles.  In  these  portions  of  the  slides  the  largest 
apatites  are  to  be  found. 

Sections  of  other  fresh  g)-anular  <likes  present  nearly  the  same  phe- 
nomena as  those  above  described.  In  most  of  them  the  diabasic  structure 
is  very  pronounced  and  the  grain  is  finer  than  in  the  case  of  the  dike  last 
mentioned.  Moreover,  in  nearly  all,  alteration  has  progressed  a  little  further. 
Green  biotite  and  brown  hornblende  in  small  quantities  are  nearly  always 
the  accompaniments  of  the  green  hornbleudic  and  chloritic  decomposition 
products  of  the  pyroxene.  They  occur  in  small  flakes  on  what  were  the 
peripheries  of  the  original  ])yroxeue  areas,  and  no  doubt  owe  their  origin 
partly  to  the  feldspar.  Upon  farther  alteration  the  brown  hornblende 
passes  into  a  chlorite,  while  the  green  mica  retains  its  }n-o})erties. 


180  THE   MAKQUETTE    IRON-BEARING    DISTRICT. 

Occasionally  the  structure  of  the  diabases  is  porphyritic  rather  than 
ophitic.  In  the  SW.  ^  sec.  23,  T.  48  N.,  R  25  W.  (Atlas  Sheet  XXXVIII), 
for  instance,  is  a  fine-grained  dike  that  may  be  called  a  diabase-poqihyrite. 
It  contains  phenocrysts  of  plagioclase  and  augite  in  a  groundmass  com- 
posed of  a  plexus  of  small  j^lagioclase  laths,  round  augite  gi'ains,  and  crystals 
of  magnetite,  with  the  intersertal  structure  as  defined  by  Rosenbusch.  A 
second  diabase -porphyrite  diff"ers  from  the  type  just  mentioned  in  the 
absence  of  pyroxene  phenocrysts  and  in  the  presence  of  magnetite  in  large 
quantities.  The  latter  mineral  is  found  not  only  in  the  little  grains  between 
the  constituents  of  the  groundmass,  but  also  in  large,  irregular  masses  scat- 
tered through  the  rock  and  in  skeleton  crystals  resembling  the  microlites  in 
basic  glasses  (fig.  G).  These  microlitic  growths  form  long,  slender,  straight 
rods  cutting  indiscriminately  through  the  grains  of  the 
T  groundmass  and  through  phenocrysts. 

JBr^  Most  of  the  basic  dikes  in  the  Northern  Complex  are 

-"^■^  j/i/v\J  epidiorites,  that  is,  they  are  rocks  showing  unmistakably 
^^^^^^m^^  the  diabasic  structure,  but  in  which  the  augite  has  been 
^  MS/  entirely  replaced  by  uralite  or  some  other  green  horn- 
rir,.  6.-M.u'netite  in  imo  bleudc.  They  arc  always  more  or  less  schistose,  and  there- 
grdiue  <  .a  asc  or  .asai  ^^^^^  ^^^^^  niucli  oldcr  tliau  the  frcsh  diabases.  They  vary 
from  one  another  mainly  in  the  form  of  their  homblendic  constituent  and  in 
the  freshness  of  their  plagioclase.  In  some  of  them  the  augite  has  been 
pseudomorphed  by  green  hornblende.  In  others  the  hornblende  is  in 
isolated  or  in  grouped  acicular  crystals,'  the  ends  of  which  often  extend  far 
out  into  the  altered  plagioclase  surrounding  the  areas  originally  occupied 
by  ophitic  augite.  In  the  least  altered  epidiorites  the  plagioclase  is  fresh, 
and  in  these  varieties  augite  cores  often  remain  as  nuclei  within  the  amphi- 
bole  areas.  As  alteration  progresses  the  plagioclase  becomes  more  and 
more  clouded  by  secondary  products  until  finally  it  becomes  an  aggregate 
of  epidote,  chlorite,  amjihibole,  and  calcite. 

Fine  examples  of  leucoxene  are  seen  in  many  specimens.     The  min- 
eral occurs  as  little  cloudy  masses  around  titanic-iron  grains,  and  also  as 

'The  greenstone-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  G.  H. 
Williams:  Bull   U.  S.  Geol.  Survey  No.  62,  1890,  fig-.  2  of  PI.  XII. 


BASIC    DIKES    IX   THE    NORTHEltX   COMPLEX.  181 


(L.i 


latter  niiucri 

;il,  rejilacin^'  it  (■tiiiipletely  in  sciine  cases. 

if   the   opaiji 

le    ii-on   oxide   form   a    iietwoi-k   in  whose 

o|,a,jiR-  -rai 

ins  of  hnieoxene.      In   the   thicker  aggre- 

passes   into    ; 

1    ih-nse   velh)\vish-l)ro\vn   mass   with  the 

psemiouiorplis  ot  the 
In  other  cases  l)ars 
meshes  are  the  white 
gates  the  leucoxene 
pleochn'iism  of  sphene. 

Tlie  character  of  the  aheration  that  changed  diabases  into  epidiorites, 
coupled  with  the  existence  of  schistosity  in  the  latter  rocks,  is  thought  to 
be  sutiicient  reason  for  ascribing  the  origin  of  the  epidiorites  to  dvnamo- 
nietamoi-phism. 

The  "diorites"of  the  Northern  Complex  are  ])roVja])lv  altered  diabases 
in  which  the  new  hornblende  has  assumed  a  comjjact  rather  than  a  fibrous 
form.  The  hornblende  crystals  in  these  rocks  are  always  frayed  out  at 
their  ends,  but  in  cross-section  they  are  comj)act  and  idioniorphic.  'I'he 
original  feldspar  has  been  entirely  replaced  b}-  a  transparent  plagioclase 
that  is  cut  through  and  through  by  slender  needles  of  hornblende,  at  whose 
ends  terminal  planes  may  often  be  detected.  Under  crossed  nicols  large 
areas  of  the  plagioclase  break  up  into  many  small  ones,  interlocking  with 
one  another  by  sutures  which  follow  the  most  intricate  courses.  This  feld- 
spathic  mass  differs  so  greatly  from  that  of  the  epidiorites  in  appearance,  in 
freshness,  and  in  its  structure  that  we  must  regard  it  as  essentially  different 
in  origin.  The  feldspar  of  the  epidiorites  is  a  decomposed  plagioclase, 
while  that  of  the  diorites  is  a])|)arentl}'  a  recrystallized  one.  Th(!  diorite 
as  it  now  exists  is  not  an  original  rock.  It  has  been  formed  from  some 
preexisting  eruptive,  but  whether  from  an  original  diorite  or  a  diabase  is 
not  certainly  known. 

The  tinal  products  of  weathering  of  all  the  basic  rocks  described  as 
occiu'rifig  in  dikes  are  chlorite,  epidote,  kaolin,  calcite,  and  quartz.  A 
number  of  dike  masses  are  known  in  the  Northern  Complex  that  consist 
princi])ally  of  these  minerals.  Most  of  them  present  the  ophitic  texture  of 
diabases,  while  in  others  the  granitic  texture  of  diorites  and  gabbros  is 
recognizable.  All  are  highly  schistose.  Chlorite  h  by  far  the  most 
abundant  component  in  them,  and  the  rocks  therefore  are  practically 
chlorite-schists.  All  that  liave  been  studied  are  unquestionably  squeezed 
eruptives. 


182  THE    MAEQUETTE   IROX-BEARIIfG   DISTRICT. 


THE    ACID    DIKES. 


The  acid  dikes  cutting-  the  rocks  of  the  Northern  Complex  are  not  so 
numerous  as  are  the  basic  ones,  liut  their  variety  is  greater.  They  include 
coarse  granites  and  granite-porphyries,  fine-grained  granites,  aj^lites,  (piartz- 
porphvries,  and  the  aplitic  form  of  quartz-diorite  known  as  malchite. 

No  descriptions  of  the  coarse  granites  and  granite-porphyries  are 
necessary.  They  are  apo]3hyses  of  the  great  granite  masses  north  of  the 
schists  and  do  not  differ  in  character  from  these.  They  are  of  the  same 
age  as  the  gneissoid  granites,  and  hence  are  older  than  the  A-arious  aplitic 
and  diabasic  dikes  that  intrude  the  granite. 

The  remaining  acid  dikes  are  usually  of  inconsiderable  size  when 
compared  with  the  great  basic  dikes  that  traverse  the  schists  and  granites. 
TheA'  are  also  of  A^arious  ages.  Some  are  foliated  and  others  are  massiA^e. 
The  former  ^^■ere  intruded  before  the  last  effects  of  pressure  had  been 
impressed  upon  tlie  schists,  and  the  latter  long  after  the  schists  Avere  made, 
for  theA'  intersect  even  some  of  the  massive  diabase  dikes.  None  of  them, 
however,  intersect  any  of  the  members  of  the  Algonkian  series.  Tlie  color 
of  the  dikes  varies  from  pinkish-gray,  through  pink,  to  a  briglit  red.  Their 
material  is  always  compact,  and  except  when  here  and  there  it  is  microlitic 
it  is  also  very  fine  grained.  Many  of  these  dikes  are  fine-grained  granites 
A\4th  no  peculiar  features.  Their  feldspar  is  usually  red  or  pink,  and  their 
princij^al  bisilicate  is  a  chloritized  biotite.  Others  partake  more  of  the 
aplitic  character.  Their  feldspathic  component  possesses  more  or  less  per- 
fecth-  quadrangular  cross-sections,  and  their  quartzes  circular  ones. 

Botli  the  granites  and  the  aplites  are  altered.  Their  orthoclase  is  kao- 
linized  and  their  biotite  is  so  completely  changed  to  chlorite  that  it  is 
oftei^i  difficult  to  determine  what  were  the  original  components.  Epidote 
is  not  infrequently  an  alteration  product  of  the  plag'ioclase  present,  and  it  is 
apjDarently  often  a  result  of  the  decomposition  of  biotite. 

A  few  dikes  of  a  fine-grained,  dark-gray  rock  are  of  interest,  since  they 
represent  the  ajjlitic  form  of  quartz-mica-diorite,  named  malchite  by  Osann.^ 
The  best  specimen  of  this  rock  came  from  a  dike  600  steps  north  of  the 

'Mittheil.  Gross.  Bad.  geol.  Landesanstalt,  A'^ol.  II,  p.  380.  Cf.  also  Microscopic  study  of  some 
Michigan  rocks,  by  H.  B.  Patton  :  Kept.  State  Board  Geol.  Snrv.  Michigan  1893,  pp.  184-186. 


THE    PEKIDOTITE.  183 

SE.  corner  of  sec.  18,  T.  48  N.,  R.  28  W.  It  is  an  aggregate  of  plagioelase, 
orthoclase,  quartz,  and  biotite.  All  tlie  components  are  much  altered. 
The  biotite  is  chloritized,  the  orthoclase  kaolinized,  and  the  plagioclase 
saussuritized.  Plagioclase  and  quartz  compose  the  greater  portion  of  the 
rock.  The  former  is  in  little  grains  with  irregular  outlines  that  exhibit  a 
tendency  to  become  quadrangular,  and  the  latter  in  grains  between  tlie 
plagioclases.     In  structure  the  rock  is  panidiomorphic. 

The  quartz-porphyry  dikes  have  already  been  studied  by  Williams. 
Tiic  rocks  are  light-colored.  They  sometimes  still  have  their  original  char- 
acters sufficiently  well  preserved  to  exhibit  the  porphyritic  structure.  In 
other  and  more  numerous  instances  the  rocks  are  schists.  Their  porphy- 
ritic orthoclases  are  broken  and  their  fragments  displaced,  their  porphvritic 
quartzes  are  granulated,  either  entirely  or  only  peripherally,  while  the 
quartz-orthoclase  mosaic  that  originally  constituted  their  groundmass  is  now 
a  schistose  aggregate  of  quartz  and  sericite.  In  not  a  few  of  the  porphyries 
plagioclase  accompanies  the  orthoclase  as  phenocrysts,  and  chlorite,  filled 
with  tiny  brown  rutile  crystals,  is  distributed  through  the  groundmass  as 
laminre  apparently  replacing  an  original  biotite. 

THE    PERIDOTITE. 

The  serpentine  and  peridotite  form  high,  ragged  bluffs  that  are  notice- 
able for  tlieir  dark  color  and  jagged  contours.  One  of  these  bluffs  forms 
Presque  Isle,  on  the  shore  of  Lake  Superior,  about  2^  miles  north  of 
Marquette.  The  principal  occurrences  of  the  two  rocks  are,  however, 
northwest  of  Ishpeming,  in  the  area  of  the  Kitchi  schists. 

TIIK    PRKSqUE   ISLE   AREA. 

The  rocks  of  the  Presque  Isle  area  (Atlas  Sheet  XXXVIII)  are  so  well 
known,  thanks  to  Dr.  Wads  worth,  ^  that  we  need  give  them  little  attention. 
They  were  all  originally  peridotites,  but  they  have  imdergone  alterations 
due  to  weathering  until  they  are  now  largely  serpentines  and  dolomites. 
Among  the  freshest  phases  of  the  rock  Iherzolites,  picrites,  and  wehrlites 
have  been  distinsfuished.^ 


'  Lithological  Stmlies,  by  M.  E.  Wadsworth,  1884,  p.  136. 

■^Report  of  the  State  Geoliigist  for  18110-91,  by  M.  E.  Wadsworth:   Kept.  State  lioard  Geol.  Surv. 
Michij,'an  1893,  pp.  134-138. 


184 


THE    MARQUETTE    IRON-BEARING    UISTEICT. 


The  peridotite  aud  its  altered  varieties  form  an  iinineuse  knob  under- 
lying Potsdam  sandstone.  Since  the  conglomeratic  lower  layers  of  the 
fragineutal  rock  contain  pebbles  of  the  peridotite,  and  since  a  diabase  dike 
in  the  latter  is  overlain  by  horizontal  layers  of  the  sandstone,  it  is  concluded 
that  the  peridotite  is  older  than  the  sandstone.  Its  age  with  respect  to  the 
Algonkian  fragmentals  is  not  yet  known.  It  is  probably  younger  than 
the  green  schists,  for  it  is  similar  in  all  essential  features  to  the  peridotite 
of  the  Opin  area,  aud  this  intrudes  the  Kitchi  schists. 

Analyses  of  the  sei'pentine  of  Presque  Isle  were  made  by  Whitney^  in 
1859.  They  are  of  those  portions  of  the  rock  that  were  decomposed  by 
fused  sodium  carbonate,  and  are  evidently  not  complete. 

Analysea  of  serpentine  from  Presque  Isle. 


I- 

II. 

III. 

36.95 
16.50 

37.25 
6.75 

14.14 

28.67 
1.16 

10.89 

Fe.O'i 



12.90 
19.52 
14.83 

MgO 

Na.O 

33.07 

.97 

10.40 

H,0 

98.86 

'IN    AKKA. 


The  rocks  in  the  vicinity  of  Opin  Lake,  in  T.  48  N.,  K.  27  W.  (Atlas 
Slieets  XXI  and  XXIV),  that  have  been  classed  as  peridotites  and  serpen- 
tines, embrace  also  dolomitic  serpentine  and  almost  pure  dolomites.  Phases 
intermediate  between  these  four  types  are  very  numerous,  so  that  there  is  no 
difficulty  in  tracing  almost  pure  peridotites  through  serpentinous  varieties 
into  pure  serpentines  and  these  into  dolomites. 

The  Opin  area  includes  a  number  of  detached  exposures,  the  larger  from 
1  to  3  miles  long,  surrounded  l)^'  green  schists  and  greenstone-conglomer- 
ates. The  combined  ureas,  each  composed  of  from  few  to  many  boss-like 
knobs,  trend  diagonally  across  the  various  belts  of  schist.  The  peri- 
dotite is  never  schistose  except  along  shear-planes.     Both  acid  and  basic 

I  Notice  of  new  localities  and  interesting  varieties  of  minerals  in  the  Lake  Superior  region, 
by  J.  D.  Whitney :  Am.  Jour.  .Sci.,  Vol.  XXVIII,  1859,  p.  18. 


THE    TEIUDOTITE.  185 

dikes  intersect  the  schists,  whereas  all  dikes,  except  the  very  freshest  of"  the 
diabases,  are  abseut  from  the  peridotite.  These  facts  would  indicate  that 
the  peridotite  is  an  irruptive  of  later  age  than  the  schists.  Dr.  Wadsworth 
has  recently  described  an  irruptive  contact  of  the  serpentine  with  the  schists, 
and  a  dike  of  the  peridotite  intruding-  diabase,  diorite,  and  felsite.  There 
can  be  no  question,  then,  that  the  peridotite  and  its  derivatives  are  yoimger 
than  the  Deer  Lake  Kitchi  schists.  From  the  relation  of  the  various  dike 
masses  to  it,  it  would  seem  to  be  the  latest  intrusion  in  this  area,  with  the 
exception  of  the  fresh  diabases,  whose  irrujotion  continued  until  the  close 
of  Upper  Marquette  time. 

The  freshest  peridotite  obtained  in  this  area  came  from  a  large  blutf  near 
the  center  of  the  E.  ^  sec.  27,  T.  48  N.,  R.  '27  W.  The  rock  is  composed 
of  fairly  well  preserved  diallage,  olivine,  magnetite,  and  plagioclase.  The 
oli^dne  is  in  well-defined  crystals,  embedded  in  large  plates  of  pale-pink, 
almost  colorless  diallage  that  nearly  fill  the  section.  In  the  small  inter- 
spaces between  the  diallages  occurs  the  plagioclase,  as  a  weakly  refracting, 
altered,  white  or  colorless  substance,  occupying  the  same  relation  with  respect 
to  the  diallage  and  olivine  as  glass  does  to  the  crystals  in  a  hypocrystalline 
rock.  The  olivine  and  diallage  are  both  serpentinized  in  part,  and  the  dial- 
lage is  in  places  uralitized  and  chloritized,  especially  near  its  contact  with 
the  plagioclase.  The  small  amount  of  magnetite  present  is  found  amongst 
these  decomposition  products,  as  are  also  a  few  flakes  of  biotite.  Calcite 
fills  cracks  in  the  other  components  and  the  spaces  left  between  them.  The 
rock  has  the  composition,  but  not  the  stracture,  of  wehrhte.  Its  analysis, 
made  by  W.  F.  Hillebrand  in  the  Survey  laboratory,  is  given  on  the  next 
page. 

From  this  analysis  it  is  seen  that  the  rock  is  more  altered  than  would 
be  judged  from  the  investigation  of  its  single  thin  section  at  hand.  It  is  also 
noticeable  that,  like  many  other  peridotites,  it  contains  small  percentages  of 
several  rare  metals,  as  titanium,  chromium,  manganese,  and  nickel,  besides 
traces  of  strontium  and  barium. 

From  most  of  the  sections  of  specimens  taken  from  the  Opin  perido- 
tite the  olivine  and  diallage  or  other  pyroxene  have  entirely  disappeared 
and  serpentine  or  dolomite  has  taken  their  places.     In  many  of  these  the 


186 


THE   MAKQUETTK    IIIOX-BEAIUNG   DISTRICT. 


original  structure  of  the  peridotite  may  still  be  detected,  while  in  others 
nothing  can  be  seen  but  fibrous  masses  of  serpentine,  irregular  areas  and 
tiny  veins  of  dolomite,  and  some  magnetite  and  earthy  products.  When 
the  serpentine  jiredominates,  the  rocks  are  well-characterized  calcareous 
serpentines;  when  the  dolomite  prevails,  they  become  almost  pure  dolo- 
mites. These  latter  rocks  are  not  so  abundant  in  quantity  as  the  serpen- 
tines are.  They  are  found  usually  as  veins  cutting  tlie  latter  rocks,  and  as 
indefinitely  defined  bands  bordering  cracks  and  joint  planes  in  them. 

Analysis  of  the  peridotite  from  the  Opin  area. 


SiO.2 

TiOj 

Al.Oa 

Cr.Oa 

Fe.,03 

FeO 

MnO 

NiO 

CaO 

SrO 

BaO 

MgO 

KiO 

Na,0 

H:0  below  110^ 
H;0  above  llO-^ 

CO, 

P.O., 

Total  . . . 


4.96 
9.13 
.12 
.21 

Trace. 

Trace. 

26.53 

.26 

.50 

.87 


No.  17972;  from  1220  paces  N.,  500  paces  W.,  of  the  southeast  oorner  of  sec.  27,  T.  48  N  ,  R.  27  W. 


FERRUGINOUS    VEINS    IN    THE    NORTHERN    COMPLEX. 


At  various  places  within  the  schist  areas  of  the  Northern  Complex 
there  are  small  masses  of  ferruginous  slate,  ferruginous  chert,  and  magnetite- 
griinerite-schist  which  are  identical  in  hand  specimen  and  in  microscopical 
character  with  similar  rocks  of  the  Negaunee  iron  formation.     The  best- 


FERRUGINOITS    VEINS    I>f   THE   NOETHEKN   COMPLEX.  187 

known  locality  at  Avhich  these  rocks  are  found  is  in  the  northern  part 
of  the  city  of  Marquette,  a  short  distance  west  of  Light-House  Point.  'Jlie 
largest  masses  of  the  feiTuginous  rocks  are  seen  on  Michigan  street,  from 
which  place  they  extend  east  and  west  for  some  distance.  At  Michigan 
street  the  rocks  are  sepsxrated  into  two  parts  by  a,  layer  of  green  schist.  A 
short  distance  to  the  east  but  a  single  mass  is  found,  and  this,  as  it  is  followed 
farther  east,  becomes  much  smaller.  It  finally  disai)pears  at  the  end  of  one 
outcrop  separated  from  others  by  an  interval  of  10  feet.  The  feiTup-mous 
rocks  at  this  place  have  a  strike  and  dip  closely  corresponding  with  those 
of  the  foliation  of  the  schists  with  which  they  are  associated,  but  when 
examitied  miniitelv  the  two  are  found  to  be  discordant. 

East  of  this  place,  on  the  neck  of  Light-House  Point,  various 
narrow  seams  of  iron-bearing  rock,  from  a  few  inches  to  a  foot  A\ide,  are 
found.  These  are  interlaminated  with  the  green  schist  of  tlie  point.  One 
of  them  can  be  traced  for  a  distance  of  100  feet  or  more,  while  the  other 
smaller  ones  disappear  witliiu  a  short  distance.  Some  of  these  narrow 
masses  have  become  soft  and  hydrated,  and  such  resemble  the  ferruginous 
material  taken  out  from  the  Eureka  mine  2  or  3  miles  to  the  west.  The 
Eureka  ore  is  a  soft  hematite  in  the  green  schist. 

The  belt  of  green  schists  from  the  Eureka  mine  to  Light-House  Point, 
in  common  with  the  entire  green-schist  area,  shows  evidence  of  extensive 
dynamic  action,  the  rocks  all  having  a  schistosity,  and  in  some  places  l)eing 
l)rokeu  up  into  lozenge-shaped  blocks,  between  which  solutions  might 
readily  pass.  It  is  believed  that  all  of  the  ferruginous  dejiosits  of  this  area, 
in  their  banding,  in  their  relations  to  the  surrounding  green  schists,  in  their 
great  variability  in  thickness,  and  in  the  rapidity  with  which  tiic}'  die  out, 
correspond  in  every  respect  to  infiltrated  veins,  and  are,  therefore,  secondary 
to  the  country  rock. 

In  sec.  2,  T.  48  N.,  R.  27  W.,  just  north  of  the  old  Holyoke  mine, 
outside  of  the  area  mapped,  there  are  also  found  within  the  gi-een  schist  of 
the  Northern  Complex  various  masses  of  sideritic  slate,  ferruginous  slate, 
feiTuginous  chert,  and  griinerite-magnetite-schist.  In  their  relations  to  the 
surrounding  rocks  they  in  all  respects  resemble  those  adjacent  to  Marquette.' 

•  In  one  of  tlie  Archean  islands,  in  sec.  23,  T.  47  N.,  R.  26  W.,  are  also  found  narrow  veins  of 
jasper,  the  widest  being  less  than  a  foot  tliick. 


188  THE    MAEQUETTE    IRON-BEAEINCi^   DISTRICT. 

The  forniiition  of  veins  of  ferniginous  materials  must  have  been  prior 
to  Mai-quette  time,  for  in  the  lowest  formation  of  the  Marquette  series,  as 
seen  on  a  succeeding  page,  are  found  fragments  of  feiTuginous  rocks  like 
the  veins  in  the  Basement  Complex.  The  Holyoke  conglomerate  at  the 
base  of  the  Huronian  series  in  this  locality  contains  very  numerous  large 
fragments  of  various  ferruginous  rocks  which  are  identical  with  the  veins 
in  the  green  schists  below,  and  there  can  1)0  no  douljt  that  the  ferruginous 
detritus  was  derived  from  the  veins. 

SUMMARY. 

In  the  preceding  jiages  it  is  shown  that  the  basement  upon  which  the 
iMarquette  sediments  were  deposited,  as  it  exists  in  the  Northern  Complex, 
consists  mainly  of  foliated  rocks,  including  greenstone-schists,  gneisses, 
gneissoid  granites,  and  syenites,  that  are  cut  through  and  through  by  intru- 
sions of  acid  and  basic  rocks  in  the  form  of  dikes  and  are  penetrated  by  bosses 
of  peridotite.  All  the  dikes,  except  a  very  few  fresh  diabases,  are  older 
than  the  upper  beds  of  the  Marquette  series.  They  are  schistose,  and 
most  of  them  are  much  altered.  The  massive  diabase  dikes  were  probably 
formed  during  Keweenawan  time.  The  peridotite  is  younger  than  the 
Cambrian  sandstone  and  older  than  the  greenstone-schists  of  the  Basement 
Complex.     Its  age  has  not  been  determined  more  accurately. 

The  gneissoid  granites  and  syenites  differ  from  one  another  in  com- 
position, the  former  consisting  essentially  of  biotite,  quartz,  orthoclase, 
plagioclase,  and  microcline,  and  the  latter  of  hornblende  and  the  feldspars. 
Both  rocks  owe  their  foliation  to  mashing,  and  both  have  had  developed  in 
them  large  quantities  of  new  minerals,  the  most  noticeable  being-  microcline, 
plagioclase,  and  muscovite.  The  gneisses  differ  from  the  granites  simply 
in  the  greater  perfection  of  their  schistosity  and  in  the  greater  quantity  of 
new  minerals  developed  in  them.  These  acid  rocks  have  the  structure 
of  plutonic  iutrusives.  The}^  cut  through  the  greenstone-schists  and  are 
intermingled  with  them  so  confusedly  that  accurate  outlining  of  the  areas 
underlain  by  the  acid  and  the  basic  rocks,  respectively,  is  practically  impos- 
sible. From  their  structm-e  it  is  evident  that  the  granites  and  syenites  were 
intruded  into  the  schists  when  these  were  at  some  considerable  distance 


SUMMARY.  189 

below  the  existing  surface.  Since,  however,  tlie  granites  were  exposed  at 
the  surface,  when  the  basal  bcils  of  the  Marquette  series  were  formed  (as 
shown  by  the  numei'ous  bowlders  of  granite  in  the  basnl  conglomerates), 
it  necessarily  follows  that  the  interval  between  the  intrusion  of  the  granites 
and  the  formation  of  the  first  of  the  Marquette  beds  was  of  great  length. 
Since,  moreover,  the  green  schists  are  older  than  tlic  granites,  it  further 
follows  that  the  schists  are  very  much  older  than  the  oldest  niendjers  of  the 
Marquette  series. 

The  greenstone -schists  studied  are  all  squeezed  surface  materials. 
They  are  nearlv  all  recrystallized  basic  tufts  or  altered  lavas.  The  few 
schists  of  doubtful  origin  were  probably  lava  flows  of  coarser  grain  than 
the  predominant  ones.  As  these  rocks  were  surface  forms,  it  is  evi- 
dent that  there  must  have  been  a  foundation  upon  which  they  were  laid 
down.  Tile  gneissoid  granites  can  not  have  composed  this  foundation, 
because  they  are  younger  than  the  schists.  The  former,  however,  are  the 
only  other  class  of  rocks,  with  the  exception  of  the  dikes,  that  have  been 
discovered  in  those  portions  of  the  Northern  Complex  studied;  hence  it 
follows  that  the  surface  on  which  the  basic  lavas  and  tuffs  were  laid  down 
has  not  yet  been  found.  It  is  barely  possible  that  the  original  surface 
rocks  have  disappeared,  as  Lawson'  has  suggested  in  explanation  of  a 
similar  set  of  phenomena  in  the  Rainy  Lake  district  of  Canada,  and  that 
the  granites  and  gneisses  are  tlieir  fused  representatives;  but  in  the  Mar- 
quette district  there  is  no  evidence  to  show  that  this  is  the  case,  and  we 
must  therefore  content  ourselves  for  the  present  with  the  statement  that  the 
basement  on  which  the  schists  were  deposited  is  unknown. - 

It  is  to  be  remarked  in  conclusion  that,  whatever  may  have  been  the 
original  condition  of  the  granites,  all  the  members  of  the  Northern  Complex 

'  A.  C.  Lawson,  Report  ou  the  geology  of  the  Rainy  Lake  regiou :  Ann.  Rept.  Geol.  and  Nat.  Hist. 
Surv.  of  Canada  for  1887-88,  Vol.  Ill  (new  ser.),  P.,  pp.  1-196;  and  Am.  Jour.  Sci.,  3d  series,  Vol.  XXXIII, 
1887,  pp.  473-480.     Also  CongrJ-s  g(5ol.  internat.,  Compte-rendii  4th  sess.,  London,  1888,  pp.  130-152. 

-Rominger's  theory  of  the  structure  of  the  district  under  consideration  is  very  similar  to  Law 
son's  theory,  so  far  as  it  concerns  the  relations  of  the  granites  to  the  other  members  of  the  Fundamental 
Complex  and  to  those  of  the  Marquette  series  (see  Chapter  I,  p.  84),  ;ind  Rominger's  statement  was 
puhlished  much  earlier  than  Lawson's.  The  same  remarks  that  apjily  to  Lawson's  suggestion  apply 
as  well  to  Rominger's  theory. 


190  THE    MAKQUETTE   IRON-BEAKINa   DISTRICT. 

exhibit  proof  that  they  once  existed  as  igneous  magmas,  from  which  they 
were  formed  by  cooling,  so  that  in  their  present  condition  they  are  all  of 
igneous  origin.  Not  a  sediment  of  any  kind  has  been  detected  among  them. 
In  this  respect  the  Northern  Complex  differs  essentially  from  the  Marquette 
Algoukian,  which  consists  almost  exclusively  of  well-preserved  sediments. 
The  relations  of  the  granites  and  the  gneisses  to  the  greenstone-schists 
are  those  that  obtain  between  the  Mareniscan  and  the  Laurentian  series 
of  Van  Hise,  the  greenstone-schists  representing  the  Mareniscan,  and  the 
granite-gneisses  the  Laurentian.  Nothing  corresponding  to  Adams's  Gren- 
ville  series  has  yet  been  discovered  in  this  district,  and  perhaps  nothing 
corresponding  to  his  Fundamental  Gneisses. 

SECTION   II.— THE    SOUTHERN   COMPLEX. 

So  far  as  our  studies  have  gone  it  has  been  found  impossible  to  map 
the  rocks  of  the  Southern  Complex  even  as  definitely  as  has  been  done 
in  the  case  of  the  Northern  Complex.  Except  in  its  eastern  portion,  there 
are  no  large  distinct  areas  in  that  part  of  the  southern  district  studied 
that  are  occupied  almost  exclusively  by  one  kind  of  rock.  Most  of  the 
area  is  occupied  by  granites,  gneisses,  hornblendic  and  micaceous  schists, 
and  greenstone-schists,  together  with  the  various  acid  and  basic  eruptives 
that  intrude  th.em. 

The  relations  existing  between  the  rocks  of  the  Southern  Complex 
and  those  belonging  in  the  Marquette  series  are  referred  to  at  the  jn-oper 
places  in  connection  with  the  discussion  of  the  lowermost  beds  of  the 
Algoukian.  Where  their  contacts  are  seen  there  are  found  marked  uncon- 
formities between  the  two  series,  as  will  be  explained  later.  At  other  places 
the  crystallines,  as  well  as  the  fragmental  rocks,  are  mashed  to  such  an 
extent  that  it  is  difficult  to  draw  a  line  between  them.  Along  such  con- 
tacts there  are  often  developed  light-colored  sericitic  schists  and  gneisses, 
whose  origin  is  problematic.  At  many  other  places,  notably  in  Rs.  27  and 
28  W.,  the  granites  and  schists  are  separated  from  the  Algoukian  sedi- 
ments by  a  strip  of  country  devoid  of  exposures.  Often  swamps  intervene 
between  the  last  outcrops  of  the  bedded  rocks  and  the  first  ones  of  the 


THE    SOUTHEKiSr   COMPLEX.  191 

schists.  At  other  times  drift  covers  the  contacts.  In  such  cases  the  rehi- 
tious  of  the  two  series  can  not  be  determined.  Nevertheless,  there  is  no 
reason  to  beheve  that  they  are  diffennit  from  those  observed  Avhere  the 
rocks  are  seen  in  actual  contact. 

DISTRIBUTION   AND  TOPOGRAPHY. 

The  topogTaphy  of  the  Southern  Complex  differs  but  little  from  that 
of  the  northern  granite  areas.  In  its  eastern  part  the  drift  is  thicker  than 
in  the  western  part,  and  consequently  the  ledges  are  frequently  small 
isolated  exposures,  single  knobs,  or  collections  of  knobs,  that  are  presum- 
ably the  tops  of  hillocks  with  several  peaks,  separated  from  one  another 
by  littlu  defiles.  The  hillocks  themselves  are  separated  by  drift  deposits, 
so  that  the  Southern  Complex  in  its  eastern  portion  consists  in  reality 
of  distinct  areas. 

Between  the  north-and-south  center  line  of  R.  27  W.  and  the  west 
line  of  R.  28  W.  the  country  is  swampy  and  ledges  are  rare.  Wlien  they 
occur  it  is  as  small,  low  outcrops  in  the  midst  of  the  swamps.  Farther 
west  hills  and  swam[)s  alternate,  and  the  surface  has  the  usual  aspect  of 
pre-Algonkian  topography. 

In  its  eastern  portion  the  rocks  comprising  the  Southern  Complex 
form  a  narrow  belt  bordering  the  Marquette  sediments  and  extending 
southward  under  a  broad  sand  plain,  above  which  here  and  there  isolated 
knobs  of  granite  protrude,  thus  indicating  the  presence  of  pre-Algonkian 
rocks  beneath  the  saiids.  To  the  west  the  belt  expands,  until,  near  the 
Michigamme  River,  it  is  many  miles  in  width.  Here  the  area  is  divided 
by  the  Republic  tongue  of  the  Marquette  rocks  into  a  large  eastern  portion 
and  a  nairow  western  one,  which,  uniting  just  south  of  the  city  of  Republic, 
merge  into  one  large  area. 

To  the  east,  near  Lake  Superior,  granites  predominate.  Westward 
from  the  lake  shore  for  10  miles  these  are  about  the  only  members  of 
the  Southern  Complex  met  with.  Farther  west  schists  become  involved 
with  the  granites  in  the  most  intricate  manner,  so  that  frequently  it  is 
impossible  to  declare  whether  the  former  or  the  latter  rocks  are  the  more 


192  THE    MARQUETTE   IROX-BEARING   DISTRICT. 

abundant.  As  the  work  in  the  Southexni  Complex  progresses  it  is  probable 
that  the  schist  areas  and  granite  areas  will  be  differentiated  from  each 
other,  and  that  a  correct  map  will  show  large  granite  areas  surrounded  by 
schistose  rocks  and  separated  from  each  other  by  areas  in  which  schists  are 
largely  predominant. 

COMPARISON   WITH  NORTHERN  COMPLEX. 

As  compared  with  that  portion  of  the  Northern  Complex  studied,  it 
is  found  that  the  southern  area  contains  fewer  greenstone-schists.  More- 
over, in  the  southern  area  hornblendic  and  micaceous  gneisses  and  schists 
are  abundant,  whereas  in  the  northern  area  they  ax'e  absent.  The  granite 
is  intrusive  in  these  schists,  and  also  in  the  few  greenstone-schists  present. 
The  relations  of  the  greenstone-schists  to  the  hornblendic  and  micaceous 
ones  are  not  known,  liut  it  is  thought  probable  that  the  latter  are  older 
than  the  former,  and  that  this  fact  would  account  for  their  absence  in  the 
northern  area,  where,  if  they  ever  existed,  they  must  be  buried  beneath 
the  tuffs  and  lava  flows  that  have  produced  the  schistose  greenstones. 

THE   SCHISTS. 

The  schists  of  the  Southern  Complex  comprise  hornblendic  and 
micaceous  schists  and  greenstone-schists  similar  to  the  greenstone-schists  of 
the  Northern  Complex,  granite-gneisses,  and  the  Palmer  gneisses,  which, 
because  they  are  so  closely  allied  to  the  granite-gneisses  are  discussed  with 
the  latter  rocks. 

The  best  exhibition  of  the  various  hornblendic  and  micaceous  schists 
is  in  the  area  lying  southeast  of  Lake  Michigamme  and  southwest  of 
Champion,  constituting  the  northeast  quarter  of  T.  47  N.,  R.  30  W.  (Atlas 
Sheet  IX).  The  district  is  covered  with  small  knobs  and  large  hills  with 
bare  tops,  on  which  the  relations  of  the  schists  and  the  granite  may  be 
easily  studied.  Occasionally  a  knob  may  consist  exclusively  of  granite  or 
of  the  schists,  but  usually  both  schists  and  granites  are  found  in  it,  the 
granite  often  occupying  the  higher  parts.  The  schists  include  both  horn- 
blendic and  micaceous  kinds,  of  which  the  latter  are  the  more  common, 
though  the  former  are  not  rare.  The  micaceous  vai'ieties  are  well  lianded 
with  lig-lit  and  dark  lavers,  measuring'  from  a  line  or  so  to  several  inches  in 


THE    SOUTHEltN   COMPLEX. 


193 


loreadth.  On  weathered  surfaces  the  bauds  show  plainly,  but  on  fresli  sur- 
faces they  are  often  scarcely  perceptible.  Where  undistui'bed  by  granite 
intrusions  the  bands  strike  about  northeast  and  dip  northwest  at  a  very  high 
angle.  Near  the  contacts  with  the  granite  they  are  much  contorted.  The 
hornblendic  schists  are  sometimes  banded,  Ijut  the  l)anding  is  not  s(_>  reguLu- 
as  in  the  case  of  the  micaceotis  rocks. 

There  can  l)e  no  question  that  the  granite  is  intrusive  in  the  schi.sts. 
Its  dikes  and  veins  cut  tlie  schists  in  all  conceivable  directions.  Perhaps 
more  frequently  than  otherwise  tlie  dikes  run  parallel  to  the  banding  of  the 


intruded  rocks,  but  they  nevertheless  often  cut  across  them,  crumpling  and 
contorting  the  bands.  The  most  easily  accessible  locality  at  which  these 
relations  may  be  seen  is  on  a  little  knob  just  south  of  the  middle  shaft  of 
the  Champion  mine,  where  the  coarse  white  granite,  so  abundant  farther 
south,  sends  broad  dikes  with  branching  arms  into  a  black,  glistening 
mica-schist  (fig.  7).  The  same  granite  a  little  to  the  southwest  contains 
numerous  large,  sharp  fragments  of  a  similar  schist,  which  it  has  evidently 
taken  up  in  its  passage  to  its  present  position. 
:vioN  xxviii 13 


194  THE  MAEQUETTE  IKON-BEARING  DISTRICT. 

Although  more  abundant  in  this  portion  of  the  southern  area  than 
elsewhere,  the  hornbleudic  and  micaceous  schists  are  not  confined  to  it. 
Small  exposures  of  them  are  found  scattered  among  the  granite  knobs 
as  far  east  as  the  east  line  of  sec.  34,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXV),  and  as  far  west  as  Republic  (Atlas  Sheet  XI),  forming  almost  as 
gTeat  a  proportion  of  the  rocks  in  this  \acinity  as  they  do  in  the  neighbor- 
hood of  Champion.  In  the  interior  of  the  area  they  are  probably  also  quite 
common.  Wherever  found,  most  of  the  schists  are  more  or  less  definitely 
banded  and  always  distinctly  foliated.  The  general  direction  of  their 
banding  varies  in  its  strike  from  north  to  northeast,  and  in  its  dip  from  45° 
northwest  or  west  to  as  much  southeast  or  east.  Usually  the  dips  are  very 
steep,  and  often  they  are  perpendicular. 

Occupying  less  extended  areas  are  the  other  foliated  rocks  of  the 
Southern  Complex.  These  are  the  greenstone-schists  and  the  various 
gneisses.  The  former  occupy  a  distinct  but  very  small  area  near  the  shore 
of  Lake  Superior  (Atlas  Sheet  XXXIX),  where  they  present  the  same 
featm-es  as  some  of  the  corresponding  rocks  in  the  Northern  Complex. 
Their  foliation  strikes  about  east  and  west,  and  their  dip  is  nearly  vertical. 
Occasionally  similar  schists  are  found  in  other  parts  -of  the  southern  area, 
interspersed  among  the  other  rocks.  Under  these  conditions  they  appear  to 
be  mainly  schistose  dikes. 

The  gneissoid  granites  are  more  common  in  the  western  portion  of  the 
Southern  Complex  than  in  its  eastern  portion,  though  they  are  found  also  in 
the  latter  area.  Their  distribution  is  quite  uniform  throughout  the  granite, 
but  their  abundance  is  inconsiderable  when  compared  with  their  abundance 
in  the  Northern  Complex.  No  definite  relations  as  to  distribvition  have 
been  determined  to  exist  between  these  gneisses  and  the  massive  granite. 

The  Palmer  gneisses  are  found  only  along  the  southern  side  of  the 
Marquette  syncline.  Their  general  distribution  is  indicated  on  the  map 
(Atlas  Sheet  IV).  Further  reference  to  them  and  to  the  gneisses  is  deferred 
until  the  gi-anites  are  discussed. 

From  the  statements  already  made  it  is  evident  that  our  information 
concerning  the  distribution  and  relations  of  the  schists  of  the  Southern 
Complex  is  very  incomplete.     So  little  detailed  work  has  been  done  in 


THE   MICACEOUS    SCHISTS.  195 

the  area  that  we  are  obhged  to  hmit  ourselves  to  descriptions  of"  the 
microscopical  features  of  the  specimens  collected  near  the  borders  of  the 
Algonkian  sediments,  and  to  content  ourselves  with  suggestions  as  to  the 
legitimate  conclusions  to  be  drawn  from  them.  For  this  purpose  we  may 
divide  tlie  southern  schists  into  tlie  mic-aceous  and  tlu'  liornblendic  varieties,, 
leaving  the  Palmer  gneisses  to  be  treated  with  the  gneissoid  granites. 

THK    MICACEOUS    SCHISTS. 

The  micaceous  schists  include  true  mica-schists,  consisting  essentially 
of  quartz  and  nmscovite,  or  (|uartz  and  biotite;  feldspathic  mica-schists, 
containing,  in  addition  to  quartz  and  biotite,  a  large  quantity  of  feldsjiar; 
and  liornblendic,  micaceous  schists,  which  differ  from  the  feldspathic  varie- 
ties in  possessing  some  green  hornblende.  These  varieties  grade  into  one 
another  insensibly,  so  that  there  is  represented  in  hand  specimens  a  com- 
plete succession  of  types  from  the  typical  mica-schists  to  rocks  that  might 
be  called  hornblende-mica-gneisses.  Even  in  a  single  hand  specimen  the 
alternate  bands  may  consist  of  feldspathic  and  nonfeldspathic  schists,  or 
of  the  latter  and  the  liornblendic  ^•arieties.  There  is  such  an  intimate  rela- 
tionship exhibited  between  all  these  rocks,  when  their  thin  sections  are 
examined  under  the  microscope,  that  there  can  be  no  doubt  as  to  their- 
genetic  connection.  The  separation  into  classes  is  merely  for  convenience, 
in  descri|)tion. 

MUSCOVITE-SCHLSTS. 

The  muscovite-schists  are  rare.  They  are  highly  foliated,  silverv-gi-ay 
rocks,  with  contorted  folia.  They  are  not  so  definitely  l)auded  as  are  the 
less  markedly  foliated  rocks,  though  bands  can  still  be  detected  in  some 
specimens.     In  general  appearance  they  are  typical  mica-schists. 

Under  the  microscope  their  thin  sections  show  only  elongated  quartzes, 
muscovite,  biotite,  limonite,  a  few  grains  of  magnetite,  and  tin}'  plates  of 
hematite.  The  muscovite  occurs  as  little  lamiiife  cutting  through  the  (]uartz 
grains  and  as  long  wisps  between  them.  It  is  to  the  existence  of  these 
long  wisps  and  of  the  elongated  quartzes  that  the  schistosity  of  the  rock 
is  due.  The  biotite,  which  is  present  in  small  quantities  only,  appears  as 
small  brown  flakes  scattered  through  the  quartzes.     Some  in-egular  areas 


196  TUE   MAEQUETTE   IliOX-BEAEING   DISTRICT. 

of  a  matted  mass  of  tiny  sericite  or  kaolin  fibers  may  represent  an  orig-iual 
feldsj^ar,  but  if  so,  no  other  evidence  of  its  former  existence  remains. 

lilOTITK-SCIIlSTS. 

The  biotite-schists  are  much  more  abundant  than  the  muscovitic  vari- 
eties. As  seen  in  the  hand  specimen,  they  vary  from  compact  or  shghtly 
schistose,  dark-gray  rocks  (No.  16922,  analysis,  p.  202),  resembling  fine- 
grained, dark  quartzites,  to  sandy,  slaty,  light-gray  ones  (No.  16913,  analysis, 
p.  202),  resembling  friable  sandstones. 

Under  the  microscope  they  sometimes  appear  almost  massive.  Usually, 
howe^•er,  their  mica  flakes  are  arranged  with  their  long  axes  approximately 
{jarallel,  and  their  other  components  are  more  or  less  elongated  in  the  same 
direction.  Quartz  is  the  principal  component.  Its  grains  are  elongated, 
and  where  they  are  in  contact  they  interlock  l)y  irregular  sutures.  They 
frequently  contain,  as  inclusions,  spicules  of  green  liornblende  and  small 
flakes  of  biotite.  Feldspar  is  also  abundant.  A  few  irregular  grains  of 
clear  plagioclase  and  kaolinized  grains  of  an  untwinned  feldspar,  probably 
orthoclase,  lie  between  the  quartzes,  but  they  are  found  only  occasionally. 
The  greater  portion  of  the  feldspar  is  altered  into  kaolin,  chlorite,  etc. 
Biotite  is  the  characteristic  component.  It  is  found  in  large  and  small 
reddish-brown  flakes  lying  between  the  quartz  grains,  and  often  including 
several  of  them.  Magnetite,  zircon,  epidote,  limonite,  and  hematite  are 
found  in  all  sections,  but  in  very  minute  quantities.  Muscovite  is  a  little 
more  plentiful,  but  this  also  is  rare.  It  is  present  in  the  kaolinized  feldspar, 
but  not  elsewhere  in  the  slides.  In  some  few  cases  the  biotite  has  been 
changed  to  chlorite,  when  it  loses  its  brown  color.  Otherwise  the  rocks 
are  very  monotonous  in  their  features  (see  fig.  8,  on  the  opposite  page). 

FELDSPATHIC    BIl  ITITE-SCHISTS. 

The  feldspathic  schists  are  more  varied  in  character,  mainly  because 
of  the  large  quantities  of  feldspar  present  in  them.  This,  by  its  alteration, 
gives  rise  to  various  secondary  jjroducts.  The  rocks  are  very  much  like 
gray  wackes  in  their  macroscopic  appearance.  They  are  fine-grained,  frag- 
mental-looking,  gray  rocks,  with  liands  of  lighter  and  darker  shades  (No. 
16765,  analysis,  p.  202).     In  thin  section  they  are  seen  to  be  composed  mainly 


THE    MICACEOUS    SCHISTS. 


197 


of  elongated  quartzes,  brown  biotite,  plagioclase,  and  ortlioclase  or  its  decom- 
position products,  kaolin,  sericite,  and  epidote.  Garnets  are  present  in  some 
sections,  and  in  others  tourmaline  occurs  in  very  small  quantity.  Tlie 
quartz  and  the  biotite  present  no  unusual  features.  The  latter  niiucnd  is 
often  chloritized,  as  is  also  some  of  the  ortlioclase,  so  that  the  (pantity  of 
chlorite  is  much  greater  in  these  rocks  than  it  is  in  the  nonfeldspathic  schists. 

The  feldspars  are  the  most  interesting  constituents.  They  are  nearly 
always  much  altered,  the  ortlioclase  more  so  than  the  plagioclase.  In  the 
triclinic  feldspar  the  twinning  bars  are  always  recognizable  and  the  material 
of  the  grains  is  often  clear.  With 
the  ortlioclase  the  case  is  different. 
Occasional  traces  of  Carlsbad  twin- 
ning are  obscurely  visible,  Vmt  the 
mineral,  is  so  clouded  with  flakes  of 
sericite,  kaolin,  chlorite,  and  brown 
biotite,  with  grains  of  epidote  and 
quartz,  needles  of  green  hornblende^ 
and  the  dust  of  magnetite,  lliat  its 
original  nature  in  most  cases  is  dif- 
ficult to  prove. 

The  arrangement  of  the  decom- 
position prodiicts  of  the  ortlioclase 
is  irregular.  Within  the  body  of  the 
mineral  they  form  a  web  of  inter- 
woven sjiicules,  in  the  interstices  of 
which  are  little  grains  of  (piartz  and  small  areas  of  the  undecomjiosed 
feldspar.  Portions  of  the  secondary  aggregate  extend  beyond  the  original 
outlines  of  the  grains  and  penetrate  between  the  pi'imary  quartzes  and  the 
biotites.  Thus  it  frequently  seems  as  though  the  rock  were  a  fragmental 
one,  since  we  find  rounded  grains  of  quartz  and  irregular  flakes  of  brown 
biotite  embedded  in  a  fibrous  groundmass  which  in  appearance  is  not  unlike 
the  material  of  a  biotite-slate.  A  close  inspection  of  the  aggregate,  liow- 
eve>i',  shows  that  the  quartz  grains  have  not  the  outlines  of  waterwom 
grains,  nor  does  the  fibrous  and  finely  granular  groundmass  in  which  they 


Fio  8  — Thm  section  of  fel.Ispathic  biotite  s< 

hist.     Xo 

.  16903. 

from  875  stops  X    125stc 

p^W  ,of  SE  cornel ' 

i.tsec.ll.T 

■.47X., 

E    30    W      Sertioil  >,1k 

)ws  typical  stnutiii 

c   of  the  ■ 

coar.Her 

schists,  rich  m  feldspai 

r.    The  li!;ht  colored 

irregular 

grains 

are  quart?    thfidoadv 

ones  feldsiiar,  and  tli 

le  dark  on 

les  bio. 

tite.    Natural  light  X  1 

55. 

198  THE   MAlfQUETTE    lEON-BEARING  DISTRICT. 

lie  bear  the  same  relations  to  them  as  does  the  fine-grained  fragmental 
matrix  to  the  larger  grains  in  a  sandy  slate.  Large  areas  of  the  matrix 
polarize  with  indefinite  outlines,  resembling  the  irregular  outlines  of  crys- 
talloids of  feldspar  in  a  granite — a  phenomenon  due  to  the  remains  of 
slightly  altered  feldspar  left  between  the  meshes  of  its  alteration  products. 
The  larger  quartzes  embedded  in  this  groundmass  of  decomposed  feldspar 
are  all  compound.  The  elongated  grains  are  made  up  of  a  coarse  mosaic 
of  smaller  .grains,  the  direction  of  whose  longer  axes  appears  to  be  inde- 
pendent of  that  of  the  larger  aggregate.  Under  crossed  nicols  these  rocks 
resemble  a  lot  of  nests  of  quartz  mosaic  in  a  groundmass  composed  of 
ill-defined  plagioclase  grains  and  large  flakes  of  brown  biotite  in  a  matrix 
of  small  grains  of  qiiartz  and  fibrous  decomposition  products  of  orthoclase. 
They  thus  simulate  very  strongly  certain  sedimentary  schists. 

As  the  quantity  of  feldspar  increases  the  supei'ficial  resemblance  of  the 
schists  to  fragmental  rocks  becomes  stronger,  for  the  alteration  products 
are  greater  in  quantity  and  the  original  outlines  of  the  feldspathic  grains 
are  more  and  more  obscured. 

HORNBLENUIC   BIOTITE-SCHISTS. 

The  hornblendic  micaceous  schists  differ  from  the  feldspathic  varieties 
in  that  they  contain  large  crystals  of  green  hornblende  that  are  idiomor- 
phic  in  cross-section  and  are  frequently  twinned.  They  form  very  much 
larger  jjlates  than  do  any  other  minei'als  in  the  rocks,  and  often  these  plates 
surround  and  inclose  a  half  dozen  or  mcu*e  grains  of  quartz  or  feldsjiar. 
The  hornblende  is  evidently  the  latest  mineral  formed  in  the  rocks  in  which 
it  occurs,  and  is  quite  certainly  secondary.  Otherwise  the  hornblendic 
varieties  are  similar  to  the  feldspathic  micaceous  schists. 

STRUCTURE. 

The  structure  of  the  muscovite-scliists,  and  of  many  of  the  biotitic 
varieties,  is  that  of  typical  schists  (see  fig.  8,  p.  197).  Others  of  the  biotite- 
schists,  particularly  those  containing  much  feldspar,  have  the  cataclastic 
structure,  which  in  many  cases  resembles  the  fragmental  structure  of  a 
sedimentary  rock.     However,  although  their  components  are  broken  and 


THE   MICACEOUS   SCHISTS.  199 

shattered,  there  can  not  be  deteeted  iuuoug-  the  frag-uieuts  any  tliat  present 
the  lea.st  evidence  of  being  waterworn.  Their  quartz  grains  are  in  all 
cases  either  ver^'  sharp-edged  fragments  or  they  interlock  with  t'.ie  other 
components  by  very  in-egular  sutures.  The  feldspars  liave  been  fractured 
also,  but  in  this  case  too  the  fragments  are  very  sharp-edged.  Occasionally 
an  altered  feldspar  has  escaped  rupture  and  has  preserved  its  original  form, 
when  it  appears  as  a  phenocryst  in  a  cataclastic  matrix. 

In  the  most  altered  phases  of  the  feldspathic  schists  tlie  thin  sections 
present  a  strikingly  sedimentary  aspect.  Rounded  grains  of  quartz  and 
feldspar  are  embedded  in  an  aggregate  of  secondary  substances,  just  as  the 
grains  of  (|uartz  in  a  sandy  slate  are  embedded  in  a  fine-grained  aggregate 
of  kaolin,  etc.  But  in  this  case  the  rounding  of  the  grains  is  plainly  due 
to  decomposition  (see  pp.  197-198),  since  numbers  of  them  that  are  side  by 
side  extinguish  simultaneously  between  crossed  nicols.  In  no  instance  have 
any  waterworn  grains  been  detected  in  any  of  these  rocks,  and  hence  none 
of  them  exhibit  any  evidence  of  a  sedimentar)'  origin,  however  uuicli  they 
may  at  first  glance  look  like  sediments. 

The  foliation,  which  all  the  micaceous  schists  exhibit,  is  the  result 
of  the  flattening  of  their  larger  quartz  and  feldspar  grains  in  a  uniform 
direction  and  the  arrangement  of  the  larger  biotite  flakes  with  their  longer 
axes  in  the  same  direction.  The  cataclastic  grains  (those  formed  by  the 
fracture  of  the  larger  grains)  are  not  necessarily  elongated,  though  many 
of  them  are  so,  and  where  they  are  their  longer  axes  are  not  always 
uniformly  orientated.  The  banding  of  the  schists  is  due  mainly  to  the 
greater  abundance  of  biotite  in  certain  planes  than  elsewhere 

AYliatever  maA'  be  the  origin  of  the  banding  of  these  rocks,  it  is  clear 
that  their  foliation  is  tlic  result  of  mashing.  The  bending  of  hu-ge  biotite 
plates,  the  cracking  of  tlie  feldspars,  and  the  granulation  of  the  quartzes,  so 
frequently  observed  in  thin  sections,  and  the  presence  of  cataclastic  grains 
in  general,  are  proof  that  the  rocks  have  been  subjected  to  crushing  forces. 
That  there  has  been  mashing  is  shown  also  b}-  the  streaming  of  biotite 
flakes  around  the  porphyritic  feldspars.  In  a  rock  (specimen  No.  1G764) 
from  near  the  XW.  corner  of  sec.  2,  T.  47  N.,  R.  30  W.  (Atlas  Sheet  IX),  for 
instance,  there  is  a  large  Carlsbad  twin  of  orthoclase,  surrounded  by  lines 


200  THE   MARQUETTli   IROlSr  BEARING   DISTRICT. 

of  biotite  flakes  tliat  are  bent  to  conform  with  the  outlines  of  the  crystal. 
Phenomena  of  the  same  kind  are  met  with  in  so  many  sections  that  the 
belief  in  a  dynamic  origin  for  the  foliation  of  tlie  schists  is  irresistible. 
The  schistosity  was  imposed  upoii  the  rocks  prior  to  the  alteration  of  the 
feldspar  in  some  cases,  and  in  other  cases  it  was  produced  subsequent  to 
this  alteration  This  is  easily  accounted  for  ou  the  supposition  that  the 
feldspars  were  partially  altered  before  the  rocks  containing  them  were 
mashed,  and  that  the  alteration  continued  after  the  mashing.  While  in  some 
cases  the  alteration  products  are  arranged  as  in  a  fine-grained  schist,  with 
the  schistosity  planes  parallel  to  the  elongation  of  the  quartzes  and  to  the 
longer  axes  of  the  large  biotite  flakes,  in  otiier  instances  no  such  general 
arrangement  is  noticeable.  In  these  cases  the  small  biotite  flakes  in  tlie 
secondary  aggregate  lie  in  all  azimuths,  except  where  their  positions  have 
been  detei'mined  by  the  structure  of  the  mineral  from  which  they  were 
derived.  In  the  section  of  rock,  No.  19034,  for  example,  the  tiny  biotites 
are  often  found  in  two  series  of  lines  crossing  each  other  approximately 
at  right  angles,  having  been  formed  apparently  in  the  cleavage  cracks  of 
feldspars. 

COMPOSITION   AND   ORIGIN. 

The  micaceous  schists  are  so  much  altered  that  the  iiatiii-e  of  the 
original  rock  from  which  they  were  formed  is  not  known.  The  existence 
of  large  crystals  of  orthoclase  with  the  outlines  of  phenocrysts  in  the  midst 
of  a  cataclastic  groundmass  would  seem  to  indicate  that  the  original  rock 
was  an  acid  j)orplivry,  but  these  are  so  rare  that  any  Ijroad  generalization 
based  upon  their  presence  must  be  of  doubtful  value.  It  is  true  that  no 
sedimentary  grains  have  been  discovered  in  any  of  the  thin  sections,  and 
this  fact  would  seem  to  point  to  a  similar  conclusion.  Ikit  all  the  rocks 
have  been  so  greatly  altered  in  structure  liy  the  changes  they  have  under- 
gone that  it  would  be  surprising  if  any  evidence  of  their  original  structure 
were  discoverable. 

From  the  evidence  of  the  microscope,  all  that  can  be  said  regarding 
the  origin  of  the  schists  is  that  they  are  more  probably  igneous  rocks  than 
sedimentary  ones.  Their  banding  may  be  accounted  for  on  the  supposition 
that  they  occurred  as  flows  of  lava,  for,  though  they  are  as  evenly  banded 


ORIGIN   OF   THE   MICAOEOI^S    SCHISTS. 


201 


as  many  modern  slates  and  sandstones,  the  micaceous  schists  are  seen  in 
the  field  to  be  interbedded  with  hornblende-schists  of  whose  igneous  origin 
there  can  be  little  doubt.  It  may  be  that  some  of  the  schists  are  altered 
tuffs,  and  that  their  banding  is  due  in  part  to  the  original  stratification  of 
the  tuffaceous  l)eds,  as  is  the  case  with  the  greenstone-schists  of  the  Northern 
Complex,  but  of  this  there  is  as  little  positive  proof  as  there  is  of  a  sedi- 
mentary origin  for  any  of  the  schists. 

Dr.  Adams'  has  attempted  to  get  some  light  on  the  origin  of  the  gneisses 
of  the  Clrenville  series  in  Ontario  by  comparing  tlieir  composition  with 
that  of  slates  and  granites.  He  calls  attention  to  the  fact  that  while  the 
average  amount  of  the  alkalis  in  granites  is  7.35  per  cent,  in  23  |)rlmitive 
slates  it  is  (mly  4.7  jter  cent,  or  two-thirds  as  great.  Moreover,  the  slates 
are  much  higher  in  alumina  than  the  granites,  while  at  the  same  time 
they  are  lower  in  silica.  The  slates  also  contain  more  magnesia  than  lime, 
whereas  the  granites  contain  more  lime  than  magnesia.  After  making  his 
comparisons  Adams  concludes  that  the  Grenville  gneisses  are  more  nearly 
like  the  slates  in  composition  than  like  the  granites.  Of  course  such  a 
comparison  as  this  is  of  doubtful  utility  as  a  means  of  determining  the 
origin  of  rocks  that  have  suffered  such  a  multitude  of  changes  since  their 
deposition  as  have  the  schists  under  consideration.  Even  if  it  were  known 
that  their  composition  had  not  suffered  much  change  under  the  influences 
of  metamorphism,  the  comparative  process  could  be  of  little  aid  in  discov- 
ering their  origin,  unless  the  composition  of  both  the  granites  and  the  slates 
which  they  yielded  were  known.  ZirkeP  has  shown  that  the  range  of 
composition  in  granites  is  very  great.  His  maximum,  minimum,  and  mean 
figures  for  their  various  constituents  are  as  follows: 


Range  of  composition  i 

n  granite 

SiO,          Al,03 

Fe,0, 

CaO 

MgO. 

K,0 

N^,0 

Maximum 

81.77       19.05 
72            16 
60.50         7.02 

7,1. 
1.50 

5.65 

1.50 

Trace. 

3.17 

.50 

Trace. 

9.25 
6.50 
.56 

6.70 
2.50 
.04 

Minimum 

■  A  further  contribution  to 
3d  series,  Vol.  L,  1895,  p.  58. 

-F.  Zirkel,  Leiirbucli  der  Petrogr.aphie,  Vol.  II 


r  knowledge  of  the  Laurentian,  by  P.  D.  Adams: 
p.  30. 


202 


THE  MARQUETTE   lEOX- BEARING  DISTRICT. 


A  schist  may  therefore  have  any  composition  within  a  very  wide  range, 
and,  ahhough  this  composition  may  be  identical  with  that  of  some  slate,  or 
even  with  the  mean  composition  of  many  slates,  the  rock  may  nevertheless 
be  a  very  slightly  altered  granite. 

Three  analyses  of  the  micaceous  schists,  one  complete  and  two  par- 
tial, have  been  made  (Analyses  IV,  V,  and  VI).  These  are  compared 
with  an  analysis  of  Cambrian  slate  from  Melbourne,  Province  of  Quebec 
(Analysis  I),  and  with  analyses  of  the  amphibole-granitite  from  Hoh- 
wald  (Analysis  III)  and  the  granitite  from  Landsberg,  in  the  Andlau 
(Analysis  II).  As  will  be  seen,  the  two  granites  vary  in  the  proportions 
of  the  alkalis  and  the  alkali  earths  present,  the  Hohwald  rock  containing 
but  4  per  cent  of  the  former,  while  the  Andlau  rock  contains  7  per  cent. 
In  each,  CaO  exceeds  MgO  in  quantity.  The  composition  of  the  slate 
is  not  very  different  from  that  of  the  granites  except  in  one  particular — 
the  percentage  of  MgO  present  is  over  five  times  that  of  the  CaO. 


Analyses  of  slate,  ffranitites,  and  acMsts. 


SiO. 64.20 

TiOi 1 

AloO, I    16.80 

Fe;03 


FeO 

MnO 

CaO 

MgO 

K.2O 

Na.,0 

H,0  at  100^  .... 
H;0  above  100° . 
P;05 


Total 


18.  038 
4.213 


1.812 
1.161 


15.43 
Undet. 
Undet. 


4.47 
Undet. 

2.80 
Undet. 
Undet. 


17.23 
Undet. 
Undet. 


.87 
Undet. 
Undet. 

.29 


89.79 


Trace. 
2.34 
1.22 
2.43 
2.55 

.22 
2.04 

.19 


I.  Cambrian  slate  from  Melbourne,  Quebec.     Analyst,  T.  S.  Hunt.     Am.  Jour.  Sci.,  3d  series, 
Vol.  L,  189.5,  p.  67. 

II.  Granititi!  from  Landsberg,  near  Barr.     Analyst,  Dr.  H.  Unger.     H.  Rosenbusch,  Die  Steiger 
Scliiefer,  etc.,  1877,  p.  147. 

III.  Ampliibole-griinitite  from  Hohwald.     Analyst,  Dr.  H.  Unger.     H.  Hoseubusch,  Die  Steiger 
Schiefer,  etc.,  p.  167. 


TDE    nORXBLEXDIC    SCHISTS.  203 

IV.  No.  16765.  Dark-colored,  finely  banded  micaceous  schist,  from  near  SW.  corner  of  sec.  .35, 
T.  48  N.,  R.  30  W.  Much  altered.  Large  feldspar  grains,  small  (juantity  plagioclase,  irregular  quartz, 
little  hornblende,  considerable  biotite.     Analyst,  George  Steiger. 

Structure  cataclastic,  approaching  sedimentary  in  appearance. 

V.  No.  16913.  Light-gray  handed  schist,  from  700  steps  N.,  1450  steps  W.,  of  SE.  corner  of  sec.  3, 
T.  47  N.,  R.  30  W.  Not  so  much  altered  .as  16765.  Feldspar,  quartz,  some  biotite,  and  a  very  little 
hornblende.     Kaolin  quite  abundant.     Analyst,  George  Steiger. 

Structure  granulated,  approaching  sedimentary  fragmental. 

VL  No.  16922.  Very  dark  gray  foliated  schist  from  520  steps  N.,  1120  steps  W.,  of  SE.  corner 
of  sec.  30,  T.  47  N.,  R.  30  W.  Banded  in  the  field,  but  not  in  hand  specimen.  Contaius  large  irregular 
quartz  grains  and  comparatively  fresh  plagioclase  and  orthoclase.     Analyst,  George  Steiger. 

Structure  foliated,  like  typical  crystalline  schist. 

Upon  comparison  of  these  analyses  it  will  be  seen  that  the  micaceous 
schists  are  in  most  respects  as  much  like  the  granites  as  they  are  like  the 
slate.  With  reference  to  the  percentages  of  CaO  and  MgO  present  in 
them,  they  are  much  more  like  the  granites.  The  granitite  contains  three 
times  as  much  CaO  and  MgO,  and  the  amphibole-granitite  tvAo  and  one- 
half  times  as  much,  while  the  slate  contains,  on  the  other  hand,  o^'er  five 
times  as  much  MgO  and  CaO.  The  biotite-schist,  No.  16922,  contains  twice 
as  much  CaO  as  MgO,  while  in  Nos.  16913  and  16765  the  excess  of  CaO 
over  MgO  is  probably  even  greater;  so  tliat  if  the  analyses  show  anything 
they  indicate  that  the  schists  are  altered  granitites  rather  than  altered  sand- 
stones or  shales.  In  other  words,  the  weight  of  evidence,  while  by  no  means 
conclusive,  is  indicative  of  an  igneous  rather  than  a  sedimentary  origin  for 
the  rocks  in  question,  and  is  in  accord  with  the  little  evidence  afforded 
by  the  microscopic  investigation  of  their  thin  sections.  Whether  the  rocks 
were  flows  of  acid  lava  interbedded  with  the  rocks  that  yielded  the  horn- 
blendic  schists,  or  whether  they  were  in  large  part  beds  of  tuff,  has  not  been 
determined.  The  even  banding  of  many  of  the  schists  may  be  thought  to 
indicate  tlie  latter  origin,  but  even  banding  is  known  to  l^e  characteristic  of 
some  lavas,  and  in  dynamically  metamorphosed  rocks,  like  the  micaceous 
schists,  it  is  known  sometimes  to  be  the  direct  result  of  mashing. 

THE   HOENBLENDXC   SCHISTS. 

Those  schists  whose  predominant  bisilicate  constituent  is  a  green  horn- 
blende may  be  divided  into  two  classes,  between  which,  however,  there 


204  THE    MARQUETTE   lEON-BEAKING   DISTEICT. 

seems  to  be  every  stage  of  gradation.  In  the  one  class  are  placed  the 
greenstone-schists,  composed  of  hornblende,  plagioclase,  and  the  alteration 
products  of  the  feldspar,  and  in  the  other  class  a  series  of  lustrous,  black, 
foliated  rocks,  which  we  shall  call  amphibole-schists.  They  consist  essen- 
tially of  green  liornblende,  fresh  plagioclase,  and  quartz.  All  of  these  rocks 
are  so  similar  to  certain  phases  of  the  green  schists  of  the  Northern  Complex 
that  their  descriptions  need  not  detain  us  long.  The  greenstone-schists  are, 
clearly,  altered  and  foliated  basic  crystalline  eruptives,  and  since  they  pass 
by  intermediate  phases  into  the  amphibole-schists,  it  is  believed  that  these 
also  are  squeezed  eruptives,  in  spite  of  the  fact  that  they  are  often  banded 
and  that  some  of  them  contain  no  inconsiderable  quantity  of  quartz. 

GREEKSTONE-SCHISTS. 

The  greenstone-schists  in  the  hand  specimen  and  in  thin  sections 
resemble  more  closely  those  schists  of  the  Northern  Complex  that  were 
derived  from  basic  dikes  and  lava  flows  than  they  do  those  derived  from 
tuffs.  In  the  hand  specimen  they  present  a  wide  variation  in  appearance. 
Some  of  them  are  fine-grained,  light  greenish-gray,  almost  massive,  or 
slightly  foliated  rocks ;  others  are  dark-gray,  fibrous  schists ;  while  still 
others  are  finely  banded,  green  and  white  schists.  The  latter  are  less 
common  than  the  other  two  varieties. 

In  the  thin  section  nearly  all  the  rocks  show  plainly  their  original 
character.  Altered  plagioclase  and  green  hornblende  are  their  principal 
components.  The  feldspar  is  changed  more  or  less  completely  into  an 
aggregate  of  epidote,  saussurite,  quartz,  and  chlorite,  with  occasionally  a 
small  admixture  of  a  micaceous  mineral.  In  addition  to  the  altered  plagio- 
clase there  is  also  present  in  many  sections  a  fine  mosaic  of  fresh 
plagioclase,  resembling  the  feldspathic  mosaic  of  many  of  the  greenstone- 
schists  of  the  Northern  Complex.  An  untwinned  decomposed  feldspar, 
which  is  thought  to  be  orthoclase,  is  also  met  with  in  a  few  sections.  Its 
alteration  products  are  mainly  sericite  or  muscovite.  The  green  amphibole 
is  in  three  forms  ;  it  exists  as  long,  slender  needles  penetrating  the  deconi- 
position  aggregates  of  the  feldspars,  as  large  plates  and  aggregates  of  flakes 
occupying  spaces  formerly  occupied  by  augite,  and  as  compact  crystals,  idio- 
morphic  in  the  prismatic  zone.     The  abundance  of  the  compact  amphibole 


THE    HORXBLENDIC    SCHISTS.  205 

seems  to  increase  as  the  schistosity  of  the  rock  becomes  more  marked. 
Ill  the  less  schistose  specimens  the  amphibole  has  been  hirgely  changed 
into  chlorite  and  epidote,  while  calcite  in  large  quantity  saturates  the  rocks. 
In  some  of  the  chlorite  plates  are  series  of  fine  rutile  needles,  cutting  one 
another  at  angles  of  60°,  as  tliough  the  chlorite  hnd  originally  been  a 
biotite.  Moreover,  there  are  occasionally  scattered  through  the  chlorite 
yellowish-brown  flakes  with  the  cleavage,  pleochroisni,  and  extinction  of 
this  mica.  Leucoxene,  spliene,  magnetite,  limonite,  and  hematite  are  met 
with  ill  most  sections,  and  fairly  lai'ge  prisms  of  a  bluisli-brown  tourmaline 
are  discovered  in  a  few. 

Dynamic  effects  are  seen  in  a  number  of  the  least  altered  schists,  but 
they  are  largely  obscured  by  the  great  quantity  of  decomposition  products 
present  in  all  of  them.  Fractured  plagioclases  are  sometimes  so  abundant 
that  the  rocks  look  like  tuff's. 

From  the  microscopical  features  of  the  rocks  and  from  the  strong 
analogy  they  bear  to  the  northern  greenstone-schists  and  the  schistose  basic 
dikes  that  intrude  them,  we  may  safely  conclude  that,  like  the  northern 
rocks,  they  are  squeezed  eruptives — lavas  and  intrusive  masses  in  the  case 
of  the  unhanded  varieties,  and  tuffs  in  the  case  of  the  banded  kinds. 

The  types  of  green  schists  described  are  the  predominant  ones  in  the 
Southern  Complex.  There  are,  however,  a  great  man)-  other  interesting- 
varieties  met  with,  all  of  which  may  be  traced,  under  the  microscope,  into 
the  types  just  described.  Certain  epidotic  varieties  deserve  mention  for  the 
great  quantities  of  this  mineral  they  contain  They  are  composed  very 
largely  of  dark-green,  imperfect  hornblende  crystals,  in  a  matted  mass  of 
smaller  chloritized  flakes  of  the  same  mineral,  and  large  and  small  areas 
of  an  almost  colorless  epidote  and  saussurite  in  plates  and  grains.  Besides 
these  minerals,  brown  hornblende  in  plates,  small  grains  of  quartz,  little 
areas  of  feldspar  mosaic,  and  some  magnetite  are  always  present,  but  of 
these  minerals  only  the  biotite  is  ever  in  large  quantity.  The  biotite 
seems  to  be  more  abundant  near  the  feldspar  areas  than  elsewhere,  and  the 
epidote  appears  to  replace  this  mineral. 

Another  type  that  must  be  briefly  refen-ed  to  is  intermediate  in  its 
characteristics  between  the  greenstone-schists  and  the  amphibole-schists  to 


206  THE    MARQUETTE    IROX-BEARING   DISTRICT, 

be  described  presently.  The  rocks  of  this  type  present  a  very  fresh  aspect. 
As  a  rule  no  alteration  products  can  be  detected  in  them.  The  rocks  are 
now  composed  of  clear  plag-ioclase  and  dark-green  amphibole,  and  usually 
some  biotite.  Tlie  plagioclase  is  in  mediumly  coarse  grains  that  interlock 
in  the  manner  of  dioritic  plagioclase.  These  are  often  dusty,  with  small 
inclusions  of  magnetite,  amphibole,  etc.  The  amphibole  is  in  large  plates, 
often  twinned,  and  nearly  always  idiomorjjhic  in  the  prismatic  zone.  The 
mineral  is  cellular,  possessing  the  structure  of  Salomon's  contact  minerals, 
and  it  is  in  its  present  form  younger  than  'the  feldspar.  The  biotite  is  of 
the  usual  reddish-brown  color.  It  occurs  in  small  flakes  that  lie  between 
the  plagioclases. 

In  structure  and  composition  these  rocks  are  "diorites,"  like  those 
described  by  Williams^  from  the  Northern  Complex,  but  they  are  believed 
to  be  altered  basic  rocks.  In  some  cases,  when  the  grain  is  a  little  finer  than 
in  the  type  described  above,  the  origin  of  the  rock  is  fairly  well  indicated. 
In  addition  to  the  components  mentioned,  there  are  often  large  grains  of  a 
decomposed  feldspar  in  the  midst  of  a  mosaic  of  fresher  ones.  The  former 
are  clouded  with  small  biotite  flakes  and  small  grains  of  quartz,  and  are 
bordered  by  a  clear  mosaic  of  jilagioclase.  In  the  more  massive  forms  of 
the  rocks  the  outlines  of  the  original  large  grains  may  be  detected.  In  the 
schistose  phases  these  gradually  disappear  as  the  schistosity  becomes  more 
marked,  until  in  the  highly  foliated  phases  all  trace  of  the  large  cloudy 
grains  disappears,  and  the  rocks  now  are  aggregates  of  green  hornblende  in 
a  mosaic  of  clear  plagioclase  grains  and  brown  biotite  flakes.  The  coarser 
schists  are  believed  to  have  the  same  origin  as  those  in  which  the  plagio- 
clase is  in  the  form  of  a  fine-grained  mosaic,  the  only  difference  between 
the  two  rocks  being  in  the  size  of  the  grains  of  the  secondary  ^plagioclase. 
Both  are  believed  to  be  dynamically  metamorphosed  forms  of  a  basic 
intrusive  rock  which  may  have  been  a  diorite,  a  gabbro,  or  a  coarse  diabase. 

AMPHIISOLE-SCllISTS. 

The  amphibole-scliists  are  distinguished  from  the  greenstone-schists  by 
the  possession  of  quartz.     This  mineral  is  sometimes  present  in  very  small 

'  The  greenstone-schist  areas  of  the  Menominee  and  Marquette  regions  of  Michigan,  by  G.  H. 
Williams:  Bull.  U.  S.  Gcol.  Survey  No.  62, 1890,  p.  14G. 


TDE    nOKNELENDIC    SCHISTS.  207 

quantity,  when  the  rocks  are  much  like  the  last  type  of  the  greenstoiie- 
schists  mentioned  above.  Sometimes  it  is  present  in  very  large  quantity, 
when  they  resemble  true  hornblende-schists.  Usually  (|uartz  and  fresh 
plagioclase  are  present  in  about  equal  amounts,  ami  in  tliis  case  the  rocks 
are  intermediate  in  character  between  the  greenstone-schists  and  the  true 
hornblende-schists. 

In  the  hand  specimen  the  rocks  of  this  class  have  a  dark-gray  or 
black,  rather  than  a  green,  tinge.  Many  of  them  are  lustrous,  black,  highly 
foliated  schists  that  are  sometimes  banded  with  very  fine  parallel  lines  of  a 
white  and  a  dark-green  color,  but  which  more  frequently  are  of  a  uniform 
dark  color;  others  are  medium-grained,  dark-gray,  dioritic-looking  rocks, 
in  which  a  foliation  is  clearly  apparent,  but  is  not  marked  in  its  perfection; 
while  a  few  are  fine-grained,  black  schists  of  a  dense,  uniform  textm-e.  In 
thin  section  differences  in  composition  and  texture  may  be  detected,  corre- 
sponding with  differences  in  the  macroscopic  appearances  of  the  rocks. 

The  less  lustrous  of  the  schists  resemble  most  closely  the  greenstone- 
schists.  Quartz  and  clouded  plagioclase  are  present  in  very  much  elongated 
grams,  between  which  are  flakes  and  masses  of  green  hornblende  and 
chloritized  biotite  in  small  quantity.  Large  plates  of  epidote  and  grains  of 
titaniferous  magnetite,  surrounded  by  leucoxene,  are  scattered  through  the 
aggxegate.  The  bands  that  are  sometimes  so  plainly  seen  in  the  hand 
specimen  are  not  clearly  defined  under  the  microscope.  They  can  be  rec- 
ognized, but  they  possess  no  distinctive  features.  The  darker  bands  contain 
more  amphibole  than  do  the  lighter  ones.  Otherwise  the  two  are  similar, 
both  in  composition  and  in  structure. 

The  lustrous  schists  are  very  fresh  looking.  Now  and  then  a  turbid 
grain  of  feldspar  is  seen  in  their  sections,  but  this  happens  rarely.  As  they 
are  now  constituted  the  rocks  differ  from  the  "diorites"  described  under  the 
greenstone-schists  in  containing  a  little  quartz.  Elongated  fresh  plagio- 
clase, much  of  which  is  untwinned,  prisms  of  compact  green  hornblende, 
and  grains  of  quartz  are  the  only  components  present  in  any  quantity.  A 
few  grains  of  epidote  and  some  of  magnetite,  and  occasionally  a  flake  of 
brown  biotite,  are  also  met  with,  but  not  in  noticeable  amounts.  In  one  or 
two  instances  the  feldspar  is  in  such  small  quantity  that  the  rocks  are 
essentially  amphibole  and  quartz  aggregates. 


208  THE   MAEQTETTi:    IlfON-BEARINU   DISTRICT. 

MICACEOUS  AMPHIBOLE-SCHISTS.  ' 

There  is  a  third  class  of  schists  that  possess  at  the  same  time  some  of 
the  characteristics  of  the  hornblendic  schists  and  others  of  the  micaceous 
schists.  Macroscopically  they  resemble  the  latter.  They  are  tinely  and 
evenly  banded  arenaceous  rocks  of  a  light-gray  color.  Under  the  microscope 
they  appear  more  like  the  hornblendic  schists.  Quartz  in  large  quantity, 
altered  plagioclase,  brown  biotite,  and  green  amphibole  are  all  present  in 
them.  The  last  three  components  vary  in  amount,  but  all  are  in  large 
quantity.  The  plagioclase  and  quartz  are  in  irregular  and  often  jagged 
grains,  elongated  in  one  direction  a  little  more  than  in  others.  The  biotite, 
however,  and  usually  the  hornblende,  always  occurs  in  long,  narrow  flakes 
between  the  other  components,  and  it  is  due  to  the  fact  that  the  longer 
directions  of  these  flakes  are  always  parallel  that  the  rocks  are  foliated. 
The  bands  differ  from  each  other  only  in  the  amount  of  hornblende  and 
biotite  in  them.  The  lighter  bands  are  devoid  of  these  minerals,  while  the 
dark  ones  contain  them  in  great  abundance.  In  one  or  two  instances, 
where  the  banding  of  the  schists  is  very  obscure,  the  structure  is  granitic 
in  so  far  as  the  quartz  and  plagioclase  are  concerned.  These  two  minerals 
occur  in  irregular  grains  that  are  separated  from  one  another  by  numerous 
flakes  of  biotite  and  hornblende.  The  latter  minerals  lie  with  their  longer 
axes  approximately  parallel  to  the  bounding  planes  of  the  quartz  and  feld- 
spar grains,  as  if  they  had  been  forced  into  this  position  by  pressure  acting 
perpendicularly  to  their  predominating  direction.  The  feldspars  are  more 
or  less  altered  to  a  mosaic  of  quartz  and  sericite  or  kaolin,  or  of  quartz  and 
clear  plagioclase. 

ORIGIN. 

All  the  hornblendic  schists  appear  to  have  been  produced  by  the  mashing 
of  some  original  basic  crystalline.  It  is  not  possible  to  ascertain  positively 
that  quartz  was  not  a  constituent  of  the  parent  rock,  but  from  the  fact  that 
it  is  so  often  a  product  of  the  decomposition  of  the  original  plagioclase  it 
is  thought  probable  that  much  of  it,  in  both  the  micaceous  and  the  non- 
micaceous  hornblendic  schists,  is  a  secondary  product.  The  biotite  and 
hornblende  are  also  secondary,  but  the  mineral  from  which  they  were 
derived  is  not  known.     It  may  very  likely  have  been  augite. 


THE  GNEISSOID  GRANITES.  209 


THE  GNEISSOID  GRANITES. 


The  granite  and  granitoid  gneiss  of  the  Southern  Complex,  like  the 
con-esponding  rocks  of  the  northern  area,  are  so  intimately  related  to  each 
other  that  they  must  be  regarded  as  different  phases  of  the  same  rock.  That 
they  are  intrusives  in  the  micaceous  and  hornblendic  schists  admits  of  no 
doubt,  as  their  dikes  cut  the  latter  rocks  wherever  found,  and,  as  has  already 
been  indicated,  fragments  of  the  schists  are  included  in  the  granite. 

The  distribution  of  the  rocks  with  respect  to  the  schists  has  already 
been  described.     No  repetition  of  tlie  description  is  necessary. 


PETROGRAPHICAL    CHARACTER. 


Macroscopicai. — As  comparcd  with  the  granite  of  the  Northern  Complex, 
that  of  the  southern  area  is  less  highly  colored.  In  the  east  pink  varie- 
ties predominate,  but  toward  the  west,  more  particularly  in  the  Lake 
Michigamme  district,  a  very  fresh  white  granite  takes  its  place,  to  the 
entire  exclusion  of  the  pink  variety.  Porphyritic  facies  are  less  common  in 
the  southern  rocks,  and  the  foliated  structure  which  is  so  pronounced  in  the 
northern  granite  is  very  much  less  marked  in  these.  Moreover,  whereas 
in  the  former  rocks  there  is  always  more  or  less  biotite,  in  the  latter  rocks 
there  are  frequently  no  bisilicates  present,  except  a  little  chlorite  in  small 
flakes  that  may  have  been  derived  from  plagioclase,  and  in  a  very  few 
cases  larger  masses  of  the  same  mineral  that  may  have  come  from  biotite. 
In  general  character,  however,  the  rocks  of  the  two  areas  are  the  same.  In 
the  ledge  the  rocks  are  white,  gray,  pink,  or  red,  according  to  the  color  and 
abundance  of  the  orthoclase  present,  but  the  red  varieties  are  rare,  and  when 
they  do  occur  their  shade  is  less  brilliant  than  that  of  the  red  granites  of 
the  Northern  Complex. 

The  southern  rocks  are  always  moderately  coarse  grained,  and  usually 
are  schistose.  Near  its  northern  contact  with  the  Algonkian  the  granite  is 
more  schistose  than  elsewhere,  although  gneissoid  phases  occur  throughout 
the  entii'e  granite  area.  In  some  places,  notably  south  and  southeast  of 
Palmer,  the  granites  are  cut  by  veins  of  soft,  yellowish-gray,  sericitic  schist 
that  are  believed  to  be  mashed  portions  of  the  granite  itself,  and  in  other 

MON  XXVIII 14 


210  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

places  they  are  crossed  by  zones  of  crushed  rock.  In  the  same  area  occur 
the  Pahner  gneisses.  These  are  highly  schistose,  light-gray,  pink,  or 
yellowish  rocks,  forming  a  narrow  belt  between  the  undoubted  gneissoid 
granites  to  the  south  and  the  Algoukian  sediments  to  the  north.  Many 
phenomena  indicate  that  these  rocks  are  but  very  much  squeezed  granites, 
in  which  case  their  foliation  is  probably  due  to  the  fact  that  they  exist 
along  the  plane  of  contact  between  the  crystalline  rocks  of  the  Basement 
Complex  and  the  sedimentary  beds  of  the  Algonkian  series,  a  zone  of  great 
acconamodatiou  during  the  folding  of  the  Marquette  rocks. 

Microscopical. — Uudcr  the  microscope  the  principal  components  of  the 
granites  fire  seen  to  be  orthoclase,  albite  and  other  plagioclases,  microcline, 
quartz,  occasionally  a  little  biotite,  and  the  alteration  products  of  the  feld- 
spars. All  these  minerals  have  the  same  properties  as  they  do  in  the  northern 
granites.  The  feldspars  are  a  little  more  altered,  but  their  decomposition 
products  are  the  same  as  those  in  the  northern  rocks.  The  orthoclase  and 
microcline  have  given  rise  to  kaolin  and  sericite,  and  the  plagioclases  have 
yielded  chlorite,  kaolin,  and  small  flakes  of.  some  micaceous  mineral.  In 
extreme  cases  these  alteration  products  are  so  abundant  that  they  entirely 
obscure  the  outlines  of  the  grains  by  whose  decomposition  they  were  pro- 
duced. The  quartz  grains  always  exhibit  undulatory  extinction,  they  are 
almost  always  surrounded  by  granulated  borders,  and  very  frequently  they 
are  filled  with  little  liquid  inclosures  containing  movable  bubbles.  The 
biotite  is  present  in  very  small  quantity,  and  its  flakes  are  nearly  always 
partially  changed  to  chlorite.  A  few  zircons,  a  little  magnetite,  some 
limonite,  and,  very  rarely,  plates  of  hematite,  are  the  only  other  minerals 
noted  in  the  rock. 

In  the  most  massive  phases  of  the  granite  the  typical  granitic  structure 
can  still  be  seen,  though  the  abundance  of  alteration  products  scattered 
through  most  of  the  sections  obscures  it  more  or  less.  The  structure  of  the 
schistose  phases  presents  the  same  features  as  does  that  of  the  gneissoid  gran- 
ites of  the  Northern  Complex.  Their  quartzes  are  granulated  and  crushed, 
and  their  feldspathic  components  fractured.  Between  the  larger  grains  is 
a  mosaic  composed  of  the  finer  fragments  of  both  quartz  and  feldspar,  and 
scattered  through  this  are  muscovite  flakes  winding  in  and  out  between 


THE    PALMER   GNEISSES.  211 

the  otlier  components.  Between  the  fragments  of  shattered  quartzes  are 
veins  of  the  same  mosaic,  and  these  are  often  completely  changed  to 
sericite  and  kaolin.  Secondary  microcline  is  not  so  common  a  constituent 
in  these  granitoid  gneisses  as  it  is  in  the  corresponding  rocks  of  the 
Northern  Complex,  althougli  it  is  present,  while  secondary  albite  appears 
to  be  more  common. 

The  microscopic  features  of  the  granites  of  the  Southern  Complex  are 
thus  similar  to  those  of  the  northern  area.  Both  rocks  are  composed  of 
the  same  minerals,  and  both  have  become  gneissoid  in  places  through 
the  influence  of  pressure.  The  southern  rocks  appear  to  be  more  altered 
than  the  northern  ones,  but  they  seem  to  have  been  tlie  same  originally. 

THE  PALMER  GNEISSES. 

The  Palmer  gneisses  comprehend  a  variety  of  highly  schistose  rocks 
of  a  gray,  white,  pink,  or  light-green  color,  showing  little  lenticular  "eyes" 
of  quartz  in  a  "hydi-omicaceous"  groundmass  that  appears  to  have  been 
nuich  mashed.  When  the  quartz  is  in  excess  the  rocks  resemble  squeezed 
cherts  or  quartzites,  and  when  the  matrix  predominates  they  resemble  fine- 
grained gneisses. 

RELATIONS    TO    ADJACENT    FORMATIONS. 

These  rocks  have  already  been  mentioned  as  forming  a  distinct  belt 
between  the  granites  and  the  sedimentary  beds  in  the  vicinity  of  Palmer 
(Atlas  Sheet  XXXII).  They  are  found  also  as  isolated  ledges  at  intervals 
as  far  west  as  Champion,  always  between  well-defined  granites  to  the  south 
of  them  and  und()ul)ted  beds  of  the  Marquette  series  to  the  north. 

The  relations  of  the  gneisses  to  the  surrounding  rocks  are  not  always 
(dear.  At  no  place  are  the  gneisses  seen  to  grade  into  the  granites,  although 
the  general  similarity  of  the  two  rocks  in  their  macroscopic  features  is  strik- 
ingly noticeable.  A  small  topograj)hic  break  usually  intervenes  between 
these  ledges  that  are  nearest  to  each  other,  and  in  this  interval  it  is  thought 
gradation  phases  may  actually  exist. 

With  respect  to  the  Marquette  beds  the  relations  of  the  gneisses  diifer. 
In  sees.  34  and  35,  T.  47  N.,  R.  26  W.,  the  schists  are  unconformably 


212  THE    MAEQUETTE    IROX-BEARIXG   DISTRICT. 

beneatli  the  conglomerates  lying  at  the  base  of  the  Marquette  series.  (See 
Atlas  Sheet  XXXV.)  On  the  west  side  of  the  large  hill  in  the  NW.  ^ 
sec.  35  the  relations  of  the  two  rocks  are  plain.  Here  the  foliation  of  the 
schist  (No.  20654)  strikes  directly  into  a  heavily  bedded  quartzite  which 
a  little  farther  to  the  north  becomes  conglomeratic.  In  the  NE.  ^  sec.  34 
the  actual  contacts  between  the  gneisses  and  the  conglomerates  are  not 
seen,  but  the  two  rocks  are  very  near  each  other,  and  the  conglomerates 
are  filled  with  large  liowlders  of  the  schists.  The  little  hill  nearest  the 
northwest  corner  of  sec.  35  is  composed  of  gneiss,  which  is  cut  through 
and  through  by  so  many  dikes  and  veins  that  it  seems  to  be  saturated 
with  granitic  material.  In  this  vicinity  the  indications  point  clearly  to  the 
fact  that  the  gneisses  are  older  tlian  the  oldest  of  the  Algonkian  rocks  in 
their  neighborhood. 

In  the  little  hill  south  of  the  Piatt  mine,  in  sec.  32,  T.  47  N.,  R.  26  W., 
the  relations  of  the  rocks  are  apparently  different.  (See  Atlas  Sheet  XXXII.) 
On  the  north  side  of  the  hill,  near  the  top,  is  a  large,  bare  ledge  of  a  yellow 
schist,  which,  in  the  hand  specimen  and  under  the  microscope,  has  the  usual 
appearance  of  the  Palmer  gneisses.  The  west  end  of  the  ledge,  however, 
is  conglomeratic,  and  the  matrix  of  the  great  conglomerate  ledg'e  on  the 
west  end  of  the  same  hill  is  identical  with  the  material  of  the  yellow  schist. 
At  this  place  the  gneiss  was  originally  a  fragmental  rock.  A  few  hundred 
yards  southwest  of  the  Piatt  mine  the  conglomerate  at  the  base  of  the 
iron-bearing  formation  is  well  exposed  in  a  number  of  large,  bare  ledges, 
and  in  it  may  be  seen  hundreds  of  large  bowlders  of  the  Palmer  gneisses. 
Evidently  we  have  in  this  area  two  entirely  different  rocks  with  the  char- 
acteristics belonging  to  the  Palmer  gneisses.  One  is  a  mashed  fragmental 
rock  at  the  base  of  the  iron-bearing  formation,  while  the  other  is  much  older 
than  this,  and  is  presumably  a  mashed  form  of  the  granites. 

A  third  area  of  the  Palmer  gneisses  deserves  mention  for  the  complica- 
tion of  relations  it  presents.  Just  south  of  Summit  Mountain,  in  the  western 
half  of  sec.  25,  and  through  the  center  of  sec.  26,  T.  47  N.,  R.  27  W.,  is  a  belt 
of  schists  of  varying  width.  (See  Atlas  Sheet  XXIX.)  It  comprises  well- 
banded,  sometimes  fine-grained,  sometimes  coarse-grained,  foliated  rocks  of 
a  light-gray  or  dark-gray  cdlor.  The  rude  bedding  which  produces  the  band- 
ing dijjs  about  60°  northeast  and  strikes  about  30°  south- of  east.     In  some 


THE   PALMEIJ   GNEISSES.  213 

places  the  banding  is  even,  while  in  otliers  it  is  much  contorted,  where 
lenses  and  veins  of  quartz  and  narrow  dikes  of  red  granite  are  interposed 
between  the  bands.  As  the  granite  area  to  the  south  is  approached  the 
number  of  granite  dikes  in  the  schists  increases,  and  these  rocks  themselves 
liecorae  more  and  more  massive.  On  the  north  side  of  Lake  Palmer  the 
reliitions  of  the  dikes  to  the  schists  are  well  seen  in  the  almost  perpendic- 
ular side  of  a  high  cliff.  Here  great  dikes  of  red  granite  cut  the  schists  in 
all  directions,  although  there  seems  to  have  been  a  preference  for  a  direc- 
tion parallel  to  their  foliation.  On  the  ledges  south  of  the  lake,  numbers 
of  na)Tow  parallel  dikes  of  the  same  red  granite  occur  between  layers  of 
hornblende-schists  and  mica-schists,  producing  on  smooth  ledges  a  banded 
structure  of  great  beauty. 

There  seems  to  be  no  question  but  that  most,  if  not  all,  of  the  gneisses 
south  of  Summit  Mountain  are  properly  members  of  the  granite-schist 
series.  The  banded  structure  that  has  been  noticed  in  most  of  them  is  due 
in  part  to  the  banded  character  of  the  schist-granite  complex  from  which 
they  were  derived. 

The  schists  with  the- characteristics  of  the  Palmer  gneisses  therefore 
include  foliated  rocks  of  Algonkian  age  (in  the  neighborhood  of  the 
Piatt  mine)  and  others  belonging  to  the  Basement  Complex.  The  line 
between  the  two,  as  drawn  on  the  maps,  is  as  accurately  located  as  is 
possible  after  making  a  very  thorough  examination  of  all  the  ledges  in  its 
vicinity.  Where  well-defined  conglomerates  occur  the  line  is  drawn  just 
beneath  these,  and  the  schists  accompanying  the  conglomerates  are  placed 
where  they  belong,  in  the  Marquette  series.  Where  no  conglomerates  are 
found  and  nothing  is  discovered,  either  in  the  field  or  under  the  microscope, 
to  indicate  that  the  gneisses  were  once  fragmental,  they  are  placed  in  the 
Basement  Complex,  and  the  line  is  drawn  above  them.  The  Palmer 
gneisses  of  the  area  represented  on  the  map  (Atlas  Sheet  IV)  are  therefore 
regarded  as  members  of  the  Basement  Complex.  (The  meaning  of  apparent 
gradations  between  unconformable  series  is  described  in  another  place; 
see  pp.  298-299). 

PETROGRAPHICAL    CHARACTER. 

The  key  to  the  origin  of  the  Palmer  gneisses  is  discovered  in  the 
study  of  the  altered   mosaic    between    the   large  fractured    fragments  of 


214  THE   MARQUETTE   IRON-BEAEING^   DISTRICT. 

quartz  and  feldspar  in  the  gneissoid  granites.  In  these  rocks  the  mosaic 
(which  in  fresh  specimens  consists  of  tiny  fragments  of  quartz,  orthoclase, 
plagiochise,  etc.,  broken  from  the  larger  grains  and  saturated  with  newly- 
deposited  microcUne  and  albite)  has  been  changed  to  an  aggregate  of  tiny 
flakes  of  kaolin,  chlorite,  and  sericite, ,  small  grains  of  quartz,  and  occa- 
sionally long  larainffi  of  muscovite,  besides  fragments  of  clouded  feld.spar. 
As  the  alteration  of  the  mosaic  proceeds  and  its  decomposition  products 
increase  in  quantity,  its  structure  becomes  less  and  less  clearly  recognizable, 
until  in  one  or  two  instances  it  can  liardl}^  be  discerned.  The  large  frag- 
ments of  feldspar  that  are  embedded  in  it  have  also  suffered  alteration,  and 
the  quartzes  have  been  crushed  until  their  positions  are  occupied  by  four  or 
five  differently  orientated  grains,  which  in  the  less  schistose  rocks  may  be 
seen  to  fit  together  into  a  single  one.  In  the  more  highly  foliated  phases 
the  parts  have  sometimes  been  moved  from  their  places  and  now  appear  as 
isolated  fragments. 

In  the  Palmer  gneisses  all  certain  traces  of  their  origin  have  dis- 
appeared. Under  the  microscope  there  is  but  little  variation  in  the  structure 
or  composition  of  their  different  phases.  Even  the  schistose  fragmental 
rocks  that  are  associated  with  the  conglomerates  are  as  nearly  like  the  true 
gneisses  in  thin  section  as  they  are  in  the  ledge.  They  may  contain  a 
greater  quantity  of  quartz  than  do  the  latter  rocks,  and  the  grains  of  this 
mineral  may  be  a  trifle  more  rounded  in  outline.  It  is  doubtful  whether 
these  rocks  would  have  been  separated  from  the  genuine  gneisses  derived 
from  the  granite  had  their  relations  in  the  field  not  been  plain.  Even  with 
the  care  that  has  been  used,  it  is  probable  that  a  few  rocks  of  fragmental 
origin  have  been  included  in  the  area  of  the  Palmer  gneisses. 

In  the  thin  sections  of  the  gneisses  quartz  grains  are  observed  embedded 
in  a  fine-grained  matrix  of  a  nearly  uniform  texture  and  composition.  The 
quartzes  are  crushed,  as  they  are  in  the  schistose  granites.  Often  they  form 
lenticules  of  a  quartz  mosaic  in  which  each  separate  grain  exhibits  the  phe- 
nomenon of  undulatory  extinction.  When  not  completely  shattered  they 
are  granulated  around  their  edges,  and  especially  at  the  ends  of  the  lenti- 
cules, where  mosaics  of  fine  grains  have  been  produced.  Portions  of  these 
mosaics  extend  out  as  long  tails  in  the  direction  of  the  foliation  of  the  rock, 


THE   PALMER   GNEISSES.  215 

winding  in  and  out  among  the  other  components,  and  aiding  in  emphasizing 
the  schistosity.  In  most  of  the  sections  examined  the  parts  of  the  crushed 
quartzes  have  been  separated,  and  into  the  crevices  between  them  the  matrix 
has  been  forced,  thus  producing  a  genuine  fragmental  structure. 

Tlie  matrix  in  which  the  quartzes  He  is  a  uniform  felt  of  kaoUn,  mus- 
covite,  a  few  flakes  of  chlorite,  a  little  biotite,  small  masses  of  calcite,  tiny 
grains  of  quartz,  and  remnants  of  feldspar.  The  sericite  and  kaolin  are  the  most 
abundant  components.  Their  leaflets  are  usually  arranged  approximately 
parallel  to  the  planes  of  foliation  in  the  rock,  except  where  they  occur  in  the 
crevices  between  fractured  quartz  and  feldspar  grains,  when  they  are  per- 
pendicular to  the  ^va\h  of  the  crack.  They  bend  aroulid  the  larger  quartzes, 
enveloping  them  in  concentric  layers,  and  wind  in  and  out  between  neigh- 
boring grains  like  the  matrix  of  many  squeezed  porphyries.  Occasionally 
the  remnants  of  feldspar  left  between  the  meshes  of  the  matrix  are  optically 
continuous  over  large  areas  with  the  outlines  of  granitic  feldspar  grains,  but 
usually  when  they  can  be  detected  they  give  evidence  that  they  too,  like  the 
quartzes,  were  fractured  and  their  parts  separated  during  the  production  of 
foliation  in  the  rock. 

By  far  the  greater  number  of  the  Palmer  gneisses  are  as  simple  in 
composition  as  those  above  described.  A  few  present  special  features  that 
should  be  mentioned.  A  number  of  specimens  collected  from  various 
points  all  along  the  belt  are  dotted  on  their  surfaces  with  plates  of  a  dark- 
green  chloritoid,^  varying  in  size  from  2  mm.  to  almost  microscopic  dimen- 
sions. A  rock  (specimen  No.  21999)  from  about  1500  steps  N.,  750  steps  W., 
of  the  SE.  corner  of  sec.  32,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXII),  is  a 
good  type  of  these.  Its  bowlders  constitute  a  large  proportion  of  those 
occurring  in  the  conglomerates  southwest  of  the  Piatt  mine.  In  the  hand 
specimen  the  rock  resembles  a  decomposed  gneiss.  In  its  thin  section  quartz 
grains  are  rare.  Only  an  occasional  one,  or  a  quartz  mosaic  with  the  outlines 
of  a  grain,  is  found  here  and  there  through  the  schistose  matrix,  which  is  a 
uniform  mass  of  sericite  and  kaolin  flakes,  with  a  little  fine-grained  quartz 
mosaic.  Embedded  in  the  matrix,  in  positions  irrespective  of  the  schistosity, 
are  large  plates  of  the  green  chloritic  mineral,  numerous  grains  and  irregular 

'  Cf.  A.  C.  Lane,  Rept.  State  Board  of  Geol.  Surv.  for  1891-92,  p.  182,  Lansing,  1893;  and  W.  H. 
Hobbs,  Am.  Jour.  Sci.,  3d  series,  Vol.  L,  1895,  p.  125. 


216  THE   MAEQUETTE    IRON-EEAllING   DISTlflCT. 

masses  of  broA^-n  rutile,  and  a  few  rhomboliedra  of  some  alniost  colorless 
carbonate.  The  cliloritoid  is  the  most  interesting  component.  It  is  in 
large  tabular  plates  with  a  cellular  structure,  and  is  filled  with  inclusions  of 
quartz,  rutile,  and  portions  of  the  rock's  groundmass.  As  usually  seen,  the 
plates  appear  as  prisms  Avith  a  distinct  cleavage  parallel  to  their  long  direc- 
tions, and  sometimes  a  parting  perpendicular  thereto.  In  the  direction  of 
the  cleavage  their  color  is  a  deep  bluish-green,  and  pei'pendicular  to  it  a 
pale  yellowish-green.  Between  crossed  nicols  the  prisms  are  all  striated 
with  longitudinal  twinning  lamellae,  whose  extinctions,  measured  against 
the  cleavage  lines,  vary  between  1°  and  21°.  The  prisms,  of  course,  are 
vertical  sections  of  the  plates,  whose  cleavage  is  parallel  to  the  base. 

Evidently  the  cliloritoid  is  the  youngest  mineral  in  these  rocks.  Not 
only  does  its  contact  structure  indicate  this  fact,  and  the  position  of  its 
plates  with  respect  to  the  foliation,  but  the  same  mineral  in  well-developed 
plates  of  the  same  habit  is  found  not  only  in  the  bowlders  of  the  gneisses 
in  the  conglomerates  near  the  Piatt  mine,  but  as  well  in  the  matrix  of  these 
rocks. 

Other  specimens  of  the  gneisses  differ  from  this  one  mainlv  in  the  size 
of  the  cliloritoid  plates.  In  some  the  plates  are  very  large,  and  in  others 
they  measure  only  a  few  tenths  of  a  millimeter  in  their  longer  directions. 
In  one  or  two  cases  the  chlorite  appears  to  be  in  bands  in  the  schists,  other 
portions  of  the  rocks  being"  without  them.  Usually  its  plates  are  dissemi- 
nated irregularly. 

In  two  or  three  sections  there  were  also  noticed  a  few  small,  ill-defined 
prisms  of  dark  greenish-blue  tourmaline,  a  mineral  whose  presence  in  rocks 
is  usually  ascribed  to  contact  or  fumarole  action.  In  the  joresent  instance 
there  is  no  evidence  of  any  kind  to  indicate  that  the  mineral  is  of  contact 
origin.  Its  grains  are  distributed  irregularly  through  the  gneisses,  without 
any  reference  to  their  foliation,  and  the  mineral  is  consequently  subsequent 
in  its  origin  to  the  production  of  the  gneissic  structure. 

COMPOSITION   AND   ORIGIN. 

The  similarity  of  the  matrix  of  the  Palmer  gneisses  to  the  altered 
mosaic  of  the  crushed  granites  and  to  the  altered  feldspars  of  the  more 
massive  phases  of  these  rocks,  and  the  discovery  of  indefinitely  outlined 


THE   rAL.AIEK    GNEISSES. 


217 


granitic  grains  of  feldspar  in  the  least  altered  of  the  gneisses,  strongly 
suggest  that  these  rocks  ai-e  very  schistose  granites  in  which  the  alteration 
of  the  feldspars  has  proceeded  so  far  as  to  destroy  their  original  outlines  and 
to  yield  a  uniform  aggregate  of  decomposition  products.  The  destruction 
(if  the  outlines  of  the  original  grains  is  as  much  due  to  the  mashing  to 
whicli  the  rocks  have  been  subjected  as  to  the  alteration  they  have  suffered, 
and  the  completeness  of  the  alteration  miist  itself  be  due  largely  to  this 
same  mashing,  which  fractured  the  feldspars  of  the  border  granites  and 
rendered  them  more  easy  preys  to  the  attack  of  decomposition  processes 
than  the  same  minerals  in  the  more  massive  gi-anites  beyond  the  limits  of 
the  peripheral  zone  of  maximum  movement. 

An  analysis  of  one  of  the  most  schistose  phases  of  the  Palmer  gneisses, 
specimen  No.  20647,  from  the  top  of  the  large  hill  in  the  NW.  ^  sec.  35, 
T.  47  N.,  R.  26  W.,  was  made  by  George  Steiger,  of  the  Survey  laboratory. 
His  results  are  as  follows: 


Analysis  of  Palmer  gneiss. 


SiOj 

TiO, 

Al.Oj 

Fe.O, 

FeO 

MnO 

CaO 

MgO 

Xo 

Na.O 

HjOatlOO'^ 

HjO  above  100-^. 
P^O^ 


Total 


These  figures  correspond  very  nearly  to  those  that  would  be  obtained 
iipon  analysis  of  a  mixture  composed  of  68.6  per  cent  quartz,  14.6  per  cent 
kaolin,  8.7  per  cent  sericite,  5.7  per  cent  plagioclase  (0.6  per  cent  anoithite 
and  5.1  per  cent  albite),  1.2  per  cent  chlorite,  1  per  cent  magnetite,  and 


218  THE   MARQUETTE   IliON-BEAlMNG   DISTRICT. 

0.2  per  cent  apatite.     They  point  clearly  to  the  fact  that  these  rocks  are 
composed  of  granitic  material  that  has  been  silicified. 

INTRUSIVES  IN  THE   SOUTHERN   COMPLEX. 

In  the  Southern  Complex,  as  in  the  Northern  Complex,  the  schists  and 
granites  are  cut  Ijv  well-characterized  dikes  and  veins  of  eruptive  material. 
The  characters  of  the  dikes  in  both  areas  are  much  alike.  They  comprise 
diabasic,  epidioritic,  and  aplitic  kinds.  The  basic  dikes  were  evidently 
formed  at  different  times,  for  some  of  them  ai-e  schistose  and  are  clearly 
altered  diabases,  while  others  are  beautifully  fresh  and  entirely  massive. 
The  latter  must  be  much  younger  than  the  former.  They  were  perhaps 
intruded  during-  Keweenawan  time,  for  they  are  identical  in  composition 
and  general  character  with  the  smaller  dikes  cutting  Upper  Marquette 
sediments,  while  at  the  same  time  none  of  them  have  been  found  pene- 
trating the  Cambrian.  Among  the  materials  of  the  fresher  dikes  may  be 
mentioned  ophitic  diabases,  olivine  dialjases,  basalts,  luster-mottled  gabbro- 
like  diabases,  and  uralitic  diabases. 

The  older  and  usually  larger  dikes  are  epidioritic  and  uralitic  schistose 
diabases,  exactly  like  similar  rocks  in  the  Northern  Complex,  and  practi- 
cally identical  with  the  material  of  the  large,  boss-like  dike  masses  in  the 
Algoukian.     (See  Chapter  V.) 

SUMMARY. 

The  rocks  of  that  portion  of  the  Southern  Complex  discussed  in  this 
volume  are  micaceous  and  hornblendic  schists,  greenstone-schists,  gneissoid 
granites,  certain  schists  that  have  been  called  "Palmer  gneisses,"  and  acid 
and  basic  dike  masses.  The  greenstone-schists,  the  granites,  and  the  dike 
materials  are  similar  in  their  essential  features  to  the  corresponding  rocks  of 
the  Northern  Complex.  All  are  igneous  in  origin.  The  greenstone-schists 
are  squeezed  basic  lavas  or  tuffs.  They  are  older  than  the  granite.  The 
dike  masses  are  in  all  respects  like  those  that  penetrate  the  northern  area. 

The  Southern  Complex  differs  from  the  northern  area  in  the  smaller 
quantity  of  greenstone-schists  in  the  former,  and  the  presence  in  it  of  the 
micaceous   and  hornblendic  schists  and  the  Palmer  gneisses.     The  latter 


SLTMMAEY.  219 

rocks  are  apparently  in  most  cases  extremely  mashed  phases  of  the  granites. 
They  occur  only  on  the  borders  of  the  granitic  areas,  between  these  and  the 
Marquette  sedimentaries.  The  gneisses  consist  largely  of  quartz,  sericite, 
plagioclase,  and  kaolin.  In  a  few  instances  rocks  witli  the  characteristics 
of  the  Palmer  gneisses  are  found  at  the  base  of  the  Marquette  series,  con- 
stituting the  matrix  of  conglomerates  in  which  the  bowlders  are  largely 
identical  with  specimens  of  Palmer  gneisses  occurring  beneath  the  con- 
glomerates. These  rocks  are  regarded  as  mashed  arkoses,  derived  bv 
disintegration  and  alteration  of  tlie  granites,  whose  mashed  and  silicified 
forms  the  true  Palmer  gneisses  are.  The  arkoses  originally  had  the  same 
composition  as  the  granites  of  whose  detritus  they  consist;  consequently 
their  altered  phases  are  practically  identical  with  the  altered  granites  them- 
selves. The  area  on  the  map  colored  for  the  Palmer  gneisses  is  underlain 
by  those  gneisses  that  are  believed  to  be  mashed  granites. 

The  micaceous  and  hornblendic  schists  are  evenly  banded  rocks  -with 
a  distinct  strike  and  dip.  Their  banding  is  often  narrow  enough  to  be 
observed  in  hand  specimens.  In  other  cases  the  banding  is  broad,  so  that 
it  is  observable  only  in  the  ledges.  In  thin  section  a  few  of  the  rocks 
are  typically  gneissic.  In  most  of  them  a  cataclastic  structure  is  strongly 
marked.  All  are  more  or  less  foliated,  and  their  foliation,  as  well  as  their 
cataclastic  structure,  is  ascribed  to  pressure.  The  hornblendic  schists  are 
shown  to  be  mashed  basic  eruptives,  and  the  micaceous  varieties  are  thought 
to  be  mashed  acid  eruptives;  but  whether  the  schists  were  originally  tutfs 
or  massive  rocks  is  not  known. 

The  schists  are  older  than  the  granites,  since  dikes  of  the  latter  rock 
intrude  the  former  in  great  numbers.  Their  relations  to  the  greenstone- 
schists  are  not  known,  since  contacts  of  the  several  kinds  of  schists  have  not 
been  observed.  If  the  micaceous  and  hornblendic  schists  are  older  than 
the  green  schists,  they  may  represent  the  basement  upon  which  the  latter 
rocks  were  laid  down.  In  any  event  the  hornblendic  and  micaceous  schists 
rejjresent  the  typical  Mareniscan  formation  as  defined  by  Van  Hise  in  his 
correlation  essay,  and  the  gi'anite  the  Laurentian. 

As  in  the  case  of  the  Northern  Complex,  no  rocks  of  sedimentary 
oi'igin  have  been  detected  in  the  Southern  Complex. 


220  THE   MARQUETTE   lliO:^f-BBAEING   DISTRICT. 

SECTIOK  III.— ISOI.ATED   AREAS   WITHIN  THE   ATjGONKIAK. 

In  addition  to  the  two  areas  of  the  Basement  Complex  which  have 
been  discussed,  there  are  isolated  patches  of  pre-Algonkian  rocks  lying 
entirely  within  the  Algonkian  area.  Some  of  these  areas  perhaps  repre- 
sent islands  within  the  Algonkian  sea,  while  others  are  portions  of  the 
pre-Algonkian  mainland  that  have  been  forced  upward  tlu'ough  the  over- 
lying rocks  by  the  forces  that  folded  and  compressed  the  latter.  These 
latter  constitute  the  axes  of  anticlinal  folds,  and  are  natui'ally  longer  in  the 
direction  of  the  strike  of  the  folds,  and  when  the  material  of  the  nuclei  is 
schistose  the  direction  of  the  schistosity  is  usually  parallel  to  the  greater 
diameter  of  the  areas.  They  are  bordered  by  fragmental  beds  belonging 
with  the  lowei-most  formations  comprised  within  the' folds. 

The  rocks  forming  the  greater  portion  of  the  isolated  areas  are  gneis- 
soid  granites  and  schistose  greenstones  that  diifer  in  no  essential  respect 
from  the  con-esponding  rocks  of  the  Northern  and  the  Southern  Complex. 
The  greenstone-schists  of  the  isolated  ar^a  south  of  Marquette  are  identical 
with  the  Mona  schists.  The  granites  consist  of  the  same  minerals  as  do  the 
other  granites  of  the  Basement  Complex,  but  they  have  become  gneissic. 
Under  the  microscope  their  constituent  minerals  are  seen  to  be  shattered 
and  crushed  to  such  an  extent  that  many  sections  look  like  those  of 
fragmental  rocks.  The  fragments,  especially  those  of  quartz,  have  been 
rounded  by  attrition,  and  the  feldspar  has  been  granulated  so  that  the 
sections  resemble  those  of  an  arkose  containing  large  waterworn  quartz 
grains.  As  alteration  progresses  the  feldspar  changes  to  a  mosaic  of  seri- 
cite,  kaolin,  and  quartz,  which  often  becomes  so  abundant  as  to  obliterate 
the  outlines  of  the  feldspar  fragments  or  to  wholly  destroy  the  grains. 
In  this  extreme  phase  of  alteration  the  rocks  present  the  appearance  of 
sericite-schists,  such  as  are  so  common  in  the  belt  of  Palmer  gneiss  in  the 
northern  border  of  the  Southern  Complex.  Since  many  of  these  sericite- 
schists  occupy  zones  of  mashing  in  the  granites,  there  can  be  no  question 
as  to  their  origin. 


CHAPTER    III. 


By  C.  R.  Van  Hise. 


THP:  LOWP]R  MARQUETTE  SERIES. 

The  Lower  Marquette  series  consists,  from  the  base  upAvard,  of  the 
following  formations:  The  Mesnard  quartzite,  the  Kona  dolomite,  theWewe 
slate,  the  Ajibik  quartzite,  the  Siamo  slate,  and  the  Negaunee  formation. 
At  the  beginning  of  Lower  Marquette  time  the  transgression  of  the  ocean 
Avas  from  the  east  and  the  north,  and  as  a  consequence  the  inferior  forma- 
tions of  the  Lower  Marquette  series  appear  only  in  the  northeastern  part  of 
the  district.  South  of  Palmer  and  westward  the  lowest  formation  found 
is  the  Ajibik  quartzite;  that  is,  the  three  inferior  formations  of  the  Lower 
Marquette  district  were  not  here  deposited,  this  part  of  the  district  then 
being  above  Avater. 

SECTION  I.— THE  MESNARD   QUARTZITE. 

The  formation  is  given  the  name  Mesnard  quartzite  because  it  com- 
poses the  larger  part  of  the  mass  of  Mount  Mesnard  south  of  Marquette, 
and  because  the  predominant  rock  is  quartzite. 

DISTRIBUTION,    EXPOSURES,    AND    TOPOGRAPHY. 

The  Mesnard  quartzite  makes  up  a  continuous  belt  adjacent  to  the 
Archeanon  the  south  side  of  the  series  (see  Atlas  Sheet  IV),  extending  from 
west  of  Lake  Mary,  sec.  9,  T.  47  N.,  R.  25  W.,  to  the  sand  plains  west 
of  Lake  Superior.  In  sees.  1  and  2,  T.  47  N.,  R.  25  W.,  the  formation 
extends  north  to  an  island  of  Archean  in  sees.  2  and  3,  and,  swinging  both 
east  and  west  of  this  island,  it  entirely  surrounds  it.     Upon  the  northern 

221 


222  THE   MAEQUETTE   IROX-BEAKIXG  DISTEICT. 

side  of  the  Lower  Marquette  series  the  Mesnard  formation  extends  continu- 
ously, soutli  of  the  Archean,  from  Lake  Superior  to  the  west  side  of  sec. 
29,  T.  47  N.,  R.  25  W.  For  several  miles  to  the  west  of  sec.  29  there  are 
no  exposures,  but  just  east  of  Carp  River  a  heavy  belt  of  quartzite  again 
appears  next  to  the  Archean  and  runs  westward  as  far  as  Teal  Lake.  The 
peculiar  distribution  of  the  formation  is  explained  by  its  folding,  which  is 
considered  below. 

On  account  of  the  resistant  character  of  tlie  quartzite,  it  constitutes, 
south  of  Marquette  (Atlas  Sheets  XXXVIII  and  XXXIX),  three  prominent 
ranges,  the  first  including  Mount  Mesnard,  the  second  being  south  of  the 
Archean  island  in  sees.  2  and  3,  T.  47  N.,  R.  25  W.,  and  the  last  being 
adjacent  to  the  Archean  to  the  south.  As  the  formation  grades  from  a  pure 
vitreous  quartzite  to  a  slate,  its  resisting  power  is  very  diverse,  and  its 
complicated  folding  gives  an  irregular  distribution  to  the  different  belts, 
so  that,  while  the  ranges  have  the  distribution  mentioned,  the  topography 
in  detail  is  exceedingly  rough.  In  crossing  the  formation  one  climbs  a  steep 
ledge,  plunges  into  a  sliarp  ravine,  then  ascends  another  bluff,  to  again 
climb  down;  so  in  crossing  a  range  one  traverses  a  series  of  exceedingly 
steep  ridges. 

FOLDING. 

At  the  east  end  of  the  district  the  quartzite  is  folded  into  two  closely 
compressed  east -west  synclines,  with  a  central  anticline,  the  quartzite 
occupying  the  entire  breadth  of  the  Algonkian  in  the  section  just  east  of 
the  State  prison.  (Atlas  Sheets  XXXVIII  and  XXXIX.)  East  of  this  line 
the  overlying  dolomite  appears  in  the  southern  syncline.  In  the  section 
running  south  from  Mount  Mesnard  both  the  northern  and  southern  syn- 
clines show  the  overlying  dolomite,  but  on  the  anticline  between  erosion 
has  cut  to  the  Archean  North,  at  Mount  Mesnard,  another  syncline  appears. 
West  of  Mesnard  the  northern  belt  of  quartzite  has  a  monoclinal  dip,  ver- 
tical or  south  at  a  very  high  angle.  When  examined  in  detail,  however,  it 
is  found  in  its  slaty  phases  to  be  rolled  into  a  set  of  minor  overfolds,  which, 
in  passing  from  the  Archean  toward  the  center  of  the  Algonkian,  show 
steadily  higher  and  higher  members.  In  the  southern  belt  the  quartzite 
north  of  Lake  Mary  constitutes  a  shallow  synclinal  trough.  (Atlas  Sheet 
XXXVII.) 


THE    MESNAKD    QUAKTZITE.  223 

PETROGRAPHICAL   CHARACTER. 

Macroscopicai. — Peti'ographically  the  formation  consists  of  conglomerates, 
g-raywackes  and  graywacke-slates,  and  quartzites,  with  all  gradations 
between  the  different  phases,  although  quartzite  is  the  predominant  rock. 
Where  rocks  of  the  formation  are  found  in  contact  with  or  close  to  the 
suiTOunding  granite,  they  are  a  coarse  granite-conglomerate,  or  a  rock 
which  may  be  called  a  recoraposed  granite  where  the  constituent  particles 
composing  the  rock  ar^  the  separate  mineral  particles  of  the  Basement 
Complex. 

On  the  south  side  of  the  Algonkian  trough  the  conglomerate  is  mag- 
nificently exposed  west  of  Lake  Mary  in  the  SW.  ^  sec.  9  and  the  SE.  ^  sec. 
8,  T.  47  N.,  R.  25  W.  It  may  also  be  well  seen  at  and  east  and  west  of 
the  line  between  sees.  1  and  2  of  the  same  township,  and  at  other  places. 
With  the  fragments  of  granite  are  apparently  many  of  vein  quartz,  and  a 
few  of  red  jasper,  of  chert,  and  of  quartz-rock.  In  some  cases  this  bowlder- 
bearing  granite-conglomerate  passes  into  a  less  conglomeratic,  reddish  rock 
which  closely  resembles  the  original  granite. 

On  the  north  side  of  the  trough  north  of  Mud  Lake,  in  sec.  29,  T.  48  N., 
R.  25  W.,  the  lowest  horizon  is  again  a  basal  conglomerate,  the  numer- 
ous fragments  being  derived  mainly  from  the  granites  and  schists  of  the 
Northern  Complex,  the  latter  being  more  abundant  because  immediately 
adjacent.  The  fragments  vary  from  those  of  minute  size  to  bowlders  2  or 
3  feet  in  diameter.     Here  no  fragments  of  chert  or  jasper  were  found. 

The  basal  conglomerate  at  Mud  Lake  usually  passes  quickly  into  inter- 
stratified  slate  and  gray wacke,  and  then  into  a  quartzite.  The  slate  and 
graywacke  are  very  closely  folded,  there  being  many  reduplications  of 
the  same  strata,  all  having,  however,  a  southern  monoclinal  dip,  and  the 
axes  of  the  little  folds  pitching  steeply.  So  close  has  been  the  compres- 
sion that  the  more  resistant  belts  of  graywacke  in  the  slate  have  been 
broken  into  a  reibungsbreccia.  In  some  places  the  folding  has  been  so 
severe  as  to  entirely  destroy  the  thin  belts  of  gi'aywacke,  producing  out  of 
them  large  numbers  of  pebbles  and  bowlders.  All  stages  of  the  transition  are 
found  between  the  continuous  belts  of  graywacke  and  the  pseudo-conglom- 
erate in  the  slate. 


224  THE    MARQUETTE    IROX-BEARING    DISTRICT. 

The  slates  and  graywackes  usually  pass  quickly  into  the  typical 
quai'tzite  of  the  formation.  Within  the  Mesnard  quartzite  is  an  interstrati- 
fied  conglomerate,  from  a  few  inches  to  40  feet  in  thickness,  in  which  are 
abundant  fragments  of  ferruginous  schist,  of  quartz,  of  chert,  and  of  jasper. 

The  quartzite  is  in  general  a  rather  pure  vitreous  quartzite,  very  mas- 
sive in  hand  specimens,  but  in  the  ledges  often  showing  distinctly  the 
bedding,  and  not  infrequently  passing  into  slaty  phases.  In  many  places 
at  the  east  end  of  the  Mesnard  range  the  original  ripple-marked  surfaces  of 
the  layers  are  observed.  The  intricate  windings  of  the  conglomeratic  chert 
and  jasper  j^ebble-bearing  layer  were  traced  out,  and  were  of  great  assistance 
in- determining  the  structure.  Where  the  folding  has  been  close  the  quartzite 
passes  into  a  very  vitreous  rock,  or  even  into  a  quartz-schist.  The  \atreous 
rock  is  produced  by  extensive  fracturing,  or  even  Ijrecciatiou,  and  the  filling 
of  the  resultant  minute  and  large  cracks  with  vein  chert  or  quartz.  The 
veins  vary  from  those  of  minute  size  to  those  several  inches  across,  and  in 
some  cases  they  anastomose  through  the  quartzite  in  every  direction.  This 
secondary  material  often  closely  resembles  the  original  stained  or  granu- 
lated quartz  grains,  but  the  rocks  as  a  whole  take  on  a  peculiar  aspect,  and 
have  been  called  cherty  quartzites. 

At  the  top  the  quartzite  passes  into  slaty  phases,  and  these  grade  into 
slate,  a  belt  of  which,  from  less  than  30  to  100  feet  thick,  separates  the 
quartzite  from  the  Kona  dolomite.  The  Mesnard  quartzite  may  then  be 
divided  into  four  members:  (1)  Conglomerate,  (2)  slate  and  graywacke, 
(3)  quartzite,  and  (4)  slate.  The  quartzite  is  the  predominant  member. 
Slates  and  graywackes  are  locally  intermingled  with  the  quartzites.  A  sin- 
gle section  showing  all  the  phases  is  rarely  found,  and  exposures  are  not 
sufficiently  numerous  to  enable  one  to  make  these  subdivisions  in  mapping. 

Microscopical. — Thc  conglomcrates  are  of  tliree  main  kinds:  (1)  Those 
adjacent  to  the  Mona  schist;  (2)  those  adjacent  to  the  granites;  and  (3) 
those  interstratified  with  the  graywacke  or  quartzite.  The  first  occurs  along 
the  northern  border  of  the  Algonkian,  the  second  along  the  sovithern  border, 
and  the  third  at  various  places  along  both  the  northern  and  southern  belts. 

(1)  The  northern  conglomerate  is  in  its  lower  parts  a  stucco  of  granite 
and  green-schist  fragments  set  in  a  sparse  matrix.    The  granitic  pebbles  and 


PETROGllAPIIICAL  CnAKAGTEE  OF  MESNAKD  QUAKTZITE.    225 

bowlders  are  well  rounded.  They  comprise  coarse-grained  nuiscovite-granite 
and  peculiar  fine-grained  granites.  The  green-schist  pebbles  have  a  very- 
wide  variety,  including  decomposed  granular  greenstones  and  various  chlo- 
ritic  schists.  Every  phase  of  the  basic  and  acid  fragments  is  matched  by 
rocks  of  the  Northern  Complex.  A  comparison  of  the  fragments  with  the 
adjacent  rocks  of  the  Archean  can  leave  no  doubt  that  the  major  part  of 
the  detritus  of  the  conglomerate  was  derived  from  this  source.  The  sparse 
matrix  between  the  pebbles  is  composed  of  well-rounded  to  subangular 
grains  of  feldspar,  of  quartz,  and  of  the  finer  complex  detritus  of  the  various 
materials  of  the  Northern  Complex.  A  few  complex  cherty  fragments  are 
seen.  In  this  matrix  the  various  feldspars  are  especially  abundant.  This 
finer  detritus  is  set  in  a  still  finer  background  of  the  same  materials,  with 
much  chlorite  and  fine  secondary  quartz. 

(2)  The  conglomerates  adjacent  to  the  Southern  Complex  have  two 
phases — those  that  are  coarse  and  distinctly  show  a  conglomeratic  char- 
acter, and  those  that  are  composed  of  finer  detritus.  The  latter  in  some 
cases  so  closely  resemble  granite  in  the  field  that  they  are  with  difficulty 
discriminated  from  it. 

The  coarser  phases  have  as  predominant  pebbles  coarse  granite,  the 
feldspar  of  which  is  much  kaolinized,  and  which  may  be  considered  a  kao- 
linic  quartz-schist;  large  iiTegular  areas  of  complex  quartz,  which  may  have 
been  derived  from  a  very  coarse  grained  granite,  or  may  have  come  from  a 
quartz-schist ;  and  complex  pebbles  of  altered,  fine-grained  biotite-gneiss.  As 
the  conglomerate  becomes  finer-grained  the  complex  fragments  decrease  in 
quantity  and  are  replaced  by  large  simple  fragments  of  quartz  and  feldspar. 
Where  the  pebbles  disappear  the  rock  in  hand  specimen  simulates  granite 
or  gneiss.  In  addition  to  the  predominant  pebbles  of  the  conglomerate, 
there  are  present  large  complex  fragments  of  ferruginous  schist,  of  chert 
or  jasper,  and  of  a  quartzite-like  rock.  The  ferruginoiis  schist  pebbles  have 
a  very  finely  crystalline,  quartzitic,  and  kaolinic  background,  through  which 
iron  oxide  is  scattered  or  concentrated  in  irregular  connecting  layers.  These 
appear  to  be  ferruginated,  decomj^osed  schistose  rocks  rather  than  true  chert 
or  jasper.  The  ferruginous  chert  or  jasper  pebbles  are  very  similar  to  those 
of  the  Negaunee  formation,  but  they  show  less  banding,  and  the  iron  oxide 
MON  xxviii 15 


226  THE   MAEQUETTE    lEOX-BEAEING   DISTEICT. 

is  scattered  through  the  homogeneous  quartzose  background  somewhat  uni- 
formly. The  major  part  of  the  supposed  quartzite  pebbles,  as  seen  in  hand 
specimen,  are  found  to  be  complex  interlocking  quartz  and  much  mashed 
and  broken  quartz-schist,  in  which  a  great  deal  of  secondary  quartz  has  infil- 
trated. In  a  less  mashed  phase  the  quartz  is  in  distinct,  closely  fitting  or 
interlocking  granules,  which  suggest  a  fragmental  character,  but  although 
carefully  searched  for,  no  evidence  could  be  found  of  enlargement  or  of 
cores,  and  it  is  probable  that  the  material  is  from  veins.  The  chert,  jasper, 
and  quartz  pebbles  may  have  been  derived  from  veins  in  pre-Marquette 
rocks,  or  jiossibly  in  part  by  the  mechanical  destruction  of  secondary  veins 
within  the  formation  itself 

The  matrix  of  these  conglomerates  consists  of  quartz  and  feldspar 
fragments,  set  in  a  background  composed  of  more  finely  pulverized  and 
kaolinized  materials  of  the  same  kind.  In  many  cases  also  this  back- 
ground contains  much  very  finely  crystalline,  cherty  quartz.  The  slides 
are  also  cut  through  b)^  veins  of  the  same  chertv  quartzose  material.  In 
some  cases  dynamic  action  has  broken  up  the  cherty  matrix  and  chert 
veins,  producing  pseudo-pebbles,  and  this  mav  be  the  source  of  some  of  the 
fragments  which  at  first  sight  appear  to  have  been  derived  from  a  pre- 
existent  cherty  rock.  The  feldspar  fragments  frequentl}-  show  interesting 
micaceous  and  quartzose  decomposition.  The  quartz  grains  are  often 
enlarged.  All  <  )f  the  grains,  whether  in  the  complex  fragments  or  in  the  back- 
ground, show  undulatory  extinction  or  fracturing.  The  same  phenomena  are 
exhibited  by  the  feldspars,  but  to  a  less  degree.  In  certain  cases  the  frac- 
tures in  the  quartz  are  in  two  systems  at  right  angles  to  each  other,  pro- 
ducing many  little  rectangular  particles  of  quartz  from  a  single  individual. 

At  places  near  the  base  of  the  formation  the  much  maslaed,  fine-grained 
conglomerate  can  not  in  hand  specimen  be  discriminated  from  the  gneiss 
below.  As  seen  in  thin  section,  the  fragmental  rocks  are  found  to  be  kao- 
linic  quartz-schists.  The  simple  and  complex  quartz  grains  usually  show 
distinct  rounding,  although  some  of  them  have  a  decided  granitic  shape.  In 
many  slides  they  are  granulated  and  greatly  elongated  in  a  common  direc- 
tion by  dynamic  action.  Feldspar  fragments,  if  present  originally,  have 
decomposed.     The  quartz  grains  are  in  a  matrix  of  finely  crystalline,  cherty 


PETEOGRAPHICAL  CHAEAGTEll  OF   MESNAIM)  (iUAKTZITE.    227 

quartz,  kaolin,  and  sericite,  tlw  tirst  beiny  often  })iv(lonuiiant.  Nnmerous 
veins  of  secondary  quartz  cut  the  matrix  and  the  coarser  yrains.  The 
gneisses  adjacent  differ  from  the  clastic  rocks  just  described  in  that  distinct 
residual,  although  much  altered,  feldspar  remains,  in  the  absence  of  abundant 
secondary  chert}-  ([uartz,  and  in  the  distinct  granitic  texture. 

(3)  The  interstratitled  conglomerates  differ  from  (2)  only  in  that  the 
predominant  pebbles  are  chert,  jasper,  quartz,  and  ferruginous  schists,  and 
that  granite  pebbles  are  sparse  or  al)sent  altogether,  although  sometimes 
much  detrital  feldspar  is  present. 

The  pure  quartzites  grade  through  a  feldspathic  quartzite  into  the  fine- 
grained conglomerates.  The  least  mashed  phase  of  the  quartzites  consists 
almost  entirely  of  well-rounded,  uniform  grains  of  quartz  of  medium  size, 
which  have  become  enlarged,  the  enlargements  interlocking  and  nearly 
filling  the  interspaces.  A  very  small  amount  of  sericite,  oxide  of  iron,  and 
independent  secondary  quartz  is  seen  between  the  grains.  In  certain  less 
pure  phases  larger  amounts  of  these  materials  are  present,  ilany  of  the 
least  mashed  quartzites  show  remarkable  pressure  effects  The  grains  which, 
have  been  least  affected  show  merely  undulatory  extinction.  From  this-, 
phase  the  grains  grade  into  those  in  which  minute  cracks  have  formed 
However,  whether  the  extinction  is  undulator}-  or  there  are  distinct  cracks,, 
the  breaking  has  been  in  two  directions  at  right  angles  to  each  other.  •  Tli& 
fractures  in  one  direction  may  be  more  marked  than  those  in  the  other,  and 
one  set  may  disappear.  Where  the  fracturing  is  distinct,  each  of  the  quartz 
grains  is  broken  into  a  large  number  of  parallel  plates,  or,  if  fractured  in 
two  directions,  into  a  very  large  number  of  minute  rectangular  blocks. 
These  fractures  are  plainly  produced  in  the  shearing  planes.^  That  thev  in 
many  cases  can  not  be  quartz  cleavage  is  shown  1))-  the  fact  that  they  pass 
in  the  same  direction  from  grain  to  grain.  Where  the  fracturing  is  most 
marked  iron  oxide  and  gas  and  water  bubbles  have  formed  in  the  openings. 

The  pure  vitreous  quartzites  also  pass  into  the  cherty  quartzites.  In 
these  the  dynamic  effects  upon  the  original  quartz  grains  are  more  pro- 
nounced.    Between  the  original  grains  and  through  them  there  has  been  a 

'  See  Principles  of  Nortli  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise :  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  ( 


228  THE   MARQUETTE   IRON-BEARIXG   DISTRICT. 

great  deal  of  secondary  clierty  quartz  deposited.  Also  numerous  veins  of 
secondary  clierty  quartz  are  present.  In  these  are  inclosed  fragmental 
grains  derived  by  dynamic  action  from  the  original  material.  In  some 
cases  the  vein  material  passes  gradually  into  the  ordinary  rock,  more  and 
more  of  the  original  fragmental  quartz  appearing,  until  the  grains  are 
merely  broken  apart,  with  secondary  quartz  between  them. 

In  one  phase  of  the  cherty  quartzite  it  appears  that  after  the  rock  had 
been  subjected  to  a  first  dynamic  movement  and  had  been  cemented  by 
cherty  quartz  a  later  dynamic  movement  broke  up  this  rock  along  certain 
zones,  thus  producing  reibungsbreccias,  the  fragments  of  which  are  com- 
posed of  simple  grains  of  quartz,  mingled  with  chert  grains.  The  whole 
was  afterward  cemented  by  a  later  infiltration  of  silica  and  oxide  of  iron. 
This  phase  suggests  that  many  of  the  chert  fragments,  and  possibly  some 
of  the  ferruginous  chert  and  jasper  of  the  quartzite,  were  produced  by 
dynamic  processes  operating  upon  a  rock  which  had  been  previously 
broken  and  cemented  by  secondary  chert  and  jasper. 

The  conglomerates,  in  their  passage  to  the  quartzites,  at  many  places 
grade  through  the  phase  of  graywackes  or  graywacke-slates,  and  the  quartz- 
ite also  grades  above  into  similar  rocks.  These  graywackes  have  a  clayey 
background,  in  which  are  set  many  small  and  medium-sized,  well-rounded 
io  subangular  grains  of  quartz  and  the  various  feldspars.  The  feldspars 
are  frequently  altered  in  part  to  kaolin,  sericite,  and  quartz.  Occasional 
complex  grains  of  cherty  quartz  are  seen.  The  matrix  consists  of  finely 
crystalline  quartz,  kaolin,  and  sericite,  with  occasional  large  flakes  of  mus- 
covite.  In  many  places  it  is  stained  with  iron  oxide.  In  the  rock 
which  has  suffered  the  least  from  dynamic  action,  undulatory  extinction  and 
fracturing  are  seen  in  the  grains  of  quartz,  but  the  pressure  has  not  been 
sufficient  to  give  a  distinct  an-angement  of  the  particles  with  their  longer 
axis  in  a  uniform  direction. 

In  a  more  mashed  phase  of  the  graywackes  the  quartz  and  feldspar 
particles  show  a  distinct  arrangement  with  their  longer  axes  in  a  common 
direction,  and,  with  this,  most  marked  imdulatory  extinction,  fracturing,  and 
even  granulation.  Some  of  the  larger  grains  show  particularly  well  the  rect- 
angular fractures  in  two  directions  spoken  of  in  connection  with  the  quartzites. 


PETEOGRAPHICAL  CHARACTER  OF  MESNARD  QUARTZITE.    229 

In  the  matrix  sericite  has  abundantly  developed,  and  the  leaflets  are 
parallel.  The  minute  spaces  formed  by  the  shattering  of  the  large  frag- 
mental  grains  and  those  in  the  background  are  filled  with  secondary  cherty 
quartz,  which  has  thoroughly  cemented  the  rock.  The  larger  fractures  are 
filled  with  cherty  quartz,  forming  veins.  In  many  of  these  are  fragmental 
grains  broken  off  from  the  main  mass  of  slate. 

In  the  phase  in  which  the  dynamic  action  was  still  more  severe  the  gray- 
wackes  were  shattered  through  and  through,  the  particles  having  moved  and 
ground  over  one  another.  As  a  result  of  this  there  Avere  left  innumerable 
minute  spaces,  which  have  been  taken  advantage  of  by  the  infiltrating  sil- 
ica, and  are  now  filled  with  secondaiy  cherty  quartz.  The  original  frag- 
mental quartz  grains  are  always  somewhat  granulated  on  their  exteriors, 
and  many  throughout,  so  that  a  quartz  grain  is  represented  by  a  lenticular 
mass  of  finely  interlocking  quartz..  In  the  matrix  the  sericite  has  developed 
in  coarser  blades  than  in  the  less  metamorphosed  rocks.  It  is  everywhere 
in  long,  narrow  leaflets  having  a  parallel  arrangement  in  the  same  direction 
as  the  elongated  quartz  grains.  Numerous  veins  are  completely  filled  with 
interlocking,  coarsely  and  finely  crystalline  quartz,  apparently  all  of  it  being 
secondary.  If  any  of  the  original  fragmental  quartz  grains  have  dropped 
in  the  crevices,  they  have  become  so  shattered  as  to  have  lost  their  rounded 
outlines. 

The  conglomerates,  quartzites,  and  graywackes  of  the  Mesnard  for- 
mation include  rocks  varying  from  those  which  are  indurated  mainly  by 
siliceous  cementation  to  those  which  are  crystalline  schists.  From  their 
macroscopical  and  microscopical  descriptions  it  is  plain  that  there  has  every- 
where been  interior  movement.  Even  in  the  least  altered  phases  of  the 
rock  every  grain  of  quartz  shows  the  effect  of  strain.  From  this  least  altered 
phase  there  are  all  gradations  to  those  phases  in  which  the  rock  is  a 
shattered  or  mashed  mass  cemented  by  cherty  quartz.  Moreover,  after 
a  first  shattering  and  cementation  there  was  a  later  folding,  which  again 
shattered  the  rock,  including  both  the  original  constituents  and  the  sec- 
ondary cherty  quartz.  This  broken  rock  was  again  cemented  Ijy  later 
infiltrating  silica. 

In  certain  parts  of  the  formation,  where  the  relief  was  largely  by 


230  THE   MARQUETTE   lEON-BEAEING  DISTRICT. 

shattering  the  rock  en  masse,  the  clastic  character  of  the  original  grains  is 
usually  still  marked,  and  they  are  easily  discriminated  from  the  secondary 
cherty  quartz.  In  other  phases  of  the  rock  the  stresses  were  relieved  by 
movement  affecting  the  mineral  particles.  The  original  quartz  and  feldspar 
grains  were  granulated,  and  the  latter  were  decomposed.  Secondary  quartz 
formed  both  in  the  interstices  and  in  veins,  and  sericite  developed.  This 
process  of  secondary  silicification  and  development  of  sericite  seems  to  be 
in  direct  ratio  to  the  severity  of  the  mechanical  movement  affecting  the 
individual  grains.  Between  the  phases  in  which  the  relief  is  largely  by 
brecciation  and  those  in  which  it  is  largely  by  mashing  there  are  all 
gradations,  an  intermediate  phase  showing  the  partial  granulation  of  the 
fragmental  grains,  their  cementation  by  silica,  and  at  the  same  time  numer- 
ous veins  of  secondary  cherty  quartz.  As  has  been  said,  the  extreme 
alteration  of  the  original  quartzose  sandstone  resulted  in  peculiar,  vitreous, 
cherty-looking  quartz-rocks,  and  that  of  the  original  feldspathic  debris 
resulted  in  a  sericite-schist.  The  facts  that  the  sandstones  became  cherty 
brecciated  rocks  and  that  the  coarse  and  fine  muds  became  schists  are 
probably  explained  by  the  brittle  character  of  tlie  first  and  the  plastic  char- 
acter of  the  second,  one  yielding  mainly  by  fracture,  the  other  mainly 
by  flow.* 

RELATIONE   TO   UNDEELYING   FORMATION. 

The  fact  that  basal  conglomerates  are  found  at  various  places  near  the 
contact  of  the  Mesnard  quartzite  and  the  Basement  Complex  has  already 
been  mentioned,  and  the  localities  at  which  these  conglomerates  occur  have 
been  mentioned.  These  contacts  are  of  such  character  as  to  indicate  that 
the  Mesnard  quartzite  is  separated  from  the  Basement  Complex  by  a  great 
unconformity.  Since  in  these  basal  conglomerates  are  numerous  pebbles 
and  bowlders  of  granites,  gneisses,  and  schists  from  the  Basement  Complex, 
the  major  part  of  the  complex  history  of  the  Archean  was  complete 
before  the  Mesnard  quartzite  was  deposited.  Erosion  had  before  this 
time  cut  so  deeply  into  it  as  to  bring  to  the  surface  in  some  places  coarse- 
grained granites  and  in  other  places  the  truncated,  foliated  layers  of  the 

'  Principles  of  North  American  pre-Cambrian  geology,  by  C.  K.  Van  Hise:  Sixteenth  Ann.  Rept. 
XJ.  S.  Gaol.  Survey,  Part  I,  1896,  pp   601-603. 


RELATIONS   OF   THE    MESXAliD    QUARTZITE.  231 

schists  and  gneisses.  In  the  locahties  where  the  1)as;il  eouglomerates  occur 
the  proof  of  the  unconformable  relations  is  c<)uclusi\c.  In  other  locali- 
ties the  granite  was  apparently  decomposed  before  the  de])osition  of  the 
quartzite,  and  here,  as  has  been  said,  it  yielded  its  sm;dl  separate  mineral 
particles  to  the  overlying  rock.  This  recomposed  rock  lias  been  thor- 
oughly cemented.  During  the  subsequent  folding  sheai-iug  has  taken  place 
along  the  junction,  resulting  in  the  development  of  parallel  schistosity  in 
the  original  granite  and  in  the  recomposed  rock.  In  such  cases  it  is  difficult 
or  impossible  to  indicate  the  exact  contact  between  the  Basement  Complex 
and  the  Mesnard  quartzite.  Such  localities  were  explained  bv  Rominger 
as  cases  of  progressive  metaniorphism,  the  granite  being  a  metamorphosed 
sedimentary  rock.  Later  he  abandoned  this  view.  One  of  the  Ijest  local- 
ities in  which  to  observe  this  apparent  gradation  l)etween  the  gneissoid 
granite  and  the  (piartz-schist  is  just  north  of  the  little  granite  knob  on 
whose  south  side  is  the  Mest  quarter  post  of  sec.  1,  T.  47  X.,  R.  25  W. 
(Atlas  Sheet  XXXIX).  This  contact  between  the  Mesnard  quartzite  and 
the  Archean  affords  an  excellent  illustration  of  the  principle  that  crys- 
talline gneissoid  granite  ma)'  grade  step  by  step  into  a  rock  which  is  an 
unquestionable  quartzite,  there  being  no  sharp  line  of  demarcation  between 
the  two,  and  yet  lietween  the  two  formations  there  reall)'  being  a  iirofound 
unconformity. 

THICKNESS. 

As  the  Mesnard  quartzite  is  the  first  formation  of  a  transgressing  sea, 
it  doubtless  originally  varied  in  thickness,  this  l)eiug  due  to  irregularities 
of  the  Archean  basement.  This  irregularity  in  the  basement  is  indicated 
by  the  fact  that  the  quartzite  belt  is  in  one  place  scarcely  more  than  150 
feet  across.  As  the  dips  are  here  vertical,  this  mav  lie  taken  as  the  thick- 
ness of  the  formation.  From  this  thickness  the  quartzite  shows  a  continuous 
exposure  at  Mount  Mesnard  f)f  700  feet,  which  with  an  inclination  of  S0° 
corresponds  to  a  thickness  of  670  feet.  In  other  places  the  belt  is  nuich 
wider  than  this,  but  here  the  increased  width  is  plainly  due  to  folding,  and 
even  at  Mount  Mesnard  the  interstratified  belts  of  slate  and  graywacke  may 
contain  minor  rolls  which  have  escaped  observation  and  the  real  thickness 
of  the  formation  be  less  than  600  feet. 


232  THE   MAEQFETTE   lEON-BEAEIXG   DISTEICT. 

INTERESTING    LOCALITIES. 

Mud  Lake. — Noi'tli  of  Mutl  Lake,  adjacent  to  the  old  road  east  and  west 
of  N.-S.  i  line,  sec.  29,  T.  48  N.,  R.  25  W.  (Atlas  Sheet  XXXVI),  i.s  a  con- 
glomerate, described  hj  Irving^  as  the  "State  Road  conglomerate."  This 
conglomerate  occurs  at  various  points  for  a  distance  of  a  quarter  of  a  mile 
east  and  west,  hanging  upon  the  southern  flank  of  the  prominent  ridge  of 
Mona  schist  running  east  and  west  through  this  and  adjacent  sections.  The 
basal  portion  of  the  conglomerate  is  very  coarse.  The  fragments  contained 
in  it  comprise  both  granite  and  green  schist.  The  granite  fragments  vary 
from  small  pebbles  to  bowlders  2  feet  or  more  in  diameter.  They  are  well 
rounded,  and  in  lithological  character  are  similar  to  the  granites  which 
occur  as  intrusives  in  the  northern  part  of  the  Mona-schist  belt.  While 
these  granite  fragments  are  abundant,  green-schist  fragments  are  still  more 
plentiful.  In  size  they  A-ary  from  small  particles  to  large  blocks.  Some  of 
them  are  distinctly  rounded,  but  many  are  angular,  being  in  shape  similar 
to  the  irregular  schistose  blocks  which  at  the  present  time  are  broken  by 
•vyeathering  agencies  from  the  main  mass  of  i\Iona  schist.  Search  was 
made  for  jasper  pebbles,  such  as  occur  in  the  conglomerate  to  the  east,  but 
without  success.  All  who  have  examined  this  conglomerate  agree  that  it 
is  a  basal  one,  being  made  up  largely  from  the  formations  immediately 
subjacent,  but  containing  a  sufficient  amount  of  material  somewhat  remote 
from  the  contact  to  show  that  it  can  not  be  a  dynamic  conglomerate.  This 
belt  of  conglomerate  is  only  a  few  feet  in  width,  and  nearly  all  of  the 
localities  are  just  north  of  the  old  State  road. 

Immediately  south  of  this  road  occur  most  interesting  exposures  of 
interstratified  slate  and  graywacke.  The  rock  varies  from  a  very  fine 
grained  slate  to  a  coarse  graywacke,  the  denser  phases  of  which  are  red 
and  felsitic-looking.  Certain  exposures  are  wholly  of  the  graywacke,  others 
of  the  slate,  and  others  are  interstratifications  of  the  two.  The  most  altered 
phases  take  on  a  schistose  structure,  and  are  difficult  in  hand  specimen  to 
discriminate  from  a  crystalline  schist.     This  rock  is  found  to  have  a  cleavage 


'  The  greenstone-scliiBt  areas  of  the  Meuominee  and  Marquette  regions  of  Michigan,  by  6.  H. 
Williams,  with  an  introduction  by  R.  D.  Irving :  Bull.  U.  S.  Geol.  Survey  No.  62, 1890,  p.  21. 


INTERESTING   LOCALITIES   OF   MESNAED   QUAIITZITE.  233 

with  a  strike  in  a  nearly  east-and-west  direction  and  a  dip  of  abont  80°  to 
the  southward.  While  the  slaty  cleavage  has  a  strike  approximately  east 
and  west  and  a  uniform  southern  dip,  when  carefully  examined  the  bedding 
layers  are  seen  to  be  in  a  series  of  sharply  compressed  anticlines  and  syn- 
clines,  with  isoclinal  southern  dips  and  steep  pitches.  At  certain  places  in 
the  gray  slate  or  graywacke  background  are  found  numerous  pebbles,  some 
several  inches  across,  of  red,  felsitic-looking  rock.  These  at  first  were 
thought  to  be  derived  from  an  extraneous  source,  but  a  careful  examination 
of  all  the  ledges  discloses  every  gradation  between  these  pseudo-conglomer- 
ates and  the  interlaminated  slate  and  red  gi-aywacke.  During  the  intricate 
folding  the  more  rigid  and  brittle  felsitic-looking  graywacke  was  broken  up; 
the  fragments  were  ground  over  one  another  and  thus  rounded;  at  the  same 
tinie  they  were  buried  in  the  slate  and  graywacke  matrix.  Step  by  step  the 
process  may  be  traced  from  the  phase  in  which  the  more  resistant  layers  are 
merely  shattered,  through  the  phases  in  which  the  fragments  are  somewhat 
separated  bitt  have  a  distinct  linear  aiTangement  corresjjonding  to  the 
original  layer,  to  those  phases  in  which  no  traces  of  the  original  coarser  gi'ay- 
wacke  layers  ag  such  are  to  be  seen.  In  their  places  are  the  dynamically 
rounded  fragments  in  the  slates. 

It  is  evident  from  the  foregoing  that  this  whole  mass  of  slate  and 
graywacke  has  been  kneaded  in  a  most  remarkalile  manner  by  the  folding 
process.  Up  to  a  certain  point  the  acconnnodations  have  been  made  by  the 
slipping  of  the  layers  over  one  another,  with  readjustment  of  the  minor 
particles  within  the  layers,  but  in  the  most  completely  j^seudo-conglomeratic 
phase  the  pseudo  pebbles  are  so  irregularly  distributed  as  to  indicate  that 
the  whole  material  must  have  been  mashed  together,  the  parallel  layers 
being  compressed  by  the  forces  lentil  the  originally  horizontal  beds  are  in  a 
series  of  nearly  vertical,  isoclinal  folds. 

Parallel  to  the  schistose  structure  of  the  slates  and  graywackes,  in 
certain  places,  are  veins  and  irregular  oval  lenses  of  impure  ferriferous 
dolomite.  These  are  taken  to  be  secondary  infiltration  or  replacement 
products. 

There  are  no  continuous  exposures  connecting  the  conglomerates  on 
the  north  side  of  the  road  with  the  slate  on  the  south  side,  but  there  is  little 


234  THE    MARQUETTE   lEONBEAEING   DISTRICT. 

doubt  that  the  slate  is  the  nonconglomeratic  phase  of  the  formation. 
This  slate,  by  becoming  more  and  more  quartzose,  passes  into  vitreous 
quartzite,  which  shows  large  exposures  just  north  of  Mud  Lake.  This 
more  resistant  rock  seems  to  have  been  upturned  without  having  received 
the  minor  plications  which  are  found  in  the  slate.  The  soft  slate  was 
between  the  heavy  beds  of  quartzite  on  the  one  side  and  the  strong  Mona 
schists  on  the  other;  and  doubtless  the  larger  part  of  the  readjustment 
which  was  necessary  when  the  layers  were  folded  together  took  place  in  the 
slates,  and  are  thus  explained  the  profound  dynamic  effects  there  seen. 

An  examination  of  the  conglomerates  and  slates  in  thin  section  fully 
confirms  all  that  has  been  said  in  i-eference  to  the  relations  of  the  rocks  as 
seen  in  the  field.  The  predominant  pebbles  from  the  State  road  con- 
glomerate comprise  almost  every  phase  of  the  peculiar  rocks  of  the  Mona- 
schist  formation  in  the  neighborhood  of  Marquette.  There  are  also 
found  abundantly  coarse-grained  granite,  the  peculiar  red  granite,  and  a 
finer-grained,  feldspathic-looking  granite,  all  of  which  in  dikes  cut  the  Mona 
schists.  The  matrix  of  the  conglomerate  is  of  the  ground-up  detritus  of  the 
same  material,  feldspar  however  being  predominant,  because  of  the  basic 
character  of  the  rocks  from  which  the  material  is  derived.  In  thin  section 
a  few  chert  fi'agments  were  found.  These  were  not  in  the  form  of  pebbles, 
but  this  occui-rence  microscopically  connects  this  conglomerate  Avith  the 
conglomerate  which  occurs  on  Mount  ()mimi.  It  has  been  stated  that 
probably  the  conglomerate  grades  up  into  the  slates  and  graywackes  on 
the  south  side  of  the  road.  The  latter  prove  in  thin  section  to  be  identical 
in  character  with  the  matrix  of  the  conglomerate.  The  quartzites  are  found 
to  be  tyi^ical  of  the  formation,  and  need  no  descrijjtion. 

Mount  omimi. — Ou  tlie  steep  northern  slope  of  Mount  Omimi  (Atlas  Sheets 
XXXVI  and  XXXVIII),  in  the  northern  part  of  sees.  33  and  34,  T.  48  N.,  R. 
25  W.,  occurs  a  conglomerate,  varying  from  10  to  40  feet  in  thickness.  This 
conglomerate  has  a  feiTuginous  gray  wacke  background,  and  contains  numer- 
ous pebbles-  of  banded,  cherty -looking  quartz,  of  white  crystalline  quartz, 
of  ferruginous  chert  and  jasper,  of  heavily  ferruginous  pebbles,  of  white 
schistose  graywacke,  and  of  green  schist.  It  is  underlain  by  coarse  gray- 
wacke  and  slate,  and  is  overlain  by  slates  and  graywackes,  interstratified 


IXTEEESTINCl    LOCALITIES   OF   MESXAIM)  QT'AKTZITE.  235 

with  quartzites,  sometimes  strongly  and  coarsely  feldspatliic.  Tliese  pass 
into  the  pure  quartzites  which  constitute  the  greater  part  of  the  bluft". 

When  this  conglomerate  was  first  examined  it  was  thought  that  it 
marked  an  unconformity,  but  a  closer  examination  shows  it  to  be  inter- 
stratified  conformably  with  the  slates  and  graywackes  below,  and  with  the 
graywackes  and  quartzites  above.  Those  below  are  precisely  similar  to 
the  slates  and  graywa.ckois  south  of  the  State  road  north  of  Mud  Lake, 
and  apparently  are  at  the  same  horizon.  In  the  upward  gradation  from 
this  to  the  quartzite  it  appears  that  the  currents  were  strong  enough  to 
locally  form  a  bed  of  conglomerate.  The  conglomerate  differs  from  that 
at  the  base  of  the  series  north  of  Mud  Lake  in  the  absence  of  abundant 
granite  and  green-schist  pebbles  and  in  the  presence  of  the  varieties  which 
have  been  given  as  characteristic  of  it.  The  conglomerate  appears  to  fol- 
low along  the  border  of  the  hill  to  the  east,  and  in  field  relations  appar- 
ently cuts  slightly  across  the  direction  of  stratification  of  the  overlying 
slates  and  quartzites,  although  no  actual  discordance  was  seen  at  any 
locality.  The  junction  of  this  conglomerate  layer  with  the  underlying- 
slates  and  graywackes  was  a  zone  of  maximum  differential  inovement  at 
the  time  of  the  folding.  As  evidence  of  this,  the  slates  are  broken  into 
thin  plates;  they  are  heavily  impregnated  with  oxide  of  iron;  the  con- 
glome_rate  itself  in  certain  places  takes  on  a  brecciated  form,  and  its  matrix, 
as  well  as  some  of  the  jiebbles,  is  heavily  impregnated  with  iron  oxide. 
The  gray wacke  pebbles  contained  in  the  lower  pai't  of  the  conglomerate 
^n-obably  have  the  same  origin  as  those  in  the  slates  north  of  Mud  Lake; 
that  is,  they  are  of  dynamic  origin.  At  various  places  the  whole  series  is 
cut  through  by  diabase  dikes. 

A  microscopical  examination  of  the  Omimi  conglomerate  shows  that 
the  majority  of  the  heavily  ferruginous  pebbles  are  decaying  fragments 
of  a  schistose  rock,  which  have  been  strongly  impregnated  by  iron  oxide, 
as  has  also  the  matrix.  None  of  the  quartzite-like  pebbles  are  certainly 
fragmental,  although  some  of  them  at  first  sight  have  a  clastic  appearance; 
but  none  of  the  grains  show  cores  or  enlargements,  and  they  interlock. 
They  appear  to  differ  from  the  cherts  only  in  that  the  (piartz  is  mcfre 
coarsely  crystalline.     Some  of  these   complex  quartz   })ebbles  are  mashed 


236  THE   MAEQUETTE   lEOXBEAEIXG   DISTEICT. 

into  quartz-scliists.  Doubtless  tlie  chert,  jasper,  and  quartzite-like  frag- 
ments are  derived  from  the  veins  of  these  kinds  which  are  found  in  the 
green  schists  of  the  Northern  Complex,  although  it  is  possible  that  a  part  of 
them  are  derived  by  dynamic  action  from  vein  chert  and  quartz  deposited 
in  the  formation  itself  before  the  final  folding.  The  feldspathic  quartzite 
contains  A^ery  abundant  simple,  large  grains  of  feldspar,  which  ai-e  in  some 
cases  distinctly  enlarged.  The  graywackes,  slates,  and  quartzites  do  not 
differ  from  the  ordinary  phases  of  the  formation. 

Mount  Mesnard. — Mouut  Mcsuard  (Atks  Sheet  XXXVIII)  is  a  large  bluff 
in  the  west  half  of  sec.  35  and  the  eastern  part  of  sec.  34,  T.  48  N.,  R.  25  W. 
In  structm-e  this  mountain  is  a  closely  compressed  syncline,  the  formations 
concerned  being  the  Mesnard  quartzite  and  the  Kona  dolomite.  This  fold 
is  so  closely  compressed  as  to  make  the  dips  everywhere  approximately 
parallel,  varying  from  80°  to  the  south  to  vertical.  The  major  part  of  the 
mountain  and  the  two  northern  of  its  higher  points  are  made  up  of  the  pure, 
vitreous,  broken  and  cherty  Mesnard  quartzite.  Between  this  and  the  Kona 
dolomite  is  a  layer  of  slate,  with  a  transition  schistose  quartzite.  The  slate, 
being  less  resistant  than  the  quartzite  or  the  cherty  dolomite,  is  marked  by 
an  irregular  longitudinal  depression.  In  the  readily  yielding  slate  are  seen 
strong  evidences  of  mashing,  the  major  readjustment  in  folding  apparently 
being  here.  Constituting  the  center  of  the  syncline  is  a  second  row  of 
points,  one  being  the  culminating  peak.  These  are  composed  of  the  vertical 
layers  of  the  closely  compressed  Kona  dolomite.  The  steep  south  brow  is 
comjiosed  of  the  slate,  and  on  the  south  flank  of  the  bluff  is  again  the 
Mesnard  quartzite,  making  the  other  limb  of  the  syncline.  This  syncline 
has  minor  corrugations  and  a  westward  pitch,  as  a  consequence  of  which 
the  fingers  of  the  Kona  dolomite  unite  toward  the  west  into  a  broad  area  of 
this  formation.  The  south  and  southeast  slopes  of  the  bluff  are  composed 
of  the  Mesnard  quartzite.  Because  of  the  westward  pitch  of  the  formations 
and  the  topography  the  belt  of  Kona  dolomite  terminates  a  short  distance 
east  of  the  culminating  peak,  as  a  consequence  of  which  the  eastern  half  of 
the  ridge  is  composed  wholly  of  the  Mesnard  quartzite  folded  back  upon 
itself 

East  of  the  State  prison  the  south  arm  of  the  Mesnard  quartzite  con- 
stitutes a  ridge,  a  point  of  Lake  Superior,  and  a  small  island  off  the  coast. 


INTERESTING   LOCALITIES   OF   MESNxVKD   QUAKTZITE.         237 

These  exposures  here  are  less  mashed  than  at  Mount  ]\Iesnard,  and  at  many- 
places  beautifully  show  ripple  marks,  especially  in  the  slaty  phases.  The 
rocks  are  vertical  or  have  a  dip  of  80°  to  the  south.  The  only  indication 
of  the  direction  in  which  they  have  been  upturned  is  given  by  the  ripple 
marks.  An  examination  of  these  shows  the  south  faces  of  the  quartzites  to 
have  the  normal  form  of  the  ripple  marks  and  the  north  faces  their  casts.' 
This  furnishes  evidence  in  support  of  the  statement  first  made,  that  these 
quartzites  are  on  the  south  side  of  the  fold  and  are  a  continuation  of  the 
southern  part  of  the  quartzite  of  Mount  Mesnard. 

Mount  chocoiay. — Ou  Mouut  Cliocolaj  (Atlas  Sheet  XXXIX),  about  3  miles 
south  of  Marquette,  are  the  extreme  eastern  exposures  of  the  Marquette 
series.  This  prominent  bluff  rises  about  150  feet  above  the  sand  plans  of 
the  Chocoiay  River,  to  the  south  and  east.  The  eastern  abrupt  face  of  the 
bluff  gives  beautiful  exposures  of  the  Mesnard  quartzite,  of  the  Kona 
dolomite,  and  of  the  underlying  green  schists  of  the  Archean.  The  major 
part  of  the  bluff  is  a  simple  syncline,  the  dips  of  the  quartzite  being 
about  60°  N.  on  the  south  side  and  85°  to  90°  S.  on  the  north  side.  The 
quartzite  exhibits  nearly  all  phases  of  the  formation,  including  the  slaty 
and  novaculitic  phases,  cherty  quartzite,  and  the  ordinary  massive  forms. 
The  Kona  dolomite  constitutes  the  center  of  the  syncline  and  the  top  of  the 
bluff.  As  usual,  between  the  quartzite  and  dolomite  is  a  thin  bed  of  slate. 
A  ravine  separates  the  Mesnard  quartzite  from  the  green  schist  of  the  Base- 
ment Complex  to  the  south.  The  two,  however,  dip  in  opposite  directions, 
the  quartzite  about  60°  to  the  north  and  the  schistose  structure  of  the  green 
schists  about  45°  to  the  south. 

On  the  western  end  of  Mount  Chocoiay  the  quartzite  formation  is  found 
to  pass  entirely  around  the  dolomite.  In  passing  from  the  north  to  the  south 
side  the  strike  varies  from  west  to  southwest,  then  to  south,  and  finally  to 
southeast,  thus  showing  that  the  whole  is  an  eastward-plunging  spicline. 
Superimposed  upon  this  major  fold  are  beautifully  exposed  minor  anticlines 
and  synclines.  Near  the  base  of  the  formation  on  the  southwestern  part  of 
the  mountain  is  found  a  thin  belt  of  conglomerate,  very  similar  to  that  on 

I  Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise.  Sixteenth  Ann.  Rept. 
U.  S.  Geol.  Survey,  Part  I,  1896.  pp.  720-721. 


238  THE   MARQUETTE    IROX-BEARING   DISTRICT. 

Mount  Omimi,  the  windings  of  which  serve  beautifully  to  show  the  minor 
folding  of  the  formation. 

Migisi  Bluffs. — Across  a  transverse  depression  west  of  Mount  Chocolay,  in 
the  north  part  of  sec.  1  and  in  sec.  2,  T.  47  N.,  R.  25  W.,  are  the  Migisi  Bluffs 
(Atlas  Sheet  XXXIX).  The  eastern  part  of  these  bluffs  is,  in  a  large  way, 
a  westward- plunging  syncline,  but,  as  in  tlie  case  of  Mount  Chocolay,  this 
larger  syncline  is  found  to  be  composed  of  many  subordinate  rolls.  Tlie 
depression  separating  Mount  Chocolay  and  the  Migisi  Bluffs  is,  then,  the 
bridge  or  anticline  of  a  large  north-south  fold.  The  complex,  plunging  Migisi 
syncline  may,  then,  be  considered  as  a  combined  effect  of  folding  in  two 
directions.  The  eastern  part  of  the  bluffs  is  composed  of  the  quartzite, 
but,  as  a  result  of  its  western  pitch,  the  Kona  formation  apjjears  in  the 
eastern  half  of  sec.  2  in  a  series  of  fingers,  each  of  which  corresponds  to  a 
minor  fold.  These  fingers  unite  toward  the  west  and  form  the  broad  belt 
of  the  Kona  formation.  Beginning  at  the  north  and  going  east  and  south 
around  the  bluff,  the  strike  changes  from  an  east  direction  to  a  southeast, 
then  to  a  south,  then  to  a  southwest,  and  finally  to  a  western  course  on  the 
southern  side  of  the  fold.  As  in  other  localities,  separating  the  quartzite 
and  the  Kona  dolomite  is  a  thin  belt  of  slate,  which  becomes  calcareous  in 
its  upper  parts.  As  on  Mount  Chocolay,  the  narrow  belt  of  ferruginous 
conglomerate,  bearing  numerous  pebbles  of  chert  and  jasi)er,  is  of  great 
assistance  in  following  the  details  of  the  minor  folds.  In  the  north  part  of 
sec.  2  appear  the  green  schists  of  the  Archean,  and  a  section  through  the 
western  part  of  the  bluffs  shows  the  complete  succession  from  the  Archean 
to  the  Kona  dolomite. 

On  the  southeastern  slope  of  the  Migisi  Bluffs,  north  of  the  quarter 
post  between  sees.  1  and  2,  near  the  section  line,  may  be  seen  the  actual 
contact  of  the  quartzite  and  granite-gneiss.  As  the  bottom  of  the  quartzite 
is  approached  there  appears  a  bed  of  conglomerate  8  or  10  feet  thick, 
containing  numerous  white  quartz  pebbles,  some  of  them  8  inches  across. 
Toward  the  south,  near  the  base  of  a  cliff,  the  exposure  becomes  less 
conglomeratic  and  changes  into  a  schistose  rock.  This  clearly  fragmental 
schistose  rock  is  in  direct  contact  with  another  schistose  rock,  which  can  be 
traced  by  gradations  into  the  genuine  granite-gneiss.      There  appeal's  to 


INTERESTING   LOCALITIES  OF  MESNARD   QUARTZITE.         239 

be  no  discordance  between  the  two  schists.  After  a  short  interval  of  no 
exposure  the  normal  granite  appears  to  the  south.  This  is  one  of  the 
localities  which  were  cited  by  Rorainger^  as  evidence  of  progressive  meta- 
niorphism  of  the  quartzite  into  the  granite.  However,  taking  the  locality 
in  connection  with  others,  it  is  certain  that  there  is  no  such  gradation, 
but  an  unconformity  between  the  two.  The  apparent  transition  may  be 
explained  by  the  disintegrated  character  of  the  granitoid  gneiss  at  the  time 
of  the  Mesnard  transgression;  or  the  intense  mashing  jjroduced  by  the 
folding  at  the  junction  of  the  two  formations  may  have  obliterated  the 
pebbled  granitic  detritus,  even  if  it  existed.  The  mashing  has  transformed 
the  clastic  rock  into  a  crystalline  schist,  and  has  metamorphosed  the  granite 
into  a  similar-looking  rock. 

In  the  northern  part  of  sec.  3  the  Mesnard  quartzite  may  be  traced  in 
continuous  exposure  aroimd  the  north,  west,  and  south  sides  of  the  Archean 
green  schist,  dipping  away  from  it  in  all  directions.  The  exposures,  there- 
fore, constitute  a  westward-plunging  anticline. 

In  thin  section  the  larger  masses  of  the  Migisi  Bluffs  present  the  ordi- 
nary phases  of  gray  wacke  and  vitreous  and  cherty  quartzites.  However,  it 
is  on  the  southeastern  slope  of  this  blutf  that  occur  the  kaolinic  quartz- 
schists  described  on  pp.  226-227.  It  has  been  seen  that  in  the  held  there 
is  difficulty  in  discriminating  between  the  mashed  fragmental  rock  and  the 
underlying  gneiss.  In  thin  section  the  two  are  separable.  The  most  altered 
phase  of  the  detrital  rock  shows  distinct  rounding  of  the  quartz  grains. 
These  are  set  in  a  fine-grained  matrix  of  kaolin,  sericite,  and  cherty  quartz. 
All  distinct  feldspathic  detritus  has  disappeared.  On  the  other  hand,  the 
gneissoid  granite  has  distinctly  a  granitic  structure,  and  even  where  most 
altered  the  feldspars,  although  much  decomposed,  may  be  recognized 

Lake  Mary. — Xortliwest  of  Lako  Mary,  in  the  SE.  \  sec.  4  and  the  NW.  J 
sec.  9,  T.  47  N.,  R.  25  W.  (Atlas  Sheet  XXXVII),  are  found  large  exposures 
of  quartzite,  dipping  away  from  the  granite  on  each  side  and  toward  each 
other  under  the  Kona  dolomite,  which  appears  as  a  westward-plunging 
syncline.     Near  the  corner  of  sees.  8,  9,  16,  and  17,  T.  47  N.,  R.  25  W., 

'  The  Marquette  iron  region,  by  Carl  Rominger.  Geol.  of  Michigan,  Vol.  IV,  Part  I,  1878-1880, 
pp.  15,  .52. 


240  THE   MAKQUETTE   lEON-BEARING   DISTRICT. 

forming  embayments  oii  the  west  aud  southwest  slopes  of  a  large  granite 
bluff,  are  magnificent  exposures  of  a  great  granite-conglomerate.  The 
pebbles  and  bowlders  of  the  rocks  are  predominantly  of  coarse  and  fine 
granite,  and  with  these  are  abundant  pebbles  of  quartz  and  green  schist, 
and  fewer  of  jasper.  Interstratified  with  the  coarse  conglomeratic  bauds 
are  fine-gramed  conglomerates,  which  are  so  thoroughly  cemented  as  to 
resemble  original  granite.  The  interlaminations  of  materials  of  different 
degrees  of  coarseness  in  places  give  the  rock  a  fine  banding,  which  makes 
it  in  a  remarkable  degree  resemble  gneiss.  Nowhere  was  the  conglomerate 
found  in  actual  contact  with  the  granite,  but  as  the  various  granitic  frag- 
ments are  identical  with  the  exposures  of  granite  below,  no  one  can  doubt 
that  the  pebbles  and  bowlders  are  from  that  source.  Southeast  of  the 
corner,  in  sec.  16,  are  exposures  of  schistose  and  feldspathic  quartzite 
resembling  gneiss.  This  feldspathic  quartzite  or  recomposed  granite  grades 
into  the  ordinary  white  quartzite. 

SECTIOT^  II.— THE   KONA  DOLOMITE. 

The  name  Koua  dolomite  is  given  to  this  formation  because  the  Kona 
Hills,  rising  from  the  east  shore  of  Goose  Lake  (Atlas  Sheets  XXXIV  and 
XXXV)  as  large  bluff's  with  precipitous  cliffs,  are  composed  of  typical  rocks 
of  the  formation,  and  because  dolomite  is,  upon  the  whole,  the  predomi- 
nant rock. 

DISTRIBUTION,  EXPOSURES,  AND   TOPOGRAPHY. 

Starting  at  Mount  Mesnard  (Atlas  Sheet  IV),  the  area  covered  by  the 
Kona  formation  rapidly  widens  in  passing  westward.  From  south  of  Mud 
Lake  the  belt  again  narrows  in  going  toward  the  west,  until  at  Morgan 
Furnace  it  is  only  about  a  sixteenth  of  a  mile  wide.  Farther  to  the  west  no 
exposures  of  this  limestone  are  found,  but  its  horizon  may  be  represented 
by  a  belt  of  slates  and  quartzites  east  of  Teal  Lake. 

On  the  south  side  of  the  Algonkian  the  formation  has  a  much  more 
in-egular  distribution.  Starting  at  the  sand  plain  just  west  of  Lake  Superior, 
it  extends  west  nearly  half  a  mile,  where  it  disappears.  About  a  mile 
to  the  westward,  southeast  of  Lake  Wabassin,  the  formation  reappears 


TOPOGRAPHY  OF  THE  KONA  DOLOMITE.  241 

and  extends,  westward  as  a  belt  a  third  of  a  mile  wide.  As  it  reaches  Carp 
River  the  formation  swings  southwest,  and  then  south  to  Tigo  Lake.  Here 
a  small  arm  goes  to  the  southeast  across  this  lake  toward  Lake  Mary,  but 
the  main  belt  continues  to  the  southward.  About  a  mile  west  of  Lake 
Mary  it  widens  out  into  a  broad  area,  varying  from  a  mile  to  2  miles  in 
width,  and  extends  to  Goose  Lake,  the  last  exposures  of  the  formation  being 
fovind  on  the  east  side  of  this  body  of  water.  Also  north  of  the  Archean 
island  in  sees.  2  and  3,  T.  47  N.,  R.  25  W.,  the  limestone  appears,  just  north 
of  the  Mesnard  quartzite,  in  a  narrow  belt.  The  real  extent  of  this  area  of 
dolomite  it  is  impossible  to  give,  as  the  Potsdam  formation  occupies  much 
of  the  valley  of  the  lower  reaches  of  the  Carp  River. 

Almost  coextensive  with  the  distribution  of  the  formation  are  the 
exposures,  they  being  abundant  and  prominent  throughout  most  of  the  area. 
However,  some  of  the  most  readily  accessible  places  at  which  the  formation 
may  be  studied  are  the  exposures  east  of  Goose  Lake  and  those  south  and 
west  of  Wabassin  Lake  (Atlas  Sheets  XXXIV  and  XXXIX). 

As  a  consequence  of  the  complicated  folding  of  the  formation,  below 
described,  combined  with  the  very  different  resisting  powers  of  the  layers, 
the  topography  of  the  formation  is  exceedingly  jagged.  The  exposures 
constitute  a  set  of  sharp  and  abrupt  cliffs,  cut  by  ravines  or  separated 
by  drift-filled  valleys.  Where  north-south  and  east-west  folds  both  occur 
the  valleys  cut  across  one  another  in  two  systems  at  right  angles,  leaving 
roughly  rectangular  masses  of  rock  between.  In  places  where  the  folds 
have  a  pitch  the  layers  may  form  semicircular  outcrops  with  vertical  walls. 
Rather  low  dips  prevail  for  much  of  the  area,  and  in  traveling  over  the 
belt  one  has  to  climb  a  series  of  steep  hills,  each  of  which  is  composed 
of  a  number  of  almost  vertical,  ragged  cliffs.  The  descent  from  the  eleva- 
tion is  of  much  the  same  character.  The  weathered  surfaces  of  the  ledges 
also  are  sharp  and  ragged  in  a  minor  way  (PI.  VII,  fig„  1).  The  cherty 
layers  form  sharp  ridges.  The  quartzite  layers  project  in  less  jagged  forms. 
Geodal  concentrations  of  quartz  protrude  from  the  surface  of  the  hmestone. 
The  dolomite  has  dissolved  from  the  cherty  and  quartzose  layers,  giving 
them  a  rough,  vesicular  ajjpearance. 

MON  XXVIII 16 


242  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

FOLDING. 

The  major  folding  (Atlas  Sheet  IV)  of  the  formation  will  be  considered 
in  connection  with  the  general  geology  of  the  district.  It  may  be  said  here 
that  the  formation  has  been  affected  by  both  east-west  and  north-south 
thrusts.  In  some  cases  the  east-west  folds  are  more  conspicuous,  in  others 
the  north-south,  while  in  still  other  areas  the  folds  are  about  equally  prom- 
inent in  both  directions,  although  even  here  the  folds  of  one  set  have  less 
amplitude  and  less  length  than  those  of  the  other  As  a  consequence  of 
the  above,  each  fold  has  a  pitch,  which  may  be  slight  or  very  steep.  Still 
fiirther  to  complicate  the  structure  of  the  area,  the  major  folds  in  each 
direction  have  superimposed  upon  them  secondaiy  folds,  and  upon  these 
are  tertiary  ones.  In  some  cases,  as  in  the  largest  belt  east  of  Goose  Lake, 
the  pressure  has  not  been  so  great  as  to  give  the  beds  very  steep  inclina- 
tions, the  dips  usually  being  not  more  than  20°,  although  occasionally  as 
high  as  50°.  As  a  consequence  of  the  nearly  equal  power  of  the  folding 
forces  in  each  of  the  directions  in  this  broad  area,  the  ledges  give  strikes  in 
all  directions. 

From  the  above  It  is  clear  that  the  deformation  of  the  Kona  formation 
is  a  beautiful  illustration  of  complex  folding.^ 

To  the  pressure  of  folding  the  dolomite  has  usually  yielded  without 
prominent  fractures  or  cleavage.  The  same  can  not  be  said  of  the  inter- 
laminated  slates,  graywackes,  and  quartzites.  In  many  places  a  bed  of  slate 
has  had  developed  across  it  a  diagonal  cleavage,  which  stops  abruptly  at 
the  limestone  layers  (fig.  9).  In  other  cases  the  cleavage  passes  into  the 
dolomite  itself,  as,  for  instance,  at  the  exposure  back  of  the  railroad  section- 
house  near  Goose  Lake.  In  some  places  the  dynamic  movements  have 
produced  a  fissility  in  two  directions,  so  that  the  rocks  break  into  polygonal 
blocks.^  In  numerous  instances  the  layers  of  chert  and  quartzite  have  been 
fractured  through  and  through  by  folding,  so  as  to  change  them  into  breccias 
resembhng  conglomerates  (PI  VII,  fig.  2,  and  PI.  VIII).  Along  the  con- 
tacts of  the  dolomite  beds  and  the  quartz  layers  accommodation  was 
necessary,  and  in  places  a  bed  of  limestone  may  be  seen  bent  into  a  series 

'  Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise:  Sixteenth  Ann.  Kept. 
V.  S.  Geol.  Survey,  Part  1,  1896,  pp.  626-631. 
=  Ibid.,  pp.  643-646. 


FOLDING  OF  THE   KON^A  DOLOMITE. 


243 


of  anticlines  and  synclines,  the  overlying  quartzite  not  being  similarly  bent, 
but  being  compressed  and  brecciated,  thus  making  a  pseudo-conglomerate. 
The  folded  dolomite  laminse  are  actually  cut  awav  to  some  extent  by  the 


■"IQ.  9.— Cluavase 


shearing  action.  The  result  is  that  the  layers  of  quartzite  cut  across- 
the  folds  of  the  limestone,  as  in  an  unconformable  contact,  and  adjacent 
to  these  truncated  layers  are  the  pseudo-conglomerates  (fig.   10).     Such 


F:a.  10— Pseudonnconformity  between  cherty  quartzite  and  truncateil  strata  of  ilolnmite. 

contacts  as  these,  found  at  many  places,  strongly  suggest  an  unconfonnity 
between  the  t\vo,  but  the  true  explanation  is  undoubtedly  that  the  apparent 
unconformity  is  merely  a  dynamic  phenomenon. 


244  THE  MAEQUETTE   IRON-BEAEIXG  DISTRICT. 


PETROGRAPHICAL    CHARACTER. 


Macroscopicai. — Petrograpliically  the  different  exposures  are  A^ery  similar. 
The  formation  is  not  a  pure  dolomite,  but  is  a  cherty  dolomite  interstratified 
with  layers  of  slate,  graywacke,  and  quartzite,  with  all  gradations  between 
the  various  mechanical  sediments  and  between  these  and  the  pure  dolomite. 
In  some  exposures  the  pure  dolomite  does  not  constitute  more  than  a  third 
to  a  half  of  the  Ijelt.  The  interstratified  slates  and  quartzites  are  of  the 
same  character  as  those  of  the  Wewe  slates  and  Ajibik  quartzites,  except 
that  they  are  ajjt  to  be  more  or  less  calcareous.  The  dolomite  beds  vary 
in  thickness  from  only  a  few  inches  to  many  feet.  But  even  the  solid  belts 
of  limestone  usually  contain  very  thin  layers,  which  in  places  are  in  part 
fragmental,  but  which  are  usually  wholly  or  in  large  part  secondary  chert. 
In  color  the  dolomite  varies  from  nearly  pure  white  to  dark-brown,  depend- 
ing upon  its  purity,  and  between  these  colors  are  various  shades  of  buff, 
purple,  pink,  and  red.  As  the  interbedded  slates  and  quartzites  also  have 
a  wide  variation  in  coloring,  the  ledges  of  the  formation  are  very  different 
in  their  aspects. 

The  dolomite  varies  from  aphanitic  to  coarsely  crystalline.  Upon  the 
weathered  surface  the  pink  and  red  varieties  usually  have  a  dark-brown 
color,  due  to  limonite.  This  indicates  that  the  carbonate  carries  a  consider- 
able quantity  of  iron,  the  oxidation  of  which  has  produced  this  outer  dark- 
colored  skin.  Where  the  dolomite  is  most  coarsely  crystalline,  as,  for 
instance,  at  Morgan  Furnace,  it  sometimes  contains  belts  from  a  fraction  of 
an  inch  to  4  inches  wide,  largely  composed  of  pink,  coarsely  crystalline, 
and  evidently  rearranged  dolomite  (PI.  VII,  fig.  1).  As  a  consequence  ot 
weathering,  the  bands  of  original  sedimentary  quartz  and  of  secondary 
chert  protrude,  giving  a  peculiar  rough,  ridgy  appearance. 

Microscopical. — Tlic  rocks  of  tlic  Koua  dolomite  comprise  coarsely  and 
finely  crystalline  dolomite,  cherty  dolomite,  quartzose  dolomite,  argillaceous 
dolomite,  dolomitic  quartzites,  dolomitic  slates,  dolomitic  cherty  quartzites, 
and  dolomitic  chert. 

Tlie  compact  and  apparently  least  altered,  purer  rock  has  a  background 
consisting  of  finely  granular   dolomite,   separate  granules  of  which  are 


PLATE    VII. 


Plate  VII.— THE  KOXA  DOLOMITE. 

Fig.  1.  Weathered  surface  of  Kona  dolomite  from  Morgan  Furnace.  The  background  i8  the  ordinary 
granular  gray  dolomite.  The  rough  protruding  layers  are  chert,  a  portion  of  which  is  iron 
stained.     Natural  size. 

Fig.  2.  Brecciated  chert  at  the  base  of  the  Kona  dolomite  on  the  east  side  of  sec.  13,  T.  47  N.,  R.  26  W 
(Atlas  Sheet  XXXIV).  At  the  bottom  of  the  Kona  dolomite  chert  was  concentrated  so  as  to 
make  a  layer  2  or  3  feet  in  thickness.  Subsequent  movement  broke  this  layer  into  a  breccia 
which  in  some  pLaces  closely  resembles  a  conglomerate.  The  broken  fragments  are  cemented 
by  later  infiltrated  quartz  and  by  specular  hematite  and  magnetite.  The  chert  in  places  is 
iron-stained,  either  by  limonite  or  by  hematite.     Natural  size. 


S  GEOLOGICAL  SURVEY 


PONOGRAPH    XXVIII    PL 


K;.   i     W-EA'i'llKKEl)  SURFACK   IN  KONA  nOl.OMITl' 

U;.  2.  BRECCIATED  CHERT  AT  THE  BASE  OK  KONA  Dni.OMlTK. 


PETEOGRAPHICAL   CHARACTER   OF   THE   KONA   DOLOMITE.     247 

largely  rhombohedra.  These  very  finely  granular  rocks  vary  into  those 
which  are  more  coarsely  crystalline,  and  the  latter  grade  into  phases  in 
which  coarse  crystals  of  dolomite  compose  most  of  the  rock.  It  is  probable 
that  in  these  coarser  rocks  there  has  been  a  recrystallization.  This  is  indi- 
cated in  one  case  by  a  remarkably  beautiful  zonal  structure,  shown  by  all 
of  the  large  rhombohedra  of  dolomite. 

The  purer  phases  of  dolomite  pass  into  those  in  which  the  dolomite 
is  heavily  stained  with  iron  oxide.  In  some  cases  on  the  weathered  surface 
is  an  outer  layer  of  heavily  ferruginous  material,  resulting  from  the  decom- 
position of  the  dolomite,  and  showing  that  the  carbonate  is  ferriferous. 

These  nonfragmental  rocks  by  gradation  pass  on  the  one  hand  into 
the  argillaceous  dolomites  or  mica-slates  and  upon  the  other  into  the 
quartzose  dolomites.  In  the  argillaceous  dolomites  the  finely  crystalline 
quartz,  feldspar,  and  other  clayey  materials  are  intimately  intermingled ' 
with  the  granules  of  dolomite.  By  a  further  decrease  of  the  dolomite  the 
rocks  pass  into  the  dolomitic  slates.  Those  which  show  the  fragmental 
material  in  a  dolomitic  background  are  placed  with  the  dolomites.  Those 
which  show  a  fragmental  background  in  which  the  dolomite  occurs  are 
placed  with  the  slates.  Where  the  detritus  is  coarse  the  rocks  are  quartzose 
dolomites.  In  these  we  have  a  large  amount  of  fragmental  quartz,  in  well- 
rounded,  enlarged  grains.  Where  the  quartz  grains  are  buried  in  a  back- 
ground of  dolomite  they  are  called  siliceous  dolomites.  By  a  decrease  of 
the  dolomite  we  have  a  sparse  matrix  of  carbonate  in  which  numerous 
quartz  grains  are  set,  and  then  the  rock  becomes  a  dolomitic  (juartzite. 
In  some  cases  the  alternations  of  coarse  and  fine  material  are  in  minute 
layers,  a  fraction  of  an  inch  across,  having  alternately  coarse  and  fine 
grains  of  quartz  and  greatly  varying  amounts  of  dolomite.  In  other  cases 
thick  beds  are  wholly  of  the  dolomitic  quartzite. 

The  rocks  of  the  formation,  whatever  their  lithological  character,  have 
been  shattered  by  dynamic  action,  and  have  frequently  become  reibungs- 
breccias  (PI.  VII,  fig  2,  and  PI.  VIII)  These  breccias,  which  where  much 
mashed  resemble  true  elastics,  differ  from  conglomerates  in  the  usual  angu- 
larity of  the  fragments  and  in  containing  no  material  from  an  extraneous 
source. 


248  THE   MARQUETTE    IRON-BEARING    DISTRICT. 

The  pure  dolomites,  where  merely  shattered,  have  been  cemented  by 
finely  crystalline  cherty  quartz,  or  by  coarsely  crystalline  dolomite,  or 
by  these  two  combined.  The  brecciated  phases  show  numerous  irregular 
complex  fragments  of  the  granular  dolomite.  The  angularities  of  these 
dissevered  fragments  are  frequently  the  reverse  of  the  fragments  adja- 
cent, proving  conclusively  that  they  have  been  broken  apart.  In  a  more 
advanced  stage  of  tlie  dynamic  action  the  complex  fragments  of  the  gran- 
ular dolomite  have  a  subangular  or  roundish  appearance,  so  that  the  rock 
as  looked  at  with  a  low  power  resembles  a  conglomerate.  These  dissev- 
ered fragments  are  united  by  cherty  quartz,  by  coarsely  crystalline  dolomite, 
or  by  the  two  interlocking.  In  some  cases  this  secondary  cherty  quartz 
has  impregnated  the  rock  through  and  through,  so  that  minute  iiTegular 
veins  of  chert  or  geodal  areas  of  quartz  are  scattered  through  the  dolo- 
mite. In  a  still  further  stage  of  silicification  but  a  small  amount  of  granular 
dolomite  may  be  seen  in  the  chert  veins.  As  a  result  of  further  silicifica- 
tion considerable  belts  of  chert  are  found  interlaminated  with  the  bands 
containing  less  chert.  Frequently  these  belts  have  oval  or  abrupt  ter- 
minations. Oftentimes  after  a  first  dynamic  action  and  silicification  the 
rocks  have  been  brecciated  again,  and  have  again  been  cemented  by  infil- 
trating silica.  In  this  case  we  have  a  cherty  dolomite  or  a  chert-breccia, 
with  a  cement  of  newer  chert.  It  is  genei-ally  possible  to  discriminate  the 
earlier  and  later  chert  by  the  slightly  different  crystalline  characters  which 
it  has,  and  also  because  the  later  chert  is  sometimes  mingled  with  oxide 
of  iron. 

The  argillaceous  and  siliceous  dolomites  have  been  brecciated  and 
cemented  in  the  same  way  as  the  purer  dolomites.  In  this  case  we  have 
both  fragmental  quartz  and  secondary  cherty  quartz  intermingled.  The 
original  quartz  grains  uniformly  show  undulatory  extinction  or  fracturing. 
Frequently  during  the  folding  the  grains  of  quartz  and  feldspar  have  been 
broken  out  of  their  background  and  have  fallen  into  the  crevices.  These 
are  surrounded  by  and  embedded  in  secondary  infiltrated  cherty  quartz 
and  dolomite. 

The  slates  and  quartzites  interstratified  in  the  Koua  dolomite  are  not 
here  described,  as  they  are  in  all  respects  similar  to  the  We  we  slates  and 


PLATE    VIII. 


Plate  VIII.— BRECCIATED  KONA  DOLOMITE. 

Fig.  1.  Brecciated  chert  in  Kona  dolomite  from  sec.  18,  T.  47  N.,  R.  25  W.  (Atlas  Sheet  XXXVII).  The 
gray  chert  is  broken  into  fragments  by  dynamic  action.  The  fragments  are  angular.  They 
are  cemented  by  chert  and  limouite.  After  being  thus  cemented  the  lock  was  again  broken 
by  later  movement.  The  rock  was  again  cemented  by  minute  veins  of  chert  and  hematite. 
Natural  size. 

Fig.  2.  Brecciated  chert  in  Kona  dolomite  from  sec.  1,  T.  47  N.,  R.  25  W.  (Atlas  Sheet  XXXIX).  The 
figure  IS  from  a  chert  layer  between  the  Mesnard  quartzite  and  the  dolomite.  The  rock 
was  brecciated  by  dynamic  action.  The  fragments  were  rubbed  against  one  another,  so  that 
many  of  them  are  partly  rounded.  They  were  then  cemented  by  chert  and  hematite.  Sub- 
sequent movement  again  slightly  shattered  the  rock,  and  the  cracks  thus  formed  were 
healed  by  secondary  silica.  The  minute  veins  thus  produced  may  be  seen  running  through 
both  matrix  and  fragments.     Natural  size. 


ONOGRAPH    XXVill,  PL 


FIG  1,    BKECri.VI 
Kid.  '1.  BRECri.Vl 


!T  IX  KOX.V  DOI.OMlTt 
IT  I,\'  KOXA  DOl.OMITl 


PETROGRAPHICAL  CHARACTER  OF  THE  KONA  DOLOMITE.    251 

the  Ajibik  quartzites  subsequently  described,  with  the  exception  that  they 
are  more  or  less  dolomitic. 

The  foregoing  study  of  the  thin  sections  of  the  Kona  formation  shows 
that  it  has  been  shattered  throughout.  From  the  field  observations  it  was 
apparent  that  the  formation  had  been  aiuch  broken  by  dynamic  action,  but 
the  completeness  of  this  shattering  and  brecciation  was  appreciated  only 
by  a  study  of  the  thin  sections,  every  one  of  the  numerous  slides  showing 
these  phenomena  to  a  greater  or  less  degree.  It  thus  appears  that  not  a 
half-inch  cube  has  escaped.  It  is  believed  that  this  indicates  that  the  rock 
when  folded  was  not  buried  under  so  great  a  load  as  to  be  beyond  the 
sustaining  power  of  the  rocks.  Upon  the  other  hand,  since  there  are  no 
prominent  faults,  and  since  the  formation  as  a  whole  is  folded  in  a  com- 
plicated fashion,  retaining  its  continuity,  it  is  thought  that  it  was  buried 
lender  a  considerable  thickness  of  strata.  It  was  therefore  in  the  zone  of 
combined  fracture  and  flowage. 

RELATIONS   TO   ADJACENT   FORMATIONS. 

The  Kona  dolomite  varies  through  a  slate  into  the  Mesnard  quartzite 
below.  This  slate  appears  to  be  a  thin,  persistent  formation.  Its  thickness 
varies  from  less  than  30  feet  to  100  feet.  In  many  places  it  appears  thinner 
than  this  smaller  number,  but  it  is  only  at  a  few  places  that  the  exact  contact 
between  the  slate  ,aud  the  formations  above  and  below  it  can  be  seen, 
there  usually  being,  however,  sufficient  room  for  the  slate  belt  between  the 
quartzite  and  the  dolomite.  This  slate  may  be  well  observed  at  Mount 
Mesnard,  where  it  forms  a  little  valley  separating  the  quartzite  peak  on  the 
north  from  the  marble  peak  on  the  south.  The  slate  may  also  be  well 
seen  just  west  of  Wabassin  Lake,  in  sec.  2,  T.  47  N.,  R.  25  W.,  where 
the  westward-plunging  syncline  of  the  Kona  formation  causes  the  slate  to 
appear  immediately  beneath  the  limestone.  This  belt  of  slate,  which  was 
once  a  shale,  probably  marks  the  time  of  deepening  waters,  when  the  con- 
ditions favorable  to  the  deposition  of  a  sandstone  changed  to  those  favorable 
to  the  formation  of  a  limestone. 

Above,  the  dolomite,  by  a  lessening  of  the  calcareous  constituent, 
gradually  passes  into  the  Wewe  slate.     The  appearance  of  this  formation 


252  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

may  have  marked  a  time  when  subsidence  had  ceased  and  the  hmestone 
had  been  built  upward  until  the  finer-grained  mechanical  sediments  could 
be  carried  by  the  waves.  That  this  is  probable,  rather  than  that  the  sea 
had  deepened  so  much  as  to  make  the  limestone  formation  impossible,  is 
indicated  by  the  fact  that  above  the  Wewe  slate  follows  the  Ajibik  quartzite, 
a  coarser  mechanical  sediment. 

THICKNESS. 

As  a  consequence  of  the  complicated  folding  of  the  district,  it  is  exceed- 
ingly difficvilt  to  give  an  accurate  estimate  of  the  thickness  of  the  Kona 
dolomite.  It  doubtless  varies  much,  perhaps  reaching  its  maximum  some- 
where near  the  central  part  of  the  area,  and  thinning  out  in  passing  to  the 
west.  South  of  Mud  Lake  the  formation  has  an  almost  continuous  exposure 
for  1,500  feet,  with  a  dip  to  the  south  varying  from  78°  to  90°.  If  there 
were  no  minor  folds,  and  calling  the  average  dip  80°,  this  would  correspond 
to  a  thickness  of  about  1,375  feet.  However,  it  is  certain  that  just  to  the 
north  of  this  lake  the  slates  are  in  a  series  of  sharp  isoclinal  folds;  and 
that  this  is  true  for  the  dolomite,  to  some  extent  at  least,  is  more  than  pos- 
sible. How  much  this  maximum  thickness  should  be  decreased  on  account 
of  this  uncertain  element  of  the  problem  it  is  difficult  to  estimate,  but  it  is 
wholly  possible  that  the  thickness  as  above  calculated  should  be  reduced 
one-half  At  Groose  Lake,  as  has  been  said,  there  is  a  continuous  exposure 
of  the  formation  for  a  considerable  distance.  Here  the  thickness  of  the 
layers  was  carefully  measured  and  found  to  be  225  feet,  with  a  possible 
eiTor  of  25  feet.  If  the  formation  is  supposed  to  have  the  same  dip  to  the 
northward  for  the  remainder  of  the  detached  exposures  along  the  east  shore 
of  Goose  Lake,  this  amount  may  be  increased  by  150  or  200  feet. 

West  of  Tigo  Lake  the  formation  is  exposed  almost  continuously  for  a 
distance  of  1,300  feet.  The  dip  here  varies  from  25°  to  40°,  averaging  per- 
haps 30°  or  35°.  Calhng  the  average  dip  30°,  this  would  give  a  thickness 
of  650  feet.  Although  the  limestone  occupies  an  area  as  broad  as  2  miles 
in  certain  places,  it  can  not  be  asserted,  on  the  present  information,  that  the 
maximum  thickness  of  the  limestone  is  more  than  700  feet,  although  it  may 
be  twice  this  amount. 


THE   KONA   DOLOMITE.  253 


INTERESTING   LOCALITIES. 


Nearly  all  of  the  peculiarities  of  theJ  Koua  dolomite  mentioned  in 
the  general  description  may  be  found  at  any  of  the  localities  in  which  the 
exposures  are  extensive,  so  that  here  there  will  be  given  but  little  more 
than  a  list  of  localities  in  which  the  exposures  are  numerous,  to  serve  as 
guides  to  those  wishing  to  study  this  formation. 

Eastern  area. — Beginning  at  the  northwest,  there  are  excellent  exposures 
near  the  old  Morgan  Furnace,  in  the  north  part  of  sec.  31,  T.  48  N.,  R.  25  W. 
(Atlas  Sheet  XXXVI).  Numerous  large  exposures  occur  on  the  bluff 
south  of  Mud  Lake,  in  sec.  32.  The  south  half  of  Oinimi  Bluff,  in  sec.  34, 
also  gives  very  numerous  exposures  (Atlas  Sheet  XXXVIII).  On  Mount 
Mesnard,  as  has  been  said,  the  crowning  peak  is  a  closely  compressed 
syncliue  of  the  Kona  dolomite.  The  central  and  higher  part  of  Mount 
Chocolay,  south  of  Marquette,  is  another  syncliue  of  the  dolomite.  On  the 
Migisi  Bluffs  (Atlas  Sheet  XXXIX),  in  sees.  2  and  3,  T.  47  N.,  R.  2.5  W., 
are  very  numerous  large  exposures  of  the  dolomite.  On  account  of  the 
western  pitch  this  formation  ends  at  the  east  in  a  series  of  fingers,  the  gen- 
eral syncliue,  as  has  been  said,  being  composed  of  minor  synclines.  The 
succession  of  the  members  of  the  formation,  the  dynamic  effects  produced 
by  the  folding,  and  the  slate  marking  the  transition  belt  between  the  dolo- 
mite and  the  Mesnard  quartzite  may  be  particularly  well  observed  here. 
Tlie  hills  often  break  off  in  eastward-facing  cliffs,  and  hj  following  from  the 
north  around  the  east  side  to  the  south  side  of  such  a  bluff  one  continues 
parallel  to  the  changing  strike  of  the  terminating  layers.  Both  east  and 
west  of  Tigo  Lake  (Atlas  Sheet  XXXVII),  in  sees  4,  5,  8,  and  9,  T.  47  N., 
R.  25  W.,  are  very  numerous  exposures  of  the  dolomite. 

Ragged  h.iis. — Numerous  typical  exposures  of  the  formation  are  found  on 
all  the  blufts  in  sees.  7,  8, 17,  andl8,T.47  N.,R.25  W.(Atlas  Sheet  XXXVII) 
On  the  south  slope  of  the  bluff  in  the  SW.  ^  sec.  17  is  a  belt  of  cherty, 
quartzitic  breccia,  which  on  its  weathered  surface  very  closely  resembles  a 
conglomerate.  This  breccia  contains  fragments  of  slate,  quartzite,  chert,  and 
marble.  It  is  underlain  by  folded  beds  of  marble,  the  minor  sinuosities  of 
which  are  truncated  by  the  breccia,  and  it  is  overlain  by  quartzite.     At  first 


254  THE   MAEQUETTE   IKON-BEAEING   DISTEICT. 

sight  the  appearance  of  an  nnconformity  is  very  strong  indeed.  (See  fig.  10, 
p.  243.)  However,  when  the  supposed  conglomerate  is  followed  along  the 
strike,  its  brecciated  character  is  found  gradually  to  disappear  and  it  changes 
into  ordinary  quartzite.  The  fragments,  instead  of  being  waterworn,  are 
distinctly  angular.  Moreover,  while  at  first  sight  there  appears  to  be  a 
wide  variety  of  fragments  in  the  breccia,  all  of  these  are  obtainable  from 
the  immediately  adjacent  beds.  It  appears  that  when  the  series  was  folded 
the  more  plastic  limestone  yielded  to  the  pressure,  in  both  a  major  and 
a  minor  way,  by  folding,  while  the  brittle  cherty  quartzite  was  fractured 
through  and  through,  the  movement  of  the  fragments  over  one  another,  and 
of  the  bed  as  a  whole,  being  sufficient  to  truncate  the  minor  waves  of  the 
marble.  In  a  large  way  the  belt  of  dolomite  and  that  of  the  quartzite  and 
breccia  are  conformable. 

In  the  west  part  of  the  SE.  ^  sec.  18  is  exposed  the  contact  between  the 
Mesnard  quartzite  and  the  Kona  dolomite,  which  here  has  a  general  strike 
approximately  north  and  south  and  a  dip  to  the  east,  but  with  minor  cross 
folds  with  east-west  axes.  At  the  top  of  the  Mesnard  is  cherty  quartzite, 
which  is  followed  by  thin  beds  of  novaculite  and  slate  before  the  impui-e 
limestone  is  reached. 

Kona  Hills. — The  most  extensive  exposures  of  the  formation  are  on  the 
Kona  Hills  (Atlas  Sheet  XXXIV),  which  rise  from  300  to  400  feet  above 
Goose  Lake,  and  make  up  a  great  series  of  bluff's  in  sees.  11,  12,  13,  and 
14,  T.  47  N.,  R.  26  W.  It  is  from  these  extensive  and  typical  exposures 
that  the  formation  is  given  its  name.  Facing  the  southeast  arm  of  Goose 
Lake  are  bold,  almost  vertical  cliffs,  200  feet  high.  At  the  point  where  the 
lake  widens  these  chffs  slope  rapidly  to  the  north,  following  approximately, 
with  a  somewhat  regular  incline,  the  dip  of  the  formation.  The  lowest 
exposure  here  found  is  a  very  impure  dolomite.  Above  this  follows  a 
succession  of  interlamlnated,  impure  dolomites,  red  and  black  slates,  cherts, 
quartzose  dolomites,  cherty  quartzites,  at  places  brecciated,  and  occasional 
beds  of  nearly  pure  quartzite,  or  even  of  conglomerate.  These  various 
strata  may  have  thicknesses  from  an  inch  or  less  to  a  number  of  feet.  The 
layers  of  quartzite,  usually  not  more  than  a  foot  or  two  in  thickness,  and 
oftentimes  less,  are  generally  interstratified  with  the  dolomitic  slates.     In 


INTEEESTING   LOCALITIES  OF  TUE   KONA   UOLOxMITE.         255 

one  place  ripple  marks  were  seen  in  the  beds  above  and  below  a  layer  of 
conglomerate.  The  close  intermingling  of  mechanical  and  nonmechanical 
sediments  suggests  that  at  the  time  of  the  deposition  of  the  lower  half  of 
these  beds  the  water  was  not  very  deep,  and  perhaps  a  shore-line  was  not 
distant.  The  colors  of  the  rocks  vary  from  the  dark  gray  of  the  slates, 
through  various  shades  of  buflP  and  brown,  to  nearly  white  in  the  case  of 
one  or  two  of  the  dolomites  or  quartzites.  Following  above  these  beds 
are  others  comprising  all  of  the  foregoing  kinds,  and  also  heavy  beds  of 
nearly  pure,  coarsely  granular  dolomite,  some  of  which  are  20  feet 
thick.  The  total  thickness  of  the  beds  thus  far  exposed  measures  about  225 
feet.  The  argillaceous  beds  are  extensively  affected  by  a  slaty  cleavage, 
which  frequently  stops  abruptly  at  the  more  massive  dolomite  or  quartzite 
beds  (fig.  9,  p.  243). 

After  an  interval  of  no  exposure,  the  next  place  north  on  Goose  Lake 
is  occupied  by  coarsely  crystalline,  nearly  pure,  pink  dolomite,  with  occa- 
sional layers  of  more  finely  crystalline  material  and  a  few  layers  of  chert. 
After  another  interval  of  no  exposure  are  very  large  outcrops  of  similar 
dolomite,  some  layers  of  which,  however,  are  very  quartzose,  and  a  few  lay- 
ers of  which  are  shaly.  The  northernmost  exjDOSure  is  a  coarsely  crystalline 
dolomite,  containing  many  nodules  of  coarsely  crystalline  quartz.  Of  the 
large  exposures  southeast  of  Goose  Lake,  probably  not  more  than  one-third 
of  the  thickness  is  composed  of  reasonably  pure  dolomite,  the  remaining 
two-thirds  being  largely  mechanical  sediments.  Farther  to  the  east  the 
proportion  of  mechanical  material  is  not  so  great. 

Of  the  numerous  large  exposures  east  of  Goose  Lake,  only  a  few 
are  platted  on  the  atlas  sheet,  and  these  are  mainly  along  the  outer  borders 
of  the  area  This  area  has,  however,  been  sufficiently  traversed  to  show 
that  there  are  everywhere  great  bluffs  of  the  dolomite. 

As  explained  in  the  general  folding  of  the  area,  east-west  and  north- 
south  forces  were  about  equally  strong,  although  the  folds  with  north-south 
axes  are,  upon  the  whole,  of  larger  dimensions  and  less  dips  than  those 
with  east-west  axes.  It  follows  that  strikes  and  dips  can  be  found  in 
almost  any  direction,  and  the  true  structure  is  perceived  by  general  study 
rather  than  by  taking  strikes  and  dips.      As  a  result  of  the  folding,  the 


256  THE   MARQUETTE   IRON  BEARING   DISTRICT. 

ledges  were  broken  in  tlie  two  directions  according  to  a  rectangular  system, 
and  the  topography  has  a  corresponding  an-angement.  The  great  bluffs 
north  of  the  south  arin  of  Goose  Lake  are  cut  by  deep  ravines  running 
in  a  north- south  direction,  or  in  a  direction  somewhat  east  of  north,  corre- 
sponding to  one  set  of  folds.  The  changing  strikes  and  dips,  showing  a 
northward-plunging  anticline  compounded  of  the  two  foldings,  may  be  seen 
along  the  face  of  the  exposures  east  of  the  south  arm  of  Goose  Lake, 
Similar  eastward-plunging  anticlines  and  synclines  may  be  observed  along 
the  west  side  of  the  north-south  valley  separating  the  exposures  of  the  Wewe 
slate  and  Kona  dolomite  in  the  southeast  part  of  sec.  13  and  the  northeast 
part  of  sec.  24,  T.  47  N.,  R,  26  W. 

Along  the  eastward-facing  cliff  of  limestone  just  west  of  the  Wewe 
slate,  in  the  southeast  part  of  sec.  13,  T.  47  N,,  R.  26  W ,  below  the  lime- 
stone, there  is  found  a  considerable  quantity  of  green  schist  which  is  cut  by 
granite  veins.  Upon  this  material  is  a  conglomerate  containing  numerous 
pebbles  of  the  subjacent  green  schist  and  granite.  This  grades  quickly  up 
into  graywacke,  and  this  above  into  the  limestone.  The  green  schist  cut 
by  granite  is  identical  in  character  with  that  of  the  Archean,  and  is  taken  to 
be  of  Archean  age.  Therefore  we  have  the  Kona  dolomite  resting  uncon- 
formably  on  the  Archean.  It  follows  that  during  the  time  of  the  deposition 
of  the  Mesnard  quartzite  to  the  east  this  part  of  the  district  was  above  the 
water,  and  that  it  was  submerged  in  Kona  time. 

SECTION  III.— THE    WEWE    SLATE. 

The  name  Wewe  slate  is  given  to  this  formation  because  it  occurs  in 
typical  development  on  the  Wewe  Hills,  southwest  of  Goose  Lake  (Atlas 
Sheet  XXXV),  and  because  the  predominant  rock  is  a  slate.  With  the 
slate  are  graywackes,  conglomerate,  mica-slates,  and  in  places  mica-schists. 

DISTRIBUTION,  EXPOSURES,  AND   TOPOGRAPHY. 

Starting  at  the  west  side  of  Goose  Lake  (see  Atlas  Sheet  IV),  the  belt 
extends  in  a  general  westerly  course  for  about  3  miles,  having,  however, 
for  this  distance  tortuous  boundaries  and  a  greatly  varying  width.  It  will 
be  seen  that  the  Kona  dolomite  begins  east  of  Goose  Lake  as  a  broad  belt. 


EXPOSURES   OF    THE   WEWE    SLATE.  257 

The  Wewe  slate,  following  above  the  limestone,  should  appear  both  to  the 
north  and  south  of  this  belt.  On  the  south,  however,  the  formation  is 
exposed  only  in  sees.  13  and  24,  T.  47  N.,  R  26  W.,  and  in  sec.  18,  T.  47  N., 
R.  25  W.,  where,  however,  it  extends  but  a  short  distance  before  it  is 
hidden  by  the  Pleistocene  sands.  The  northern  arm  of  the  slates  shows 
outcrops  in  sees.  11  and  12,  T.  47  N.,  R.  26  W.,  and  very  numerous  out- 
crops west  of  the  Kona  dolomite  in  sees.  5,  6,  7,  and  8,  T.  47  N.,  R.  25  W. 
In  this  area  the  slate  belt  swings  from  an  easterl}-  course  to  a  northerly, 
and  finally  to  a  westerly  one,  and  extends  along  the  southern  side  of  the 
northern  limestone  for  an  unknown  distance  westward.  There  are  no 
exposures  in  this  area,  and  whether  it  dies  out  before  the  slates  and  quartz- 
ites  east  of  Teal  Lake  are  reached  is  uncertain.  The  black  slate  occurring 
at  a  somewhat  persistent  horizon  between  thick  beds  of  quartzites  in  sees. 
32  and  33,  T.  48  N.,  R.  26  W.,  may  be  the  most  westerly  representative  of 
the  northern  belt.  Farther  west  the  formation  was  not  deposited,  since  in 
Wewe  time  the  sea  encroaching  from  the  east  had  not  overridden  that  part 
of  the  district. 

The  slate  being  a  less  resistant  formation  than  the  Kona  dolomite 
below  or  the  Ajibik  quartzite  above,  is,  in  general,  marked  by  valleys,  and 
consequently  the  exposures  are  few  for  much  of  the  area  of  the  Ix'lt.  The 
two  exceptions  to  this  statement  are  the  numerous  prominent  exposures  in 
sees.  5,  6,  7,  and  8,  T.  47  N.,  R.  25  W.,  and  the  exposures  west  of  Goose 
Lake.  The  appearance  of  the  first  set  of  outcrops  is  due  to  the  cutting 
action  of  Carp  River,  which  flows  over  the  ledges  in  a  number  of  rapids 
and  cascades.  Tlie  many  exposures  west  of  Groose  Lake  are  due  to  the 
fact  that  here  was  the  westward  limit  of  the  shore-line  at  this  time,  and 
therefore  the  sediments  deposited  at  this  place  were  coarser  and  were  later 
changed  to  graywacke  and  conglomerate,  and  thus  became  more  resistant. 
Also  they  gain  in  prominence  by  the  presence  of  several  resistant  Archean 
islands,  which  they  surround. 

FOLDING. 

The  broad  l)elt  of  slate  running  north  and  east  from  Groose  Lake,  then 
swinging  to  the  north  and  west,  has  no  especially  interesting  folds,  as  the 
slate    everywhere  dips    away  from  the   Kona  dolomite  below,  and  thus 

MON  xxviii 17 


258  THE   MARQUETTE  IKON-BEAEING  DISTKICT. 

forms  a  great  -westward-plungiug  syiicline,  with  the  eastern  termination  in 
sees.  5,  6,  7,  and  8,  T.  47  N.,  R.  26  W. 

However,  the  folding  in  the  two  areas  east  and  west  of  Goose  Lake  is 
interesting  and  pecuhar.  In  sees.  13  and  24,  T.  47  N.,  R  26  W.,  and  sees. 
18  and  19,  T.  47  N.,  E.  25  W.,  the  shite  has  been  affected  by  both  an  east- 
west  and  a  north-south  folding.  The  north-south  pressure  has  folded  the 
slates  into  a  series  of  minor  rolls,  and  the  same  layer  is  repeated  many 
times.  The  east-west  pressure  has  bowed  the  slates  into  anticlines  and 
synclines.  The  character  of  this  folding  is  particularl}^  well  shown  by  the 
almost  continuous  sections  which  are  observable  along  the  east  parts  of 
sees.  13  and  24  and  along  the  west  side  of  the  southeast  arm  of  Goose 
Lake  (Atlas  Sheets  XXXIV  and  XXXV).  A  major  anticline  causes  the 
little  east-west  folds  to  plunge  to  the  eastward  on  the  east  side  of  the  area, 
and  to  the  westward  on  the  west  side.  The  slate  originally  arose  above 
the  Kona  dolomite  of  Kona  Hills,  but  has  been  removed  from  it  by  erosion. 

West  of  Goose  Lake  (Atlas  Sheet  XXXV)  it  has  been  said  that  the 
slate  covers  a  belt  of  greatly  varying  width,  in  which  are  Archean  islands. 
The  largest  of  these  areas  covers  a  considerable  part  of  the  central  por- 
tion of  sec.  23.  Another  area  is  southwest  of  this,  in  sees.  22  and  23, 
and  two  other  areas  occur  in  sec.  22,  one  at  the  center  of  the  section  and 
the  other  in  the  center  of  the  southwest  quarter.  The  conglomerates, 
slates,  and  quartzites  in  sec.  23  and  in  part  of  sec.  24  have  a  quaquaversal 
an-augement  around  the  oblong  Archean  area  of  sec.  23.  In  other  words, 
the  slates  and  quartzites  constitute  a  part  of  a  northwest-southeast  anticline 
which  plunges  both  to  the  east  and  to  the  west  from  the  center  of  sec.  23. 
The  strikes  about  this  and  the  other  areas  are  northwest-southeast  except 
at  the  ends  of  the  areas ;  the  dips  are  all  to  the  northeast,  showing  that  the 
folds  have  been  pushed  over  from  the  northeast  or  pushed  under  from 
the  southwest.  The  dispersed  distribution  of  the  small  Archean  patches 
and  the  fact  that  basal  conglomerates  cover  a  considerable  area  are  taken  to 
indicate  that  there  are  several  subordinate  folds  in  this  part  of  the  district. 

PETROGEAPHICAL    CHARACTER. 

Macroscopicai. — For  the  arcas  north  and  east  of  Goose  Lake  the  rocks  of 
the  formation  are  slates  and  graywackes.     Southwest  of  Goose  Lake  the 


PETROGRAPIIICAL   CHARACTER   OF    THE    WEWE    SLATE.      259 

lower  part  of  the  formation  becomes  a  (juartzite  or  (juartzite-cong-lomerate. 
Tliese  conglomerates,  rejjosing  as  .tliey  do  upon  the  gneissoicl  granites,  are 
very  largely  composed  of  detritus  derived  from  them  (fig.  11).  Immedi- 
ately adjacent  to  the  Archean  cores  on  the  Wewe  Hills,  in  the  centers  of 
sees.  22  and  23,  the  basal  rocks  are  no  more  than  a  mass  of  granite  blocks, 
cemented  by  fine  debris  of  the  same  material.  An  intermediate  rock  is  a 
coarsely  banded  feldspathic  quartzite  which  in  the  field  very  closely  resem- 
bles the    original    gneissoid   granite    (PI.  X,    fig.   1).     From    these    basal 


of  sec.22,T.47H'.,E.: 


members  there  are  all  gradations  to  graywackes,  novaculites,  and  slates. 
The  slates  in  places  contain  pebbles  or  bowlders  of  many  kinds,  and  thus 
become  slate-conglomerates.  In  the  higlier  part  of  the  formation  the  slates 
and  graywackes  pass  by  interstratifications  and  gradation  into  the  Ajibik 
quartzite. 

The  ordinary  detritus  of  the  formation  diff"ered  from  very  fine  mud 
to  coarse,  sandy  mud,  and  there  were  frequent  alternations  of  the  various 


260  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

phases.  As  the  result  of  the  compacting  and  modification  of  these  beds 
we  have  shale,  slate,  novaculite,  and  graywacke.  The  color  of  these  rocks 
varies  from  red  to  black,  with  various  shades  of  buff  and  brown,  depending 
upon  the  quantity  and  condition  of  the  iron  oxide.  While  many  minor 
alternations  occur,  one  part  of  the  Wewe  formation  may  be  as  a  whole  finer- 
grained  than  another  part.  For  instance,  at  the  exposures  in  the  southeast 
part  of  sec.  13,  T  47  N.,  R.  26  W.,  the  black,  finer-grained  phases  of  the 
slates  occupy  a  higher  horizon  than  the  coarser,  novaculitic-looking  phases. 

As  a  consequence  of  the  folding,  certain  of  the  slates,  and  especially 
those  that  are  fine-grained,  have  had  developed  in  them  a  slaty  cleavage. 
Also,  along  the  zones  of  sharpest  folding  and  of  mashing,  the  rocks  pass  into 
mica-slate,  or  even  into  a  rock  approaching  a  mica-schist.  In  some  cases 
they  approach  knotenschiefer  in  appearance.  As  a  consequence  of  the 
slaty  cleavage  and  schistosity,  in  many  ledges  it  is  difficult  to  determine 
the  true  strikes  and  dips.  However,  the  tnie  bedding  is  usually  indicated 
by  frequent  alternations  of  darker  and  lighter  colored  materials.  Often 
parallel  to  the  bedding  are  cherty-looking  layers,  which  frequently  have  a 
lenticular  character,  the  oval  areas  lying  end  to  end,  with  intervening  slate, 
or  overlapping.  When  followed  closely,  they  are  found  in  places  to  cut 
in  a  minor  way  across  the  bedding.  Often  they  branch  into  two  or  more 
parts,  or  send  out  stringers  into  the  slate.  In  other  cases  the  clierty  or 
quartzose  layers  follow  the  schistosity  rather  than  the  bedding.  Finally, 
the  slates  and  gi-aywackes  are  usually  cut  by  numerous  veins  running  in 
all  directions.  A  close  examination  shows  that  whether  these  cherty  parts 
follow  the  bedding  or  the  schistosity,  or  cut  the  rock  at  random,  they  are 
secondary  infiltrations. 

In  many  places  the  orogenic  movements  have  been  so  powerful  as  to 
shatter  the  rock  through  and  through  (fig.  12,  p.  263,  and  PI.  IX,  fig.  1),  or 
even  to  produce  breccias  (fig.  13,  p.  263,  and  PI.  IX,  fig.  2),  the  fragments 
of  which  are  in  some  places  tolerably  well  rounded  by  dynamic  action, 
so  as  to  form  pseudo-conglomerates.  The  fragments  vary  in  size  from 
minute  ones  to  great  blocks  several  feet  in  diameter.  The  shattered  rocks 
have  been  cemented  by  vein  quartz,  jaspery  quartz,  and  hematite,  some- 
times one  and  sometimes  two  or  three  together  (PI.  IX,  fig.  1). 


PLATE    IX 


Plate  IX.— BRECCIATED  WEWE  SLATE. 

Fig.  1.  Shattered  "Wewe  slate  from  the  NE.J  sec.  21,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXII).  The 
cherty  slate  was  shattered  by  a  first  movement  which  opened  cracks  in  various  directions. 
These  were  filled  with  secondary  quartz.  The  rock  was  again  shattered,  and  the  openings 
thus  formed  were  tilled  by  secondary  quartz,  limonite,  and  hematite.  Besides  this  sh.at- 
tering  there  was  movement  between  the  individual  mineral  particles,  which  granulated  the 
rock.  In  the  interspaces  between  the  particles  chert  and  hematite  were  deposited.  By 
observing  the  figure  closely  innumerable  minute  brilliant  flecks  of  the  latter  may  lie  seen. 
Natural  size. 

Fig.  2.  Brecciated  Wewe  slate  from  the  same  locality  as  flg.  1.  The  erogenic  forces  locally  shattered 
the  rock  into  a  rubble.  The  broken  fragments  were  cemented  by  secondary  quartz,  which 
in  the  figure  occupies  as  much  space  as  the  material  of  the  original  slate.  In  some  places 
the  slate  fragments  themselves  are  broken  along  two  regular  sets  of  planes  inclined  to  each 
other,  which  doubtless  m  each  case  represent  shearing  planes,  both  sets  being  produced 
simultaneously,  just  as  in  the  case  of  building  stone  crushed  under  the  testing  machine. 
That  these  sets  of  planes  do  not  intersect  each  other  at  right  angles  is  doubtless  largely 
explained  by  the  structure  of  the  slate,  which  controlled  to  some  extent  the  direction  of 
fracture  and  thus  prevented  the  breaking  from  always  occurring  along  the  maximum  shear- 
ing planes.  After  the  rock  was  brecciated  and  cemented  as  above  described,  a  later  movement 
again  slightly  shattered  it.  The  cracks  thus  formed,  running  through  slate  fragments  and 
matrix  alike,  are  filled  with  secondary  silica.  As  in  fig.  1,  the  slate  fragments  are  impreg- 
nated with  secondary  hematite.     Natural  size. 


S  GEOLOGICAL  SURVE 


ONOGRAPH     XXVIII,  PL 


ATTKIIKI)  WKWH  SI.ATl': 
IKCCl.VrisI)  WKWK  Sl.ATl 


PETROGRAPHICAL  CHARACTEE   OF   THE   WEWE    SLATE.       263 


At  one  exposure  the  veins  of  hematite  are  later  than  the  white  quartz, 
and  the  jasper  is  Later  than  the  hematite;  and  some  of  the  fragments  have 
around  them,  in  concentric  parallel  zones,  quartz,  liematite,  and  jasper, 
although  even  at  this  place  the  quartz  entirely  fills  some  of  the  spaces. 
Where  the  veins  of  hematite  and  jasper  are  of  considerable  size  they 
can  not  be  discriminated  from  the  hematitic  jasper  of  the  iron-bearing 
formation.  In  places  the  amount  of  hematite  is  so  great  in  the  breccia 
that  the  material  has  been  prospected  for  ore.  The  secondary  charac- 
ter of  the  jasper  and  hematite  in  the  ease  of  these  breccias  can  not  be 
doubted,  and  this  has  a  bearing  upon  the  origin  of  the  jasper  and  hematite 
of  the  iron-bearing  formation.     These  breccias  are  discriminated  from  true 


Fig.  12. — Shattered  slate  cemented  by  rein  quartz,  from 
NE.  i  sec.  21,  T.  47  N.,  E.  26  '\V. 


Fin  13  — Brecciated  slate  c 

i  localitj 


conglomerates  by  the  fact  that  all  of  the  fragments  are  derived  from  the 
slate.  Also,  the  breccias  vary  into  slate  by  imperceptible  stages,  both 
along  the  strike  and  across  it;  and  finally,  while  many  of  the  fragments 
have  been  rounded  so  as  to  resemble  those  produced  by  water  action,  others 
have  an  irregular  character  which  is  not  consonant  with  a  water  origin. 

Microscopical. — The  malu  varieties  of  rock  discriminated  in  thin  section 
are  basal  conglomerates  and  quartzites,  graywackes,  no\-aculites,  slates,  and 
slate-conglomerates. 

The  quartzites  and  conglomerates  differ  from  each  other  only  in  that  the 
conglomerates  have  larjre  frag-ments.     In  other  ^^'ords,  the  cong-lomerates 


2G4  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

have  a  quartzite  base.  Tlie  complex  fragments  found  in  the  conglomer- 
ates in  each  locality  are  predominantly  of  the  particular  rock  immediately 
subjacent,  but  with  these  are  fragments  derived  from  other  sources. 
These  fragments  comprise  white  mashed  granite,  described  on  p.  220 
as  sericitic  quartz-schist;  white  mashed  granite  containing  large  crystals 
of  feldspar;  pink  granite;  gneissoid  granite;  a  peculiar,  very  feldspathic 
pegmatite;  fine-grained  chloritic  schist  or  gneiss;  sericite-schist  or  gneiss; 
quartz  pebbles;  and  other  varieties  of  rock.  All  of  these  pebbles  show 
dynamic  effects.  Many  of  them  have  been  broken  and  cemented  by 
finely  crystalline  and  secondary  quartz.  Microcline  cleavage  is  also 
developed  in  the  potash-feldspars.  The  quartz  grains  uniformly  show 
undulatory  extinction;  many  of  them  are  distinctly  fractured,  and  these 
fractures  are  in  some  grains  according  to  a  rectangular  system.  The  quartz 
pelibles  are  found  to  consist  of  intricately  interlocking  or  closely  fitting-, 
roundish  granules  of  quartz,  but  in  no  case  do  any  of  these  distinctly  show 
a  fragmental  character,  and  they  are  believed  to  have  been  derived  from 
granite  or  from  vein  quartz.  The  chloritic  and  sericitic  schists  and  gneisses 
have  in  some  cases,  at  first  glance,  a  fragmental  appearance,  but  the  more 
closely  they  are  studied  the  more  do  they  appear  to  be  completely  crys- 
talline rocks.  To  describe  the  fragments  of  the  conglomerates  in  detail 
would  be  a  repetition  of  the  description  of  the  rocks  of  the  Basement 
Complex. 

The  quartzite  or  quartzite  background  of  the  conglomerates  contains 
an  abundant,  very  finely  crystalline  groundmass  of  sericite,  kaolin,  and 
quartz,  with  a  httle  chlorite,  and  is  often  impregnated  with  iron  oxide.  In 
this  groundmass  are  simple  and  complex  grains  of  quartz  and  less  abundant 
grains  of  the  various  feldspars,  and  as  the  rocks  become  coarser-grained 
these  pass  into  the  complex  areas  composed  of  quartz  and  feldspar.  The 
groundmass  of  these  rocks  and  that  of  the  fragments  contained  in  them 
are  the  same,  and  the  structure  is  somewhat  similar  to  the  mashed  gneissoid 
granites  or  sericitic  quartz-schists  of  the  Archean.  (See  p.  220.)  Also, 
many  of  the  simple  and  complex  quartz  grains  have  a  granitic  appearance, 
having  been  but  little  waterworn;  but  some  of  the  grains  show  a  distinct 
waterworn    character,  and    they   are   rarel}^  enlarged.     In    the  quartzites 


PETROGRAPHICAL   CHARAOTEE   OF   THE    WEWE    SLATE.       265 

there  are  usually  alteruating  layers  of  finer  and  coarser  material,  while  in 
the  gneissoid  granite  the  lamiure  are  all  alike.  The  reconiposed  rocks 
contain  a  uuxch  larger  ainovmt  of  secondary  iron  oxide  than  the  schistose 
granites,  and  in  the  folding  they  have  been  more  broken,  thus  producing 
distinct  cracks  and  minute  cavities,  which  have  become  filled  with  finely 
crystalline,  secondary  quartz.  The  thin  section  thus  enables  us  to  discrim- 
inate the  recomposed  rocks  from  the  original,  mashed,  gneissoid  granite. 
Many  of  the  feldspars  of  the  quartzites  are  sufficiently  fresh  to  show 
distinctly  their  twinning,  but  all  of  them  are  more  or  less  kaolinized. 
Frequently  the  feldspars  have  largely  or  wholly  decomposed  into  a  com- 
plex, interlocking,  finely  crystalline  mass  of  sericite  and  quartz,  chlorite 
and  quartz,  biotite  and  quartz,  or  combinations  of  these.  In  an  interme- 
diate stage  there  is  with  these  residual  feldspar.  Often  during  or  subse- 
quent to  this  decomposition  much  secondary  iron  oxide  has  entered,  and 
in  these  cases  we  have  in  ])lace  of  the  feldspar  grains  an  interlocking  mass 
of  iron  oxides,  quartz,  and  sericite. 

By  a  change  in  the  cliaracter  of  the  grouudmass  and  a  decrease  in 
the  size  of  the  fragmental  grains  the  quartzites  pass  into  the  graywackes. 
The  same  constituents  are  pi-esent  in  the  groundmass  of  the  latter  as  in 
that  of  the  quartzites,  but  chlorite  is  abundant,  and  intermingled  witli  the 
groundmass  are  very  small  fragmental  grains  of  quartz  and  feldspar,  and 
frequently  a  large  amount  of  secondary  iron  oxides,  chiefly  hematite  and 
magnetite,  often  with  distinct  crystal  outlines.  In  some  cases  a  film  of 
oxide  of  iron  is  around  each  of  the  individual  grains  of  quartz.  In  the 
background,  as  the  rocks  become  mashed,  the  leaflets  of  sericite  and 
biotite  have  a  tendency  to  a  parallel  arrangement.  The  coarser  quartz 
grains  uniformly  show  undulatory  extinction  or  fracturing,  frequently 
according  to  a  rectangular  system.  The  smaller  quartz  grains,  where 
buried  in  an  abundant  matrix,  and  therefore  not  pressed  against  one 
another,  are  freer  from  these  pressure  eff"ects,  and  in  some  of  them  pressure 
efi'ects  are  not  seen  at  all.  The  quartz  grains  are  much  more  frequently 
enlarged  than  in  the  basal  quartzites.  The  feldspars,  while  often  rather 
fresh,  show  all  phases  of  decomposition  to  sericite,  biotite,  or  chlorite,  and 
to  quartz  with  iron  oxide  impregnation,  described  in  the  conglomerates. 


26fi  THE   MARQUETTE   IR0:N^  BEAEI:N^G   DISTRICT. 

By  a  decrease  in  the  size  of  the  coarser  fragmental  grains  the  gray- 
wackes  pass  into  the  shxtes.  In  these  slates  the  decomposition  of  the  feld- 
spar grains,  because  of  their  smaller  size,  is  much  more  common.  On 
account  of  the  more  plastic  character  of  the  slates,  there  is  frequently 
developed  in  them  a  slaty  cleavage  or  schistose  structure,  the  ordinary 
cleaved  slates  passing  into  mica-slates,  and  occasionally  into  sericite-schists. 
In  passing  from  the  less  mashed  to  the  most  mashed  phases  there  is  an 
increase  in  the  regularity  of  the  arrangement  of  the  sericite  leaflets  in  a 
uniform  direction.  As  in  the  graywackes,  the  rocks  are  usually  impreg- 
nated to  a  greater  or  less  degree  by  iron  oxide,  and  frequently  very 
heavily  so.  The  iron  oxide  includes  limonite,  hematite,  and  magnetite,  the 
two  latter  often  being  in  large  part  in  well-defined  crystals,  and  sometimes 
in  veins.  Frequently  the  slates  consist  of  layers  of  differing  degrees  of 
coarseness,  sometimes  a  half  dozen  fine  and  coarse  laminiT?  lieing  observed 
in  a  single  section.  In  these  cases  the  .coarser  bands  are  more  likely  to  be 
heavily  iron-stained,  the  accommodations  apparently  having  formed  cracks 
and  crevices  to  a  greater  degree  than  in  the  interlaminated  finer  and  more 
plastic  layers. 

The  slates  and  graywackes  at  times  become  conglomeratic,  so  that 
whole  exposures  are  slate-conglomerate,  or  else  the  conglomerate  layers 
are  interstratified  with  the  ordinary  slate  and  graywacke.  These  slate- 
conglomerates  bear  exactly  the  same  relation  to  the  slates  and  graywackes 
that  the  basal  conglomerate  does  to  the  quartzite — that  is,  there  are  pebbles 
and  bowlders  in  the  slate  or  graywacke  background.  These  pebbles  and 
bowlders  are  identical  in  lithological  character  with  those  of  the  basal 
conglomerate,  but,  upon  the  whole,  they  are  better  rounded.  In  certain 
places  the  later  movements  which  these  slate-conglomerates  have  under- 
gone have  brecciated  them,  so  that  with  the  water-rounded  fragments  are 
apparent  pebbles  of  slate  and  graywacke.  A  close  examination  of  these 
in  the  field,  and  especially  in  thin  section,  shows  that  they  have  angular 
forms  and  are  clearly  produced  by  the  brecciation  of  the  rock  itself.  This 
occurrence  was  particularl}^  confusing,  as  the  rock  is  an  undoubted  con- 
glomerate, and  yet  a  conglomerate  which  is  partly  autoclastic. 


PETROGRAPHICAL  CHARACTER  OF  THE  WEWE   SLATE.       267 

The  novaeulites  are  similar  to  the  slates  and  graywackes,  except  that 
they  are  largely  composed  of  very  small,  rounded  grains  of  quartz  and 
fewer  of  feldspar,  of  a  somewhat  uniform  size,  with  a  very  sparse  matrix 
of  sericite,  kaolin,  and  ferrite.  In  the  field  these  uniformly  granular 
fine-grained  rocks  were  not  discriminated  from  the  secondary  chert  veins 
and  layers,  but  in  thin  section  they  are  wholly  different,  having  the  grains 
distinctly  rounded  and  not  closely  fitting,  and  having  the  sparse  matrix 
above  described.  The  cherty  material,  upon  the  other  hand,  consists  of 
finely  granular,  perfectly  fitting  quartz,  free  from  the  clayey  constituents, 
and  where  iron  oxide  is  present,  it  is  usually  concentrated  to  a  greater  or  less 
degree  in  bunches  or  layers,  rather  than  uniformly  disseminated  between 
the  particles,  as  in  the  novaeulites. 

The  quartzites,  interstratified  with  the  higher  members  of  the  formation, 
are  in  all  respects  like  the  Ajibik  quartzites  hereafter  described. 

The  graywackes,  slates,  and  novaeulites,  as  has  been  indicated  (pp. 
260-263),  have  frequently  had  developed  in  them  a  slaty  cleavage  or  schis- 
tose structure,  and  have  been  broken  through  and  through  by  dynamic 
action.  As  a  result  of  this,  crevices  and  cracks  have  formed  parallel  to  the 
bedding,  parallel  to  the  secondary  structures  which  intersect  the  bedding-, 
in  directions  independent  of  either  of  these,  and  between  the  individual 
particles  of  the  rocks  themselves.  These  cracks  and  crevices  have  been 
largely  cemented  by  finely  crystalline,  perfectly  fitting  grains  of  quartz, 
which  in  hand  specimen  has  a  cherty  appearance.  In  other  places  coarsely 
crystalline  vein  quartz  has  entered.  During  the  readjustments  cracks  have 
largely  formed  parallel  to  the  bedding,  and  secondary  cherty  layers  have 
formed  in  this  direction.  In  hand  specimen,  in  some  cases,  they  might  be 
regarded  as  truly  interbedded  layers,  but  when  examined  in  thin  section 
the  secondary  character  of  this  vein  chert  is  undoubted.  This  is  .shown 
hj  the  fact  that  within  it  are  fragments  of  the  original  slate,  and  also  from 
these  apparent  quartz  bands  smaller  veins  of  cherty  quartz  ramify,  cutting 
the  slate  in  all  directions.  Moreover,  as  examined  in  hand  specimen,  these 
cherty -looking  layers  often  have  a  lenticular  character,  the  oval  layers  lying 
end  to  end  or  overlapping.     In  one  case,  where  the  secondary  coarsely 


268  THE   MARQUETTE   lEON  BEAEI^'G  DISTRICT. 

crystalline  quartz  is  present,  we  have  the  clearest  evidence  of  two  separate 
movements,  since  tlie  crystalline  quartz  shows  undulatory  extinction  and 
fracturing,  sometimes  according  to  the  rectangular  system.  When  the  rocks 
have  not  only  been  broken  but  interior  movement  has  occurred  tln*oughout 
their  mass,  the  entering  quartz  has  taken  advantage  of  all  of  these  spaces, 
thus  recementing  the  rock  (PI.  IX).  In  some  cases,  in  the  background 
of  the  slate,  this  secondary  quartz  seems  to  be  almost  as  plentiful  as  the 
original  material,  occurring  in  little  oval,  complex  areas,  in  minute  stringers 
ramifying  through  the  coarser  veins,  and  in  single  individuals  between  the 
fragmental  constituents.  While  the  cementing  of  the  shattered  rock  has 
been  mainly  a  process  of  silicification,  it  has  been  indicated  that  a  large 
amount  of  oxides  of  iron  has  also  entered.  In  some  instances  these  oxides 
of  iron  are  the  main  constituents  of  the  cementing  material,  but  usually 
they  are  subordinate  to  the  secondary  quartz.  Where  both  are  present 
they  are  not  uniformly  intermingled  but  are  more  or  less  concentrated  in 
irregular  areas  or  bands.  As  another  result  of  the  shattering  of  the  rocks, 
the  layers  have  been  faulted  in  a  minor  degree. 

In  an  extreme  stage  of  fracturing  the  rocks  pass  into  genuine  auto- 
clastic  rocks  or  reibungsbreccias.  In  some  of  these  the  angular  fragments 
of  the  slate  are  separated  by  reticulating  veins  of  coarsely  crystalline  quartz, 
finely  crystalline  chert  or  jasper,  and  hematite  (fig.  12,  p.  263,  and  PI.  IX, 
fig.  1).  In  other  cases  the  secondary  material  makes  a  continuous  ramifying 
mass,  within  which  are  complex  bands  and  fragments  of  the  original  slate 
or  the  separated  individual  grains  (fig.  13,  p.  263,  and  PI.  IX,  fig.  2).  The 
extreme  stages  of  brecciation  more  usually  occur  in  the  graywackes,  the 
finer-grained  phases  being  more  plastic  and  yielding  more  readily  to  pres- 
sure, and  thus  developing  into  slates  and  schists.  In  some  of  the  coarser 
graywackes  the  relief  appears  to  have  occurred  along  zones  of  irregular  width, 
and  here  the  grains  have  been  loosened  from  one  anothei*.  These  zones  are 
indicated  by  abundant  iron  impregnation,  and  are  sharply  separated  from 
the  layers  at  the  sides,  which  have  not  suffered  so  much  from  movements. 

No  better  case  is  known  to  me  of  the  phenomena  characteristic  of  the 
zone  of  combined  fracture  and  flowage^  than  is  exhibited  by  the  Wewe 

'Principles  of  Nortli  American  pre-Cambrian  geology,  by  C.  E.  Van  Hise:  .Sixteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  601-603,  654-656. 


RELATIONS   OF  THE   WEWE   SLATE.  2G9 

slate.  The  softer  layers  were  at  one  time  certainly  in  the  zone  of  flowage, 
and  under  these  conditions  cleavage  developed  in  the  normal  planes.  Later 
some  of  these  slates  passed  into  the  zone  of  fracture  for  them,  and  a  fissility 
secondary  to  the  cleavage  formed  along  shearing  planes.  The  stronger 
rocks  exhibit  beautifully  all  the  phenomena  characteristic  of  deformed  rocks 
in  the  zone  of  fracture. 

RELATIONS   TO   ADJACENT   FORMATIONS. 

In  all  the  exposures  north  and  east  of  Goose  Lake  the  inferior  forma- 
tion is  the  Kona  dolomite.  This  dolomite  generally  pasess  upward  into 
the  slate  by  a  gradual  disappearance  of  the  calcareous  material.  The 
lower  and  central  portions  of  the  formation  are  pure  slates  or  graywackes. 
In  some  cases  the  basal  horizon  of  the  slate,  or  the  upper  horizon  of  the 
dolomite,  is  a  chert-breccia,  undoubtedly  of  dynamic  origin,  but  resem- 
bling a  conglomerate  (PI.  VII,  fig.  2).  Such  breccias  may  be  well  seen  at 
the  contact  between  the  slate  and  the  Kona  dolomite  in  the  southeast  part 
of  sec.  13,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXIV).  The  slate  at  this 
particular  locality  becomes  coarser-grained  in  passing  toward  the  base, 
grading  first  into  a  novaculite,  then  into  a  graywacke,  and  then  into  a 
brecciated,  cherty  quartzite.  The  chert-breccia  at  the  contact  a^jpears  to 
have  been  produced  from  secondary  belts  of  chert,  which  liave  appeared 
within,  and  perhaps  have  replaced  calcareous  layers  in  the  quartzite.  When 
the  rock  was  folded  the  brittle  cherty  layers  were  broken  into  fragments. 
This  pseudo-conglomerate  might  possibly  be  taken  by  a  careless  observer 
as  evidence  of  a  physical  break  between  the  Kona  dolomite  and  the  Wewe 
slate. 

Southwest  of  Goose  Lake  (Atlas  Sheet  XXXV),  below  the  slate,  are 
islands  of  Archean  rocks.  It  has  been  said  that  here  conglomerates  have 
an  extensive  development  adjacent  to  the  Archean  cores.  In  sec.  23 
T.  47  N.,  R.  26  W.,  and  near  the  central  part  of  sec.  22,  T.  47  N.,  R.  26  W., 
contacts  are  exposed  between  the  Archean  and  the  conglomerates,  but 
no  contacts  were  seen  adjacent  to  the  area  in  the  southern  part  of  the 
SW.  ^  sec.  22,  althoug-h  large  exposures  of  conglomerate  were  found  near 
those  of  the  granite. 


270  THE  MARQUETTE   IRON-BEARING   DISTRICT. 

At  tlie  west,  soutliwest,  and  south  of  the  western  bluff  of  the  Archean 
of  sec.  23  the  basal  conglomerate  is  well  exposed  in  direct  contact  with 
the  underlying  crystalline  I'ocks.  At  the  west  foot  of  the  hill  is  a  solid 
ledge  of  the  white,  mashed,  schistose  Archean  granite.  It  is  in  contact 
with  and  mantled  on  both  sides  by  the  conglomerate,  which  is  mainly 
composed  of  material  exactly  like  the  original  rock.  The  fragments  and 
matrix  of  the  conglomerate  so  closely  resemble  the  granite  that  its  recom- 
posed  character  scarcely  shows — so  intensely  mashed  is  the  rock — except 
upon  the  weathered  surface,  where  may  be  seen  rounded,  protmding  frag- 
ments of  the  granite,  varying  in  size  from  small  ones  to  great  blocks.  In 
passing  eastward  along  the  south  slope  of  the  bluff  the  white  granite  of  the 
Basement  Complex  takes  on  a  different  character,  here  being  less  altered, 
and  containing  pink  augen  of  the  original  feldspar.  In  the  field,  as  well 
as  from  microscopical  study,  it  is  plain  that  it  is  a  mashed  granite.  Adja- 
cent to  this  granite  the  conglomerate  contains  predominant  peljbles  of  a 
corresponding  kind.  As  further  evidence  of  this  unconformity,  the  white 
and  pink  mashed  granite  is  cut  through  and  through  by  veins  of  red  granite, 
which  are  nowhere  observed  to  cut  the  conglomerate. 

The  contact  is  again  seen  in  the  valley  to  the  south,  where  the  recom- 
posed  rock  on  a  little  ridge  projects  east  as  an  arm  into  the  area  of  the 
Archean.  Here  the  conglomerate  has  not  been  so  much  mashed.  The 
sparse  clayey  matrix  is  stuccoed  with  fragments  of  the  red  granite  and  the 
white,  kaolinic  quartz-schist  (mashed  granite)  from  the  Archean.  Many  of 
these  macroscopically  closely  resemble  chert.  The  conglomerate  appears 
also  to  contain  fragments  derived  from  a  slate  or  g-raywacke.  The  upper 
part  of  the  conglomerate  contains,  besides  pebbles  of  granite  and  gneiss, 
many  pebbles  of  white  quartz,  some  of  which  macroscopically  appear  to  be 
derived  from  a  quartzite;  also  rare  pebbles  of  chert  and  jasper,  and  many  ot 
a  slaty  or  schistose  rock.  The  matrix,  usually  white  or  pale-green,  is  ordi- 
narily slate,  graywacke,  or  quartzite,  but  oftentimes  it  is  so  fine-grained  as 
to  have  a  novaculitic  appearance. 

In  sec.  22  also  the  actual  contact  between  the  gneissoid  granite  Archean 
axis  and  the  conglomerate  is  seen.  Here  are  magnificent  exposures  of  great 
bowlder  conglomerates,  the  granitic  fragments  of  which,  of  varying-  sizes. 


EELATIONS  OF  THE   WE  WE   SLATE.  271 

are  close  together,  so  that  there  is  but  a  sparse  matrix.  In  some  cases  this 
recomposed  rock  so  closely  resembles  granite  that  it  is  with  difficulty  that 
its  true  character  is  certainly  determined.  In  cases  of  doubt,  however,  the 
weathered  surface  enables  one  to  distinguish  between  the  original  and  the 
recomposed  rocks,  as  here  the  granite  fi'agments  protrude  from  the  face  of 
the  conglomerate.  This  granite  stucco  varies  up  into  slate-conglomerate 
of  differing  degrees  of  coarseness,  and  finally  slate  is  found  containing  only 
small  pebbles  of  granite.  In  some  cases,  in  the  finer  conglomerate,  the 
particles  of  the  recomposed  rock  are  almost  wholly  single  grains  of  quartz 
and  feldspar,  or  are  small  complex  grains  of  granite.  These  show  a  lami- 
nated arrangement,  and  in  the  hand  specimen  the  recomposed  rock  (PI.  X, 
fig.  1)  very  closely  resembles  the  original  gneissoid  granite.  As  higher 
horizons  are  reached  the  slate  and  slate-conglomerate  pass  up  into  feld- 
spathic  quartzites,  novaculites,  slates,  and  graywackes  of  various  hues, 
similar  to  those  in  sec.  23,  and  finally  above  them  appear  the  pure  vitreous 
quartzite  of  the  Ajibik  formation.  In  a  number  of  places  the  actual 
gradations  are  seen,  and  the  formation  line  between  the  two  is  somewliat 
arbitrarily  di-awn 

THICKNESS. 

On  account  of  the  complicated  character  of  the  folding  of  the  slates, 
graywackes,  and  conglomerates  southwest  of  Goose  Lake,  it  is  impossible 
to  give  even  an  approximate  estimate  of  the  thickness  of  the  formation. 
Here,  adjacent  to  the  shore-line,  it  is  natural  to  expect  it  to  have  a  greater 
thickness  than  to  the  eastward,  and  it  is  believed  that  the  thickness  is  very 
considerable.  In  sec.  22  (Atlas  Sheet  XXXV)  there  are  almost  continuous 
exposures  of  the  slate,  all  apparently  north  of  the  northernmost  anticline, 
and  all  dipping  50°  to  60°  the  same  way  for  a  breadth  of  1,300  feet.  This 
would  correspond  to  a  thickness  of  about  1,060  feet.  To  this  would  neces- 
sarily be  added  the  thickness  of  the  conglomerate,  which  should  appear 
below  the  slate  and  graywacke.  This  area  is,  however,  near  the  northern 
end  of  a  northwest-southeast  anticlinal  dome,  and  the  slate  shows  much 
brecciation,  well-developed  slaty  cleavage,  and,  when  studied  closely, 
numerous  minor  rolls;  so  it  Is  entirely  possible  that  the  real  thickness  of 


272  THE  MARQUETTE   IRON  BEARING  DISTRICT. 

the  formation  is  not  more  than  a  third  of  the  above  estimate.  In  the  east 
part  of  sees.  13  and  24  (Atlas  Sheets  XXXIV,  XXXV),  where  there  are 
numerous  rolls  of  the  slate  and  quartzite,  a  close  examination  showed  that 
there  is  probably  exposed  a  thickness  of  slates  not  exceeding  100  feet.  At 
the  numerous  exposures  in  sees.  5,  6,  7,  and  8,  T.  47  N.,  R.  25  W.  (Atlas 
Sheet  XXXVII),  there  is  little  opportunity  for  an  accurate  estimate  of  the 
thickness.  The  calculated  thickness  west  of  Goose  Lake  is  probably  a 
maximum,  and  that  east  of  Goose  Lake  may  be  considered  a  minimum. 
The  average  thickness  of  the  formation  may  perhaps  be  as  much  as  500  feet. 

INTERESTING    LOCALITIES. 

Makwa  Hills. — Begiimiiig  at  the  north  and  west,  the  first  locality  in  which 
the  AVewe  slate  may  be  present  is  in  the  quai-tzite  range  north  and  east  of 
Teal  Lake  (Atlas  Sheet  XXX).  In  the  center  of  the  quartzite  formation 
is  a  belt  of  slate,  which  is  probably  equivalent  to  some  part  of  the  Wewe 
slate  to  the  east,  but  with  what  part  it  should  be  equated  it  is  impossible 
to  say.  In  passing  from  this  place  toward  the  east  there  are  no  exposures 
for  several  miles.  The  belt  is,  however,  supposed  to  persist,  but  to  lack 
exposure  because  of  its  feeble  resistance. 

Eastern  area. — At  the  castem  eiid  of  the  great  westward-plunging  syncline 
occur  numerous  outcrops  of  this  formation  (Atlas  Sheets  XXXVI  and 
XXXVII).  The  exposures  here  are  for  the  most  part  found  along  the 
small  streams  and  on  the  Carp  River,  the  cutting  action  having  been  suffi- 
cient to  remove  the  overlying  drift.  The  rocks  have  a  slaty  cleavage,  but  the 
bedding  is  usually  determinable.  In  the  southeast  part  of  sec.  31  the  rocks 
strike  east  and  west  and  dip  south.  In  the  east  part  of  sec.  5  the  strikes  are 
mostly  north  and  south,  and  in  sec.  6  they  are  again  approximately  east 
and  west,  thus  following  the  folding.  In  the  SE  ^  sec.  6,  along  and  near 
the  Carp  River,  are  the  best  exposures.  The  slate  south  of  the  river  is  here 
overlain,  with  a  slight  discordance,  by  the  Ajibik  quartzite.  The  character 
of  this  break' will  be  discussed  later  in  connection  with  that  formation. 
Lithologically  the  slates  vary  from  very  fine  grained  argillaceous  rocks  to 
coarse  graywackes.     In  color  the  nonferruginous  phases  grade  from  gray 


INTERESTING    LOCALITIES   OF   THE   WEVVE    SLATE.  273 

or  greenish-gray  to  black.  Many  of  them  are,  however,  heavily  ferrugi- 
nous, and  these  are  dark-red,  bright-red,  or  brown.  In  some  cases  the 
amount  of  hematite  is  so  considerable  that  test-pitting  has  been  done.  In 
many  of  the  coarser-grained  black  slates  are  seen  numerous  fragmental 
particles  of  mica,  the  leaflets  being  generally  arranged  parallel  to  the 
bedding.  Certain  of  the  black  slates  have  a  carbonaceous  appearance, 
and  in  these  is  seen  very  abundant  iron  sulphide  in  innumerable  small 
crystals. 

West  of  the  exposures  in  this  vicinity  none  are  found  along  the  Wewe 
belt  for  2  miles.  In  sees.  11  and  12,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXIV),  however,  occur  typical  exposures  of  the  slate,  sepai'ating  the 
Kona  dolomite  below  and  the  Ajibik  quartzite  above,  and  thus  showing 
that  the  belt  is  persistent. 

Goose  Lake. — Tlic  uoxt  cxposiircs  of  tlio  fonuation  are  those  about  Groose 
Lake.  The  first  locality  which  pi-esents  exceptional  interest  is  in  the  NE.  J 
sec.  24  and  the  SE.  i  sec.  13,  T.  47  N.,  R.  26  W.  (Atlas  Sheets  XXXIV  and 
XXXV).  At-this  place  there  are  continuous  exposures  of  the  slate  from 
the  Kona  dolomite  below  to  the  Ajibik  quartzite  above.  This  exposure 
has  a  large  number  of  minor  rolls,  with  strikes  approximately  east-west 
or  south  of  east,  and  with  axes  plunging  to  the  east  or  south  of  east  at 
angles  from  10°  to  20°. 

At  the  bottom  of  the  formation,  or  at  the  top  of  the  Kona  dolomite,  is  a 
chert  and  novaculite  breccia,  many  of  the  chert  fragments  being  rather  well 
rounded  by  movement.  It  was  at  first  thought  that  this  was  a  conglomerate, 
and  that  possibly  there  Avas  a  break  between  the  Kona  dolomite  and  the 
Wewe  slate  (PI.  VII,  fig.  2).  This  breccia  grades  up  into  interlaminated 
fine-grained  gray  and  felsitic-looking  red  novaculites  and  graywackes,  these 
into  red  and  black  slates,  these  into  black  slate,  and  this,  by  numerous  inter- 
stratifications,  into  the  Ajibik  quartzite.  An  estimate  of  the  thickness  of 
the  various  beds  between  the  Kona  dolomite  and  the  Ajibik  quartzite  is 
as  follows:  Novaculite  and  graywacke,  30  to  50  feet;  red  and  black  slates, 
25  feet;  black  slate,  10  feet;  interstratifications  of  slate  and  quartzite,  15 
feet;  thus  making  a  maximum  thickness  of  100  feet.     On  account  of  the 

MON  XXTIII 18 


274  THE  MAEQUETTE   IRON-BEAEING  DISTEIGT. 

complicated  folding  of  the  beds,  it  was  difficult  to  make  the  determinations 
at  all  accm'ate,  as  the  same  layer  is  reproduced  in  exposure  several  times. 

Beginning  at  the  north  and  at  the  bottom  of  the  exposures,  the  lime- 
stone plunges  under  the  novaculite  with  a  dip  of  50°  to  the  south.  In 
passing  toward  the  south,  while  the  same  layer,  as  has  been  said,  may  be 
repeated  by  the  folding,  on  the  whole  higher  and  higher  members  appear. 
The  whole  is  a  part  of  an  east-of-south-dipping  stratum,  which,  however,  is 
itself  bent  into  a  number  of  secondary  folds.  If  one  sights  along  the  axes 
of  the  folds  toward  the  west,  he  sees  that  the  slate  will  rise  above  the  Kona 
dolomite,  the  same  as  it  does  where  the  two  are  in  contact  to  the  north. 

The  folding  of  the  Wewe  slate  and  Kona  dolomite  in  this  ^'icinity  is 
almost  an  ideal  case,  illustrating  the  types  of  folds  and  observations  to 
be  made  in  districts  of  complex  folding.  The  use  of  topography,  tops  of 
anticlines,  bottoms  of  synclines,  and  the  pitch  of  one  set  of  folds  to  obtain 
the  dips  of  the  cross  set  are  all  shown.^ 

The  movements  of  the  Wewe  slate  have  produced  a  cleavage — in  cer- 
tain places  something  of  a  schistose  stnicture,  and  in  the  novaculitic  layers, 
as  has  been  said,  a  breccia.  The  pseudo-conglomerate  at  the  bottom  of  the 
formation  was  at  first  supposed  to  be  a  true  conglomerate,  and  was  thought 
to  mark  a  possible  unconformable  break  between  the  slates  and  the  dolo- 
mite (PL  VII,  fig.  2).  The  strata  were,  however,  found  to  be  strictly  con- 
formable, and  the  chert  and  novaculite  fragments  dynamic  rather  than 
waterworn  pebbles.  Traced  along  the  strike,  the  autoclastic  rock  gradually 
passes  into  the  continuous  layers.  It  appears  probable  that  the  fine  sand  at 
the  base  was  interstratified  with  calcareous  layers,  that  the  carbonate  was 
leached  out  and  replaced  by  chert,  and  that  when  folded  the  rock  was 
broken.  As  further  evidence  that  this  rock  is  a  pseudo-conglomerate, 
the  novaculites  higher  in  the  formation  at  many  places  have  been  broken 
through  and  through  in  a  similar  manner  and  changed  into  breccias, 
the  fragments  of  which  are  cemented  by  secondary  cherty  quartz.  In 
the  more  argillaceous  rocks  a  slaty  cleavage  has  everywhere  developed, 
which   sometimes   passes  into  partial  schistosity.     These  phenomena  are 

'  Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise :  Sixteenth  Ann.  Rept. 
U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  626-631. 


INTEEESTING  LOCALITIES   OF   THE  WEWE   SLATE.  275 

particularly  marked  in  those  layers  iiiterstratified  with  the  Ajibik  quartzite, 
the  cleavage  and  schistosity  stopping  abruptly  at  the  quartzite  beds.  Many 
of  the  brecciated  slates  and  novaculites  are  heavily  ferruginous,  the  iron 
l)eing  largely  concentrated  in  veins.  The  extensive  dynamic  phenomena 
showj;  by  the  formation  about  Goose  Lake,  and  the  complicated  folding 
of  the  slates,  would  seem  to  indicate  that  in  the  general  folding  of  the  dis- 
trict the  major  accommodations  and  readjustments  necessary  have  occurred 
mainly  in  the  weak  slate  rather  than  in  the  strong  Kona  dolomite  below 
or  the  Ajibik  quartzite  above. 

An  examination  of  the  thin  sections  confirms  the  field  observations. 
While  the  fragmental  character  of  the  coarse  slates  is  perfectly  d'istinct, 
the  numerous  roundish  fragmental  grains  being  very  apparent,  each  indi- 
vidual shows  nndulatory  extinction  or  fracturing,  as  a  result  of  the  great 
deformation.  In  proportion  as  the  rocks  are  fine-grained,  recrystallization 
has  gone  on,  some  of  them  becoming  sericite-schists,  which  at  first  sight 
might  not  be  thought  to  be  fragmental.  There  is  abundant  evidence  of 
extensive  deposition  of  silica  and  iron  oxide,  these  materials  being  present 
both  as  veins  and  in  the  background.  No  complete  description  of  the  slides 
will  be  given  here,  as  they  are  similar  to  the  other  slides  of  the  formation 
in  the  general  area  of  Goose  Lake,  and  a  description  of  these  is  given  at  a 
previous  place  (pp.  265-269). 

wewe  Hills. — Wcst  of  Goose  Lake  (Atlas  Sheets  XXXII  and  XXXV) 
occur  the  most  extensive  exposures  of  the  formation.  The  positions  of 
ledges  actually  observed  are  given  on  the  detailed  sheets,  but  tliese  by  no 
means  represent  all  of  the  exposures,  but  merely  those  which  have  been 
examined. 

Along  and  adjacent  to  the  shore  of  the  southwest  arm  of  Goose  Lake 
there  are  almost  continuous  exposures  of  mica-slate,  graywacke,  and 
novaculite,  from  the  old  charcoal  kilns  nearly  to  the  southeast  end  of  the 
lake.  The  rocks  here  are  slaty  or  schistose,  brecciated  and  cherty — in  short, 
in  most  respects  are  similar  to  those  in  sees.  13  and  24  above  described. 

The  most  interesting  exposures  of  the  formation  are  those  on  the 
Wewe  Hills  about  the  Archean  islands  in  sees.  22  and  23.  Here  are  found 
at  a  number  of  localities  great  basal  conglomerates,  which  pass  up  into  the 


276  THE   MARQUETTE  lEONBEARING  DISTRICT. 

■slate  and  graywacke,  with  occasional  interstratified  conglomeratic  phases. 
The  first  and  largest  of  these  islands  is  that  near  the  center  of  sec.  23. 
This  is  an  oval  area,  with  its  greater  diameter  in  a  northwest-southeast 
'direction.  It  is  almost  entirely  surrounded  b}^  abundant  exposures  of  the 
Wewe  formation,  but  those  of  the  gi'eatest  interest  are  along  the  southwest 
border.  Just  north  of  the  quarter  line  is  a  great  basal  conglomerate,  in 
contact  with  and  resting  upon  a  white  schistose  granite  microscopically 
resembling  quartz-schist.  The  fragments  and  matrix  of  the  conglomerate 
are  almost  wholly  from  the  granite,  and  the  rock  is  so  firmly  cemented  that 
fresh  fractures  break  across  the  matrix  and  pebbles,  so  that  its  recomposed 
character  scarcely  shows,  except  upon  the  Aveathered  surface.  Where 
weathered,  there  may  be  seen  well-rounded  fragments  of  the  granite,  from 
those  of  small  size  to  great  bowlders,  protruding  from  the  matrix.  A  thin 
belt  of  this  conglomerate  mantles  the  granite  for  some  distance  along  the 
Ijrow  of  the  bluff,  and  here,  besides  the  white  granite,  are  also  found 
fragments  of  granite  bearing  pink  feldspar  crystals  and  fragments  of  red 
granite.  The  matrix  of  the  conglomerate  is  an  ordinary  quartzite.  In  the 
core  area  the  pink  feldspar-bearing  granite  was  found  associated  with  the 
white  granite,  and  the  red  granite  cuts  both. 

A  short  distance  south  of  the  quarter  line  of  the  section,  on  the  south- 
west slope  of  the  bluff,  great  exposures  of  conglomerate  are  again  found  in 
contact  with  and  immediately  adjacent  to  the  granite.  Here,  on  close 
examination,  it  is  perfectly  clear  where  the  schistose  granite  ends  and  the 
schistose  conglomerate  begins.  The  latter  vauies  from  a  coarse  conglom- 
erate, bearing  abundant  granitic  debris,  to  a  fine-grained  conglomerate  in 
which  the  fragmental  particles  are  mainly  single  quartz  and  feldspar  grains. 
This  conglomerate  in  its  upper  part  is  interlaminated  with  slate  and  gray- 
wacke  phases.  As  a  consequence  of  the  intense  folding  to  which  the  rock 
lias  been  subjected,  it  has  become  brecciated,  so  that  with  the  genuine 
■  detrital  fragments  derived  from  the  granite  are  also  angular  to  subangular 
fragments  of  the  slate  and  graywacke. 

Farther  to  the  southeast  is  a  small  creek,  and  across  this  to  the  south- 
west, on  the  slope  of  a  great  bluff,  is  again  found  the  sericitic  schistose 
granite,  Avhich  is  directly  overlain  by  conglomerates  containing  pebbles  of 


I^^TEEESTING    LOCALITIES    OF    THE    WEWE    SLATE.  277 

the  underlying  rock.  Thi.s  conglomerate  is  interlaminated  with  ferruginous- 
slate  and  graywacke.  Here,  as  at  the  first  locality,  it  is  difficult  to  deter- 
mine certainly  the  exact  point  at  which  the  recomposed  rock  ends  and  the 
schistose  granite  begins. 

As  has  been  explained  above,  this  area  is  a  northeastward-dipping 
isoclinal  fold.  These  conglomerates  and  slates  therefore  appciu'to  di})  under 
the  gneissoid  granite  on  the  southwest  side  of  the  area,  and  to  dip  away 
from  it  on  the  northeast  side.  Superimposed  upon  the  major  Ibid  are 
minor  corrugations.  As  a  consequence  of  this,  just  south  of  the  quarter 
line  of  sec.  23  a  tongue  of  quartzite  projects  into  the  granite  area  to  the 
southeast,  so  that  a  section  here  passes  from  the  Wewe  slate  to  the  granite, 
then  to  the  Wewe  slate,  then  to  the  granite,  and  final!)'  to  tlie  Wewe 
slate. 

These  folds  are  cross  folded,  and  consequently  pitch  either  to  the  south- 
east or  to  the  northwest,  and  the  gneissoid  granite  plunges  under  the  slate, 
and  is  thus  an  isolated  area.  The  intense  mashing  has  pniduccd  in  the 
original  granite,  as  has  been  said,  a  strongly  marked  schistose  structure,  so 
that  the  original  white  granite  has  been  transformed  to  a  rock  which  resem- 
bles a  quartz-schist.  In  a  similar  way  the  detrital  rocks  have  been  sul^jected 
to  mashing,  with  a  consequent  development  of  a  crystalline  structure,  so 
that  it  would  not  be  surprising  if  the  whole  were  regarded  as  a  conforma- 
ble series,  dipping  to  the  northeast.  However,  in  working  along  the  contact 
carefully,  the  conglomerates  and  the  occasional  localities  in  which  the 
demarcation  between  the  Wewe  formation  and  the  Archean  is  clear  show 
that  the  slate  is  later  than,  and  is  composed  of,  the  broken  granitic  material. 

The  Wewe  slates,  both  to  the  northeast  and  to  the  southwest  of  the 
Archean  area,  grade  upward  by  interlaminations  into  the  Ajibik  quartzite, 
just  as  east  of  Goose  Lake.  Here,  as  there,  the  placing  of  the  boundary 
line  between  the  two  formations  is  somewhat  arbitrary,  the  rock  being 
regarded  as  belonging  to  the  slate  where  the  slaty  phases  are  predominant. 
This  passage  of  the  slate  into  the  quartzite  on  the  southwestern  part  of  the 
bluff,  because  of  the  overturning  of  the  strata,  occurs  in  going  from  appar- 
ently higher  to  lower  members.  The  upper  phases  of  the  Wewe  slate  are 
peculiar  iron-stained  novaculites. 


278  THE    MARQUETTE    IROI^-BEARINa   DISTRICT. 

An  examination  of  the  thin  sections  enables  one  to  discriminate  with 
great  certainty  between  the  schistose  granites  which  have  taken  on  the 
character  of  kaolinic  quartz-schists,  and  the  conglomerates.  In  the  most 
mashed  phases  of  the  granite,  the  feldsjjars  have  been  entirely  decomposed, 
the  broken  granitic  quartzes  resting  in  a  kaolinic,  sericitic,  and  siliceous 
background.  In  the  conglomerates,  while  many  of  the  complex  fragmental 
grains  have  a  distinct  granitic  appearance  and  are  much  affected  by  dynamic 
action,  the  waterworn  character  of  some  of  them  is  distinct.  Also  the  con- 
glomerates have  alternating  layers  of  finer  and  coarser  material,  while  the 
laminse  of  the  granite  are  all  alike.  Finally,  the  recomposed  rock  has 
allowed  more  secondary  iron  oxide  to  enter  than  the  granite. 

In  the  center  of  sec.  22  are  a  few  outcrops  of  the  Archean  basement, 
which  together  form  an  oblong  area.  West  of  this  area,  making  up  the 
larger  part  of  a  considerable  ridge,  are  great  outcrops  of  conglomerate. 
The  great  bowlders  and  smaller  fragments  of  granite  and  gneiss  are  so 
thickly  set  in  a  sjjarse  matrix  as  to  form  a  stucco.  This  conglomerate,  as 
seen  ujjon  the  glaciated  surface,  presents  the  most  magnificent  example  of  a 
basal  conglomerate  known  in  the  district  (fig.  11,  p.  259).  At  one  place  in 
this  conglomerate  occurs  a  small  exposure  of  the  gneissoid  granite  Avhich  is 
surrounded  on  all  sides  by  the  conglomerate. 

The  fold  here  is  again  an  isoclinal  anticline,  the  strikes  being  about 
northwest-southeast  and  the  dips  to  the  northeast.  Also  the  fold  is  cross 
folded,  so  that  from  the  crest  it  plunges  to  the  northwest  and  to  the  south- 
east. In  going  to  the  northeast  or  southwest  from  the  center  of  the  bluff 
one  passes  to  higlier  horizons,  although  the  dips  are  continuously  to  the 
northeast.  The  exposures  to  the  southwest  are  more  nearly  continuous, 
and  here  the  coarse  conglomerate  is  seen  to  vary  into  fine  conglomerate, 
this  into  coarse  feldspathic  graywacke,  and  this  into  slate,  there  being,  how- 
ever, ^'arious  interstratifications  of  these  materials.  The  coarse  feldsjiathic 
graywacke — that  is,  the  phase  which  is  made  up  mainly  of  the  constituent 
minerals  of  the  granite — takes  on  at  times  a  gneissoid  appearance  which  is 
remarkably  similar  to  that  of  the  original  gneissoid  granite  making  the 
center  of  sec.  22  (PI.  X,  fig.  1).  In  fact,  at  first  they  were  not  discriminated 
in  the  field,  and  were  regarded  as  the  same.     A  study  of  the  thin  sections, 


PLATE    X. 


Plate  X.— WEWE  SLA,TE  AND   SIAMO  SLATE. 

Fig.  1.  Recomposed  rock,  resembling  granite,  from  the  Wewe  slate  near  the  center  of  sec.  22,  T.  47  N., 
E.  26  W.  (Atlas  Sheet  XXXV).  The  specimen  is  taken  from  near  the  base  of  the  forma- 
tion. The  underlying  Archean  rock  is  granite.  The  discrete  mineral  particles  of  the 
granite  form  the  detritus  of  the  figures.  These,  when  cemented,  produced  a  rock  very 
similar  iu  appearance  to  a  gneiss.  Indeed,  in  the  hand  specimen  it  is  almost  impossible  to 
discriminate  this  rock  from  true  gneissoid  granite  of  the  Archean,  but  in  thin  section  the 
fragmental  character  of  the  specimen  figured  is  in  strong  contrast  with  the  completely 
crystalline  character  of  the  gneissoid  granite.  The  recompoeed  rock  has  been  somewhat 
broken  by  dynamic  action,  and  along  the  cracks  veins  have  formed.     Natural  size. 

Fig.  2.  Ferruginous  Siamo  slate,  showing  overthrust  fault,  from  the  top  of  the  formation  in  sec.  35, 
T.  48  N.,  R.  27  W.  (Atlas  Sheet  XXVII).  The  specimen  is  cut  diagonally  across  the  bedding, 
so  that  the  layers  appear  to  be  wider  than  they  really  are.  The  finely  laminated,  greenish- 
gray  portion  is  typical  of  the  less  altered  varieties  of  the  Siamo  slate.  At  the  bottom  and 
top  of  the  formation  this  material  is  frequently  iuterlaminated  with  iron-stained  layers, 
and  the  figure  shows  a  typical  case  of  this  kind.  Though  the  ferruginous  bands  approxi- 
mately follow  the  bedding,  they  out  across  it  in  such  a  way  as  to  show  that,  while  the 
percolating  waters  were  controlled  iu  a  large  way  by  the  bedding,  to  some  extent  they 
went  across  it.  A  study  of  the  thin  section  shows  that  the  ferruginous  layers  usually 
develop  where  there  was  originally  siderite.  In  one  of  the  gray  bands  an  overthrust 
fault  is  beautifully  shown.  This  has  sharply  broken  the  harder,  more  siliceous  layers 
and  lias  carried  with  it  the  weaker  layers  between  the  harder  ones.  However,  both  above 
and  below,  the  fault  passes  into  a  flexure.  The  specimen  was  evidently  iu  the  zone  of 
combined  fracture  and  flowage,  the  readjustment  of  the  harder  layers  being  by  fracture 
and  that  of  the  softer  layers  by  flowage.  This  fault,  although  on  a  minute  scale,  illustrates 
perfectly  how  a  major  fault  may  disappear  below  by  passing  into  a  flexure.     Natural  size. 


.S  GEOLOGICAL  SURVEY 


_\ 


'[C,   1     liKI'OMl'OSKI)  KOCK 
■IG-  2.  KKHKrC.lNOl'S  Sl.WK 


.IXC.  CIJAXI 
SIIOWINC, 


■|.:,|.'i;()M  WKWK  si,.vri 
)\-i;i-iTiinrsT  v.\ri;r 


INTERESTING   LOCALITIES   OF   TBE   WEWE    SLATE.  281 

however,  shows  the  completely  crystalline  character  of  the  (ine  and  the 
recomposed  character  of  the  other. 

Northwest  of  this  conglomerate  bluff,  making  another  considerable  set 
of  bluffs  in  the  northwest  part  of  sec.  22  and  the  northeast  part  of  sec.  21, 
are  the  typical  cherty  and  brecciated  Wewe  slates  and  graywackes.  Con- 
sequent upon  the  northwest  plunge  of  the  fold,  the  higher  members  are 
found  on  the  southwest,  northwest,  and  northeast  sides  of  the  exposures, 
tlie  coarsely  conglomeratic  phases  being  limited  to  the  southeastern  hills, 
though  distinctly  conglomeratic  phases  occur  at  higher  horizons.  The 
slates,  gi-ay wackes,  and  novaculites,  their  cleavage  and  foliation,  their  brec- 
ciation,  silicification,  and  ferrugination  (figs.  12  and  13,  and  PI.  IX)  are  very 
similar  to  those  phenomena  described  in  the  ledges  east  of  Goose  Lake, 
in  sees.  13  and  24.  Parallel  to  the  bedding  are  either  cherty  or  novacu- 
litic  layers,  which  are  traversed  by  veins  of  quartz.  The  usual  strike  is 
N.  15°  W.  to  N.  40°  W.,  and  the  dip  at  an  angle  of  50°  to  60°  north  of  east. 
The  bedding  is  usually  cut  by  a  foliation,  which  strikes  about  N.  50°  W. 
and  has  a  vertical  dip.  The  breccias  are  more  extensively  developed  in 
the  locality  under  consideration  than  anywhere  else.  Many  of  the  ledges 
are  traversed  in  all  directions  by  veins  of  white  quartz,  but  the  majority 
of  these  are  parallel  to  the  schistosity.  Near  the  northwest  part  of  the 
exposures,  on  one  of  the  more  prominent  bluffs,  the  extreme  stage  of 
dynamic  action  is  represented  by  a  remarkable  reibungsbreccia.  The 
fragments  are  all  of  the  black  slaty  or  cherty  rock.  They  vary  in  size 
fi'om  minute  ones  to  great  blocks  several  feet  in  diameter.  The  whole 
is  receinented  mainly  by  vein  quartz,  but  in  part  by  hematite  and  jaspery 
quartz.  The  veins  of  the  latter  are  later  than  the  white  quartz  veins,  and 
where  of  some  width  the  bands  of  ore  or  jasper  could  not  be  discriminated 
from  the  feiTuginous  jasper  of  the  Negaunee  formation.  This  breccia 
differs  from  a  true  conglomerate  in  that  the  cementing  material  is  of  a  vein 
character,  the  fragments  all  of  one  kind,  and  usually  exceedingly  angular. 
The  black  and  gray  schist,  set  in  the  quartz  veins  and  ornamented  by  the 
specular  hematite  and  red  jasper  in  smaller  quantity,  makes  the  exposure  a 
beautiful  one.  The  fragments  of  schistose  slate  often  have  around  them 
parallel  zones  of  quartz,  hematite,  and  jasper,  although  usually  the  quartz  is 


282  HE   MAKQUETTE   mON-BEARi:NG  DISTKIOT. 

alone.  Between  the  brecciated  slates  and  those  in  which  there  is  merely 
a  development  of  slatiness  or  schistosity,  with  secondary  feiTugination  and 
silicification,  there  are  all  gradations,  so  that  there  is  positive  proof  of  the 
brecciated  character  of  the  rocks. 

About  the  other  two  Archean  islands  in  sees.  22  and  23,  are  interesting 
exposures  of  conglomerate,  slate,  and  graywacke,  but  as  these  are  not  very 
different  from  those  already  mentioned,  they  will  not  be  further  described. 
The  conglomerates,  slates,  and  graywackes  are  particularly  well  developed 
about  the  corner  between  sees.  21,  22,  27,  and  28,  and  also  to  the  east  and 
northwest  of  this  point. 

SECTION  IV THE  AJIBIK  QUARTZITE. 

The  formation  is  given  the  name  Ajibik  quartzite  because  the  predom- 
inant rock  is  quartzite,  and  because  typical  exposures  of  it  occur  on  the 
bold  Ajibik  Hills  northeast  of  Palmer  (Atlas  Sheet  XXXII). 

DISTRIBUTION,    EXPOSURES,  AND    TOPOGRAPHY. 

Beginning  at  the  south  arm  of  Groose  Lake  (Atlas  Sheet  IV),  the  forma- 
tion occupies  a  broad  belt,  which  narrows  in  sec.  23,  swings  south  of  the 
Wewe  slate,  and  then  gradually  increases  in  width  to  sec.  28,  T.  47  N., 
R.  26  W.  From  this  place  one  arm  extends  to  the  west  for  nearly  a  mile, 
but  the  main  arm  swings  to  the  north,  west  of  the  Wewe  slate,  and  then 
east,  north  of  the  same  formation.  West  of  Goose  Lake  the  belt  again 
becomes  broad,  and  an  arm  projects  to  the  southeast  between  two  Archean 
islands,  being  bounded  on  both  the  east  and  the  west  by  the  Wewe  slate. 
The  main  belt,  reaching  Goose  Lake,  extends  north  of  this  area  for  a  mile, 
then  swings  eastward,  which  course  it  follows  for  2  or  3  miles,  then  swings 
to  the  northeast  to  Carp  River.  Here  it  is  faulted,  but,  reappearing  again 
north  of  the  river,  it  continues  its  course  east,  then  north,  then  west  in 
sec.  6,  T.  47  N.,  R.  25  W.  It  follows  this  western  course  to  the  quartzite 
range  east  of  Teal  Lake,  the  southern  part  of  which  it  constitutes.  West 
of  Teal  Lake  it  reappears,  here  being  in  contact  with  the  Archean,  and 
follows  along'  this  formation  to  Lake  Michigamme. 


EXPOSURES   OF  THE   AJIBIK  QUARTZITE.  283 

South  of  tlie  Negaunee  formation,  in  sec.  35,  T.  47  N.,  R.  26  W.,  there 
appears  a  quartzite,  placed  with  the  Ajibik  quartzite,  wliich  extends  west- 
ward ahriost  continuously  to  sec.  31.  The  belt  here  swings  to  the  north, 
northeast,  north,  and  finally  west  again,  about  an  anticline  in  the  Archean, 
and  tlien  extends  in  a  general  westerly  cour.se  to  sec.  20,  T,  47  N.,  R.  28  W.; 
thence  northwest  to  near  Humboldt,  in  sec.  12,  T.  47  N.,  R.  29  W.  Expo- 
sures of  quartzite  reappear  at  the  base  of  the  Lower  Marquette  series  on 
both  sides  of  the  Republic  and  Western  tongues.  It  is  doubtful  whether  this 
western  part  of  the  quartzite  is  really  the  time  equivalent  of  the  remainder  of 
the  Ajibik  quartzite.  Throughout  the  district  it  is  natural,  almost  inevitable, 
that  at  the  base  of  the  sedimentary  series  there  should  have  been  deposited 
a  conglomeratic  quartzite.  It  is  therefore  not  impossible — indeed,  it  is 
probable — that  the  westward  part  of  this  belt  of  quartzite  belongs,  in  age, 
with  the  lower  part  of  the  Siamo  slate  as  developed  to  the  east,  rather 
than  with  the  Ajibik  quartzite.  However,  as  this  quartzite  constitutes  a 
continuous  lithological  formation,  and  as  there  is  no  basis  upon  which  to 
make  the  equation,  and  as  above  it  there  occur  the  representatives  of  the 
Siamo  slate,  at  least  as  far  west  as  sec.  28,  T.  47  N.,  R.  27  W.,  the  whole 
formation  is  here  considered. 

On  account  of  the  resistant  character  of  the  quartzite.  at  various  places 
it  becomes  one  of  the  chief  topographic  features  of  the  district.  South  of 
the  southeast  arm  of  Goose  Lake  the  bold  quartzite  exposures  rise  steeply 
from  the  lake,  and  from  the  sand  plain  to  the  east  and  south.  The  series 
of  ledges  composing  the  quartzite  belt  are  almost  continuous  to  the  west- 
ward, everywhere  rising  abruptly  from  the  valley  to  the  south,  and  in  sees. 
27,  28,  and  29,  T.  47  N.,  R.  26  W.,  the  quartzite  constitutes  the  Ajibik  Hills, 
a  bold  east-and-west  ridge,  with  precipitous,  south-facing  exposures.  This 
ridge  rises  about  200  feet  from  the  valley  of  Ajibik  Creek.  On  the  north 
side  the  ridge  falls  away  less  steeply  to  the  exposures  of  the  Siamo  slate. 
While  this  ridge  has  the  general  features  above  given,  in  a  smaller  way 
it  is  exceedingly  rough,  a  north-and-south  traverse  ascending  precipitous 
bluffs,  to  almost  immediately  descend  into  a  steep  ravine,  the  other  side 
of  which  must  be  climbed  but  to  repeat  the  performance      As  has  been 


284  THE  MARQUETTE   IRON-BEARING  DISTRICT. 

said,  in  sec.  28  the  ridge  branches  into  two  parts,  one  of  which  extends 
west  about  a  mile.  The  main  belt  swings  to  the  north  into  sec.  21, T.  47  N., 
R.  26  W.  Here  there  are  again  numerous  huge  ledges  of  the  quartzite. 
Following  along  the  course  of  the  belt  to  the  northeast,  between  sees.  22 
and  23  there  are  again  numerous  large  exposures.  Continuing  to  the  north, 
the  formation  has  a  position  between  the  Wewe  slate  and  the  Siamo 
slate.  The  quartzite,  being  the  more  resistant  rock,  occupies  the  higher 
lauds,  between  lower  lands  to  the  south  and  to  the  north.  In  the  valley 
of  Carp  River  the  outcrops  are,  however,  less  numerous  than  to  the 
southwest,  although  sufficiently  abundant  to  show  that  the  belt  is  certainly 
continuous  to  sec.  36,  T.  48  N.,  R.  26  W.  From  this  point  exposures  are 
not  abundant  until  the  quartzite  east  of  Teal  Lake  is  reached,  where  again 
they  are  numerous.  From  this  place  they  extend  almost  continuously 
along  the  ridge  to  sec.  33,  T.  48  N.,  R.  27  W.  From  here  west  to  Michi- 
gamme  the  outcrops  are  not  abundant,  but  are  found  at  a  number  of  places 
close  to  the  Archean. 

Where  the  formation  appears  south  of  the  Negaunee  iron  formation, 
in  sees.  34  and  35,  T.  47  N.,  R.  26  W.,  there  are  ledges  of  quartzite  and 
conglomerate.  West  for  some  distance  the  topographic  features  are  given 
by  the  Archean  to  the  south  and  the  jaspery  iron  formation  to  the  north, 
so  that  the  quartzite  usually  occupies  a  valley  between  these  two  formations, 
but  with  frequent  exposures  in  sees.  31,  32,  and  33.  West  of  the  Volunteer 
mine  the  formation  appears  as  a  conglomerate  below  the  iron-bearing 
member.  In  sec  30,  to  the  north,  there  are  a  number  of  large  and  typical 
ledges.  West  of  this  place  the  quartzite  is  again  in  the  valley  between 
the  Archean  to  the  south  and  the  iron  formation  to  the  north,  there  being 
only  a  few  outcrops.  The  rock  is  found  facing  the  granite  near  the  center 
of  sec.  28,  T.  47  N.,  R.  27  W.,  and  somewhat  unusual  slaty  phases,  inter- 
bedded  with  amygdaloids,  are  found  near  the  top  of  the  formation  in 
sees.  27  and  28.  Several  exposures  are  found  in  sec.  19,  west  of  which  are 
no  outcrops  until  the  vicinity  of  Humboldt  is  reached,  where  exposures 
are  again  found  south  of  the  iron  formation.  The  remaining  outcrops  are 
considered  in  Chapter  IV. 


FOLDING   OF   THE   AJIBIK  QUAKTZITE.  285 

FOLDING. 

Tlie  topograpliic  features  and  the  exposures  are  closely  dependent 
upon  the  foldhio-  to  which  the  quartzite  has  been  subjected.  Beginnmg 
south  of  Goose  Lake,  the  quartzite  constitutes  an  eastward-plunging  anticline 
over  the  Wewe  slate  in  sec.  23  (Atlas  Sheet  XXXV).  To  the  north  this 
anticline  is  quickly  followed  by  a  syncline,  so  that  the  section  from  north 
to  south  includes  a  southern  anticline  and  a  northern  syncline.  Following 
the  belt  westward,  the  formation  constitutes  the  southward  slope  of  an 
anticline,  the  crown  of  which  is  to  the  north  in  the  area  of  the  "Wewe  slate 
and  Archean  islands.  The  belt  is  continuous  to  sec.  28,  T.  47  N.,  R.  26  W. 
(Atlas  Sheet  XXXII),  where,  still  constituting  one  side  of  an  anticline,  it 
swings  northwest  and  then  north.  The  westward-projecting  arm,  which 
runs  into  the  NW.^  sec.  28  and  the  NE.^  sec.  29,  is  due  to  a  subordinate 
anticline  which  springs  up  on  the  slope  of  the  main  anticline.  The  greater 
breadth  of  the  formation  in  sec.  28  is  due  to  this  same  cause.  The  west- 
ward-projecting arm  is  a  westward-plunging  anticline,  so  that  the  quartzite 
soon  disappears  under  higher  formations.  In  the  center  of  this  anticline  a 
small  area  of  Archean  appears.  The  main  belt  of  the  formation  (Atlas 
Sheet  IV)  swings  to  the  northward,  thence  northeast,  thence  east  to  Carp 
River,  and  thence  north  and  west  to  Teal  Lake.  This  main  belt  is  thus  a 
part  of  the  great  westward-plunging  syncline  of  the  eastern  half  of-  the 
district,  dipping  to  the  north  along  its  southern  arm,  to  the  south  along 
its  northern  arm,  and  to  the  west  at  the  eastern  end  of  the  syncline. 

West  of  Teal  Lake  (Atlas  Sheet  XXVII)  the  regularly  bedded,  typical 
quartzite  in  the  lower  horizons  is  found  to  be  somewhat  plicated,  then 
more  plicated,  and  finally  very  closely  plicated  into  a  series  of  minor  cross 
folds,  with  axes  plunging  sharply  to  the  south,  following  the  general  dip 
of  the  formation. 

In  sees.  30  and  31,  T.  48  N.,  R.  28  W.,  and  in  sec.  25,  T.  48  N,,  R.  29  W., 
the  quartzite  swings  to  the  north,  and  here  the  chai-acteristic  folding  of  the 
district  is  well  illustrated  (Atlas  Sheets  XV  and  XVIII).  The  formation  is 
infolded  in  the  most  complicated  fashion  with  the  granite  and  gneiss  of  the 
Archean,  the  whole  being  a  set  of  isoclinal  overfolds  with  southern  dips.    The 


286  THE   MARQUETTE  IKOX-BEARING  DISTRICT. 

fragmental  rock  occupies  the  valleys  and  the  granite  the  elevations.  These 
valleys  open  out  to  the  west  and  close  to  the  east,  the  granite  thus  forming 
amphitheaters  about  the  quartzite.  This  is  due  to  the  fact  that  the  south- 
dipping  isoclinal  folds  have  a  steep  westward  pitch.  As  a  result  of  this 
complex  folding,  an  island  of  granite  appears  surrounded  by  the  Ajibik 
quartzite  in  sec.  30,  and  another  island  of  granite  occurs  within  the  Siamo 
slate  above  the  quartzite  in  sec.  31.  Consequently,  the  north-south  subor- 
dinate rolls  are  of  sufficient  size  to  form  at  the  anticlines  islands  of  granite 
within  the  sedimentary  rocks. 

Running  southeast  between  the  Archean  area  of  sec.  23  and  the 
Ai^chean  area  of  sec.  22,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXV),  is  a 
northwestward-plunging  syncline  of  the  quartzite,  making  an  arm  project- 
ing from  the  main  area.  Tliis  belt  may  extend  farther  than  map]>ed  and 
connect  with  the  belt  to  the  south. 

With  the  exception  of  a  single  swing  about  the  Archean  .anticline  in 
sec.  30,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXII),  the  southern  belt  of 
quartzite  has  a  general  northward  dip  away  from  the  Archean  and  under 
the  iron  formation.  The  exposures  in  sees.  27  and  28,  T.  47  N.,  R  27  W. 
(Atlas  Sheets  XXVI  and  XXIX),  when  cursorily  examined,  appear  to  have 
a  uniform  northward  dip,  but  when  examined  closely  the  upper  members 
of  the  formation,  which  are  here  slates,  are  found  to  be  pressed  into  a  sharp 
set  of  overfolds  with  northern  dips.  These  folds  are  not  horizontal,  but 
plunge  steeply  (PI.  XXXV,  fig.  1).  Acompanving  these  minor  rolls  ai'e, 
doubtless,  major  rolls.  This  is  indicated  by  the  fact  that  interstratified  with 
the  slate  are  apparently  three  belts  of  amygdaloid ;  but  as  the  rocks  of  these 
belts  are  all  exactly  alike,  and  as  amygdaloid  is  absent  elsewhere  in  the  for- 
mation, it  is  more  than  probable  that  this  is  the  same  lava  flow,  reduplicated 
by  the  northward-dipping  overfolds.  In  the  Republic  tongue  and  in  the 
tongue  to  the  west  the  Ajibik  quartzite  is  in  a  closely  compressed  synchne. 

PETROGRAPHICAL    CHAKACTER. 

Macroscopicai. — Tlic  Ajibik  quartzitc  has  two  main  areas — a  western  one, 
in  which  it  rests  directly  upon  the  Archean,  and  an  eastern  one,  in  which 
it  is  underlain  by  the  Wewe  slate  (Atlas  Sheet  IV).    This  difference  is  fully 


PETKOGEAPniCAL  CnAEACTEE  OF  THE  AJIBIK  QUARTZITE.        287 

explained  iu  another  connection  by  the  transgression  of  the  sea  from  the 
east.  The  Ajibik  area  in  contact  with  the  Archeau  extends  west  from 
the  Teal  Lake  quartzite  range  on  the  north,  and  from  sec.  35,  T.  47  N., 
R.  26  W.,  on  the  south,  to  the  west  end  of  the  district.  Also  there  is  here 
included  the  area  in  sec.  29,  T.  47  N.,  R.  26  W.  The  eastern  area  comprises 
the  remainder  of  the  formation. 

Where  the  formation  rests  directly  upon  the  Archean  its  basal  part  is  a 
conglomerate  or  recomposed  rock,  the  material  of  which  is  derived  mainly 
from  the  immediately  subjacent  rocks.  In  short,  the  conditions  of  for- 
mation are  the  same  as,  and  the  phases  of  the  basal  rock  identical  with, 
those  of  the  Mesnard  quartzite,  described  on  page  223.  This  is  entirely 
natural,  as  the  two  are  in  fact  but  parts  of  the  first  deposit  of  the  trans- 
gressing sea.  The  basal  conglomerates,  slates,  and  graywackes  for  this 
part  of  the  area  qu.ickly  grade  up  into  quartzite  which  does  not  differ  from 
that  of  the  remainder  of  the  formation. 

In  the  eastern  part  of  the  district,  as  the  Wewe  slate  passes  into  the 
Ajibik  quartzite  there  is  usually  an  intermediate  phase,  or  interstratifica- 
tions  of  the  two.  In  many  places  the  slate  varies  into  a  coarse  graywacke, 
this  into  a  feldspathic  quartzite,  and  this  into  the  ordinary  quartzite.  In 
other  cases  the  transition  phase  is  a  white  or  green  novaculitic  quartzite. 
With  these  are  sometimes  red  and  brown  iron-stained  kinds.  In  places  the 
nonfen-uginous  and  ferruginous  varieties  show  the  most  curiously  c(inqjlex 
relations,  one  appearing  in  the  other  in  the  most  indiscriminate  manner,  as  if 
in  extremely  irregular  inclusions  or  patches.  The  iron  staining  is  evidently 
a  secondary  process,  and  the  differing  effects  have  been  produced  by  the 
varying  depths  to  which  the  solutions  have  penetrated.  In  one  exceptional 
locality,  in  sec.  6,  T.  47  N.,  R.  25  W.  (Atlas  Sheet  XXXVII),  the  basal 
member  of  the  quartzite  is  a  conglomerate  interstratified  with  slate,  the 
fragments  of  the  conglomerate  being  mainly  from  the  Wewe  slate. 

The  central  part  of  the  formation  in  its  ordinary  phases  is  a  typical, 
rather  pure,  vitreous  quartzite.  In  some  places  this  quartzite  becomes 
conglomeratic  and  bears  small  pebbles  of  white  quartz  or  red  jasper.  In 
other  places  it  is  interstratified  with  belts  of  mica-slate  or  graywacke. 
In  many  places  the  formation  was  subjected  to  dynamic  forces.     In  the 


288  THE   MAEQUETTE  IKON-BEARING  DISTEICT. 

least-marked  stage  of  alteration  the  quartzites  were  simply  broken  to  a 
greater  or  less  amount,  and  the  crevices  thus  formed  were  cemented  with 
finely  crystalline  cherty  quartz,  or  with  oxide  of  iron,  or  both.  In  a  further 
stage  of  the  process  the  quartzites  were  fractured  through  and  through, 
and  in  places  they  pass  into  I'eibungsbreccias.  In  the  numerous  ramif}^- 
ing,  braixching,  and  intersecting  cracks,  silica  and  iron  oxide  infiltrated. 
The  silica  in  places  took  on  cherty  or  jaspery  form.s,  and  in  other  places 
it  crystallized  as  a  vein  quartz.  The  secondary  material  may  locally  be 
so  abundant  as  to  compose  a  large  part  of  the  rock,  and  rarely  considerable 
belts  of  chert  or  vein  quartz  and  iron  oxide  may  be  seen.  In  proportion 
as  the  fractui-ing  and  the  amount  of  secondary  cherty  silica  increase,  the 
rocks  assume  a  peculiar  vitreous  aspect.  The  iron  oxide  crystallized  as 
hematite  and  magnetite,  the  latter  now  la'rgely  changed  to  martite. 

In  their  very  general  brecciation,  with  consequent  considerable  areas 
of  pseudo-conglomerates,  in  the  secondary  veining,  both  with  coarsely  and 
finely  crystalline  quartz,  and  in  the  large  quantity  of  secondary  hematite 
and  magnetite,  these  quartzites  differ  from  the  Goodrich  quartzite  of  the 
Upper  Marquette  series.  Apparently  in  some  cases  the  brecciation  was 
produced  before  the  rocks  became  thoroughly  indurated,  while  the  frag- 
ments had  a  sandy  matrix,  in  which  case  the  individual  grains  were 
broken  asunder,  and  the  whole  has  been  indurated  by  secondary  infil- 
trating silica  and  iron  oxide.  In  some  localities  very  peculiar  d^'uamic 
effects  are  observable.  As  a  consequence  of  the  folding  a  most  curious 
spheroidal  fracturing  has  occurred,  resulting  in  roundish  pebble-like  and 
bowlder-like  forms.  Iron  oxide  has  infiltrated  along  the  cracks,  and  has 
especially  affected  the  more  fractured  and  broken  matrix,  so  that  the 
spherical  pieces  appear  like  pebbles  derived  from  a  different  rock.  In  the 
most  brecciated  phase  we  have  a  pseudo-conglomerate  consisting  of  white 
spheroids  of  quartzite  in  an  iron-stained  quartzite  matrix;  a  close  exami- 
nation shows,  however,  that  many  of  the  supposed  pebbles  are  not  entirely 
surrounded  by  the  matrix,  each  being  really  attached  at  some  place  to  it. 
Following  along  the  pseudo-conglomerate  belt,  we  pass  from  this  most  con- 
glomeratic-looking phase  to  that  in  which  there  is  less  and  less  dynamic 
effects,  and  the  rock  by  gradation  passes  into  the  ordinary  quartzite  of 


PETROGRAPHICAL  CHARACTER  OF  AJIBIK  QUARTZITE.      289 

the  area.  In  an  intermediate  pliase,  while  conchoidal  fractures  are  seen, 
they  do  not  wholly  separate  different  parts  of  the  rock,  so  that  what  would 
have  been  separate  fragments  had  the  fractures  gone  further  are  but  a  half 
or  a  third  separated  from  the  quartzite  background.  The  most  extremely 
alterated  quartzite,  instead  of  being  brecciated,  was  mashed  throughout, 
and  as  a  result  passed  into  a  biotitic  or  muscovitic  quartz-schist,  or  into 
coarse,  completely  crystalline,  typical  chlorite-schists,  biotite-schists,  and 
muscovite-schists. 

In  the  northern  and  eastern  parts  of  the  district  the  quartzites  grade 
upward  by  interstratifications  into  the  Siamo  slate.  In  the  southern  and 
southwestern  parts  of  the  district  the  formation  grades  in  a  similar  man- 
ner into  the  nonfragmental  Negaunee  iron  formation.  In  sees.  27  and  28, 
T.  47  N.,  R.  27  W.,  the  intermediate  phases  are  slates  like  those  of  the 
Siamo  formation. 

Microscopical. — Wlicrc  tlic  Ajlbik  quartzite  rests  upon  the  Ai-chean,  and 
therefoi-e  has  a  conglomerate  or  feldspathic  quartzite  at  its  base,  it  is  very 
similar  to  the  basal  conglomerates  of  the  Mesnard  quartzite  and  Wewe  slate 
described  on  pages  224-227,  263-265.  The  basal  rock  in  some  jDlaces  is 
a  distinct  conglomerate,  and  in  others  is  composed  mainly  of  the  separate 
mineral  constituents  of  the  adjacent  underlying  rocks.  At  many  places 
the  basal  horizon  has  been  so  much  mashed  as  to  pass  into  a  crystalline 
schist.  In  these  places,  instead  of  the  conglomerate,  Ave  have  chloritic, 
sericitic,  Ijiotitic,  or  muscovitic  schists,  and  in  the  most  extreme  stage  of 
alteration  the  rocks  pass  into  typical  mica-schists,  the  leaflets  of  biotite  and 
muscovite  being  of  large  size  and  having  a  parallel  arrangement.  In  this 
phase  the  quartz  grains  are  wholly  granulated;  the  new  quartz  which  has 
developed  is  similar  in  appearance  to  the  granules;  and  the  original  feld- 
spar is  wholly  decomposed,  its  place  being  taken  by  the  muscovite,  biotite, 
and  secondary  quartz.  In  certain  of  the  schist-conglomerates,  while  the 
matrix  is  completely  crystalline,  in  hand  specimens  the  mashed  and  greatly 
elongated  conglomerate  pebbles  may  still  be  recognized. 

Where  the  formation  underlying  the  Ajibik  quartzite  is  the  Wewe  slate, 
there  are  apt  to  be  interlaminated  with  the  lower  horizons  of  the  quartzite, 

MON   XXVIII 19 


290  THE  MAEQUETTE   IRON-BEAEING  DISTRICT. 

biotitic  and  sericitic  slates  and  graywackes  which  are  in  every  respect 
similar  to  those  described  (pp.  265-269)  under  the  Wewe  formation. 

In  the  purest  and  least  mashed  phase  of  quartzite  the  rocks  are  com- 
posed almost  wholly  of  rounded  grains  of  quartz  of  somewhat  uniform  size, 
which  are  beautifully  enlarged,  the  enlargements  filling  the  entire  inter- 
sjDaces.  But  even  in  this  quartzite  the  grains  uniformly  show  undulatory 
extinction,  and  some  of  them  are  distinctly  fractured.  Where  the  dynamic 
effects  are  somewhat  stronger,  between  and  in  connection  with  the  enlarge- 
ments of  the  quartz  grains  there  is  a  fine  mosaic  of  independent  interstitial 
quartz,  and  with  this  there  is  a  beginning  of  the  arrangement  of  the  grains 
with  their  longer  axes  in  a  common  direction.  Very  frequently  the  fractures 
of  the  gi-ains  pass  directly  across  the  cores  and  the  enlai'gements,  showing 
that  the  fracturing  occurred  after  the  second  growth  of  the  quartz  grains. 
Occasionally  with  the  simple  quartz  grains  there  are  finely  complex  grains 
of  quartz,  which  appear  to  be  derived  from  chert.  In  a  phase  intermediate 
between  the  quartzites  and  the  graywackes  there  is  present  with  the  quartz 
a  greater  or  less  amount  of  kaolin,  sericite,  and  chlorite.  In  some  cases 
these  become  rather  abundant,  so  that  the  rocks  are  chloritic  or  sericitic 
quartzites.  Not  infrequently  the  quartzites  are  feldspathic,  and  in  some 
cases  this  mineral  has  undergone  to  a  greater  or  less  degree  the  usual 
decomposition  into  mica  and  quartz,  or  into  chlorite  and  quartz.  Where 
the  decomposition  is  complete,  in  place  of  the  round  grains  of  feldspar  we 
have  an  interlocking  mass  of  sericite  and  quartz,  biotite  and  quartz,  or 
chlorite  and  quartz,  as  the  case  may  be.  At  one  place  the  feldspar  grains 
are  as  distinctly  enlarged  as  the  quartz  grains.  The  quartzites  usually  con- 
tain a  small  amount  of  iron  oxide,  which  marks  the  cores  of  the  original 
quai-tz  grains  and  is  intermingled  with  the  new  quartz. 

In  the  quartzites  where  the  dynamic  forces  were  still  stronger  the  indi- 
vidual grains  of  quartz  are  broken  apart,  or  the  rock  is  fractured  tlu-ough 
and  through,  or  even  changed  into  a  reibungsbreccia.  In  the  larger  crev- 
ices and  cracks  is  vein  quartz  or  iron  oxide — in  some  one  alone,  in  others  the 
two  together,  although  the  quartz  is  more  abundant.  These  veins  in  some 
cases  are  coarsely  crystalline  quartz;  in  others  they  are  finely  crystalline, 
cherty,  or  jaspery  quartz,  and  with  either  of  these  are  iron  oxides.     These 


PETliO(iKAPIIICAL   CHARACTER   OF   AJIBIK    QUAKTZITE.      2i)l 

ferruginous  cliert  and  jasper  veins  often  have  the  iron  oxide  and  the  quartz 
arranged  in  bands  or  irregularly  distril)uted,  and  the  veins  are  exactly  similar 
to  the  jaspilite  of  the  Negaunee  formation.  ( )fteii  the  vein  material  is  mingled 
with  fragmental  quartz,  the  grains  having  been  broken  from  the  rock  and 
fallen  in  the  crevices.  Where  the  individual  grains  of  the  rock  were  sun- 
dered, the  parts  were  cemented  by  the  secondary  quartz  and  iron  oxide  exactly 
as  were  the  larger  spaces.  The  recognizable  original  grains  of  quartz  show 
sti'ong  dynamic  effects,  all  of  them  giving  undulatory  extinction,  and  many 
of  them  being  broken  into  several  individuals,  or  even  wholly  granulated. 
In  some  cases  the  cracks  are  in  two  sets  at  right  angles  to  each  other,  the 
cracks  of  each  set  having  a  parallel  arrangement.  The  areas  in  which  the 
grains  were  rent  asunder  and  those  in  which  they  were  not  are  very  irreg- 
vilar,  and  in  the  field  the  first  are  usually  separated  from  the  second  by 
stains  of  iron  oxide.  In  those  cases  in  which  the  secondary  quartz  is 
abundant  and  the  primary  quartz  was  granulated,  so  that  it  no  longer  has  a 
clastic  appearance,  we  have  an  intricately  interlocking  mass  of  quartz  grains 
of  various  sizes  in  which  the  original  material  can  not  be  discriminated 
from  that  which  has  come  in  later.  In  some  places  the  whole  rock  is  com- 
posed of  small,  closely  fitting-  granules  of  quartz.  The  granulated  material 
is  commonly  finer  or  coarser  than  that  of  the  interlocking  and  intersecting 
veins,  and  in  the  latter  iron  oxide  is  usually  abundant.  These  rocks,  in 
which  the  evidence  of  fragmental  origin  has  disappeared,  and  yet  which  do 
not  have  a  schistose  structure,  are  called  quartz-rocks.  All  of  these  phases 
are  so  similar  to  the  jaspilite  of  the  Negaunee  formation  that  the  two  could 
not  be  separated  in  thin  section.  However,  these  extremely  altered  i-ocks 
are  traced  into  those  which  are  less  modified,  there  first  appearing  a  few 
distinctly  clastic  grains,  then  clusters  of  them,  until  we  have  an  intermediate 
variety  in  which  perhaps  half  of  the  section  shows  fragmental  quartz  buried 
in  a  crystalline  matrix. 

Resulting  from  the  differing  modifications  of  the  original  sandstone, 
we  therefore  have  in  the  formation  quartzite,  cherty  quartzite,  ferruginous 
quartzite,  feiTuginous  cherty  quartzite,  quartz-rocks,  quartzite-breccia,  vein 
quartz,  vein  chert  and  jasper,  and  other  phases. 

The  rather  peculiar  autoclastic  rocks  which  resemble  quartzite-couglom- 
erates  were  mentioned  in  the  macroscopical  description.     Tlie  jjebble-like 


292  THE   MARQUETTE   IKOif-BEAKING   DISTRICT. 

areas,  which  were  beheved  to  be  due  to  spheroidal  fracturing,  are  clearly 
shown  to  be  of  this  character  in  the  thin  section.  Instead  of  having  smooth 
exterior  boundaries,  as  would  be  expected  in  waterworn  pebbles,  there  are 
minute  irregularities,  such  as  would  be  produced  by  fracturing.  The  sphe- 
roids are  found  to  be  pure  vitreous  quartzites,  which  are  wholly  cemented 
by  the  enlargement  process,  or,  more  rarely,  by  this  combined  with  finely 
crystalline,  interstitial  quartz.  These  pebble-like  areas  rest  in  a  background 
composed  of  quartz  grains,  which  are  set  in  a  matrix  composed  of  finely 
crystalline  quartz,  iron  oxide,  and  sericite.  It  is  apparent  that  the  individ- 
ual grains  of  this  part  of  the  rock  were  broken  apart,  and  thus  allowed  the 
secondary  materials  to  enter,  whereas  in  the  xmcrushed  pebble-like  areas 
the  space  was  fully  occupied.  It  is  clear  that  before  this  rock  was  brecciated 
it  was  indurated  by  the  enlargement  process. 

In  the  macroscopical  description  a  locality  Avas  mentioned  where 
the  lowest  horizon  of  the  Ajibik  quartzite  bears  slate  fragments.  Here  the 
lower  beds  consist  of  interstratified  slates,  graywackes,  and  conglomerates, 
which  quickly  pass  up  into  ferruginous  quartzite,  and  this  into  the  ordinary 
vitreous  rock.  The  slates  are  composed  of  interstratified  coarse  and  fine 
materials,  which  differ  chiefly  from  each  other  in  that  the  coarser  layers 
contain  numerous  large  fragmental  grains  of  quartz,  usually  simple,  but 
sometimes  complex,  and  sometimes  cherty.  The  matrix  is  clayey  material, 
so  fine  that  it  is  difficult  to  determine  the  constituents,  but  sericite,  quartz, 
chlorite,  feldspar,  and  ferrite  are  present.  The  conglomeratic  layers  also 
bear  fragments  of  the  underlying  Wew^e  slate.  However,  these  fragments 
when  closely  examined  ai'e  seen  not  to  be  sharply  outlined,  as  is  usual  with 
ordinary  pebbles,  but  are  greatly  elongated  and  have  minutely  irregular 
borders,  the  projections  of  which  fill  the  interspaces  of  the  quartz  grains. 
This  suggests  that  the  underlying  slate  was  not  nnicli  indurated  at  the  time 
it  yielded  the  fragments  to  the  quartzite,  being  rather  a  compacted  clay 
than  a  solid  rock. 

For  those  parts  of  the  area  -where  the  Ajibik  formation  is  overlain  by 
the  Negaunee  iron  formation  the  lower  formation  grades  into  the  higher,  or 
beds  which  belong  lithologically  in  the  two  formations  are  interstratified. 
In  passing  from  the  lower  to  the  higher  formation;  where  the  lowest  rock  of 


PETimURAPHICAL  CHAKACTEU  OF   AJIBIK   QUARTZITE,      293 

the  Negaimee  forinatiou  is  jasper,  the  change  takes  place  by  the  dying  out 
of  fragmental  quartz  and  the  appearance  of  hematite,  magnetite,  and  finely 
crystalline  quartz;  where  the  overlying  formation  is  griinerite-magnetite- 
schist,  the  minerals  which  appear  are  magnetite,  griinerite,  and  often  garnet. 
Occasionally  the  intermediate  phase  is  a  ferruginous  slate,  like  the  transi- 
tion liorizon  of  the  Siamo  and  Negaunee  formations.  In  the  southwest 
part  of  the  area— that  is,  in  the  Republic  and  Southwest  tongues — the  folding 
and  consequent  mashing  were  so  severe  as  to  transform  the  Ajibik  quartzite 
formation  into  a  completely  crystalline  schist.  Even  the  pure  quartzitic 
phases  now  show  no  distinctly  fragmental  grains  of  quartz,  bxit  consist 
mainly  of  coarsely  crystalline  interlocking  quartz,  in  which  are  small 
amounts  of  griinerite,  garnet,  chlorite,  biotite,  and  muscovite.  In  some  cases 
the  chlorite  developed  from  the  griinerite  and  garnet.  While  the  quartz 
grains  show  undulatory  extinction  and  fracturing,  the  dynamic  effects  are 
not  so  great  as  would  be  expected,  and  the  appearance  of  the  section 
strongly  suggests  that  the  rock  was  largely  recrystallized.  Where  the 
sandstones  were  less  pure  there  developed  from  tliem  coarse-grained, 
typical  biotite-schists,  muscovite-schists,  and  chlorite-schists,  often  garnet- 
iferous.  In  these  rocks  we  have  a  somewhat  uniformly  granular  quartzose 
background,  through  which  developed  the  biotite,  muscovite,  and  chlorite. 
There  is  a  tendency  for  the  micaceous  minerals  to  be  concentrated  into  lay- 
ers, the  less  micaceous  zones  perhaps  corresponding  to  the  original,  more 
quartzitic  laminae.  Occasionally  the  quartzose  bands  have  a  distinct  oval 
or  lenticular  character,  as  if  each  represented  a  greatly  mashed  and  granu- 
lated quartz  pebble.  The  mica  bends  around  these  areas,  joining  at  their 
ends,  thus  presenting  a  mesh-like  appearance,  but  differing  from  a  mesh  in 
that  the  leaflets  of  mica  do  not  intersect.  In  some  of  the  slides  the  biotite, 
muscovite,  and  chlorite  are  all  in  large  blades  with  a  parallel  arrangement- 
In  other  cases  the  sericite  is  in  part  in  innumerable  minute  leaflets.  In 
certain  of  the  chlorite-schists  the  chlorite  leaflets  are  minutely  puckered  by 
the  folding  in  some  places,  and  in  other  places  the  stress  has  been  relieved 
by  minute  faulting  diagonal  to  the  schistosity.  Thus  we  have  a  cleavage 
in  one  direction  parallel  to  the  schistosity  and  a  fissility  diagonal  to  this. 
By  a  dying  out  of  the  micaceous  element  and  the  appearance  of  griinerite 


294  THE   MAKQUETTE   lEON-BEARlNG  DISTRICT. 

and  magnetite  these  schists  pass  into  the  Negaunee  formation.  In  some 
cases  there  are  iuterstratified  tyjjical  biotite-schists  and  griinerite-magnetite- 
schists.  These  biotite-schists  are  ordinarily,  however,  strongly  garnetiferous. 
The  garnet,  as  usual,  developed  in  large  individuals,  which  include  very 
numerous  granules  of  quartz.  Where  the  garnet  appears  the  biotite  is 
very  sparse,  so  that  we  have  a  ramifying  background  of  biotite  and  quartz, 
in  which  are  large  ganiet  individuals,  including  quartz  and  a  small  amount 
of  biotite.  In  the  schist-conglomerate  south  of  Republic  the  matrix  is  a 
completely  crystalline  mica-schist,  and  in  their  shapes  and  relations  to  the 
matrix  tlie  mashed  granite  pebbles  are  similar  to  the  quartz  areas  just 
described. 

RELATIONS   TO   ADJACENT   FORMATIONS. 

For  the  part  of  the  belt  running  from  Goose  Lake  to  near  Teal  Lake 
the  quartzite  occupies  a  place  between  two  slates.  It  was  suggested  that  the 
mud  of  the  Wewe  slate  began  to  deposit  because  by  the  upward  building  of 
the  limestone  the  waters  became  too  shallow  for  limestone  formation.  A 
continued  shallowing  of  the  water  may  have  gone  on  by  the  upbuilding  of 
the  slate  until  it  became  so  shallow  as  to  permit  the  waves  to  carry  coarse- 
grained sand,  when  the  sandstone  was  deposited  which  was  indurated  later 
into  the  Ajibik  quartzite.  In  places  it  may  be  that  local  elevations  occurred, 
raising  the  mud  above  the  water,  so  that  when  the  waves  next  overrode  it, 
it  yielded  fragments  of  compacted  mud  to  the  basal  horizon  of  the  quartzite. 
This  is  indicated  by  the  fact — discovered  by  Mr.  A.  E.  Seaman — that  in 
sec.  6,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXVII),  south  of  Carp  River,  the 
quartzite,  with  a  conglomerate  at  its  base  containing  slate  fragments,  rests 
with  slight  discordance  upon  the  slate.  Also  iuterstratified  with  the  quartzite 
for  a  few  feet  from  the  base  are  thin  belts  of  conglomerate  which  bear  frag- 
ments of  slate  identical  in  character  with  the  slate  below.  To  account  for 
the  full  thickness  of  the  sandstone,  it  is  supposed  that  subsidence,  if  inter- 
rupted at  all,  soon  began  again.  After  a  time  it  appears  that  the  rate  of 
subsidence  was  greater  than  the  rate  of  upbuilding,  so  that  following  the 
sand  deposits  there  was  another  time  of  mud  deposits.  Further  indicating 
such  a  subsidence  is  the  fact  that  above  this  shale  followed  the  nonfraormental 


RELATIONS  OF  THE   AJIBIK   QUARTZITE.  295 

iron-bearing  formation.  In  the  eastern  part  of  the  district  the  quartzite 
grades  above  into  a  slate,  and  below  it  rests  upon  another  slate. 

In  the  area  west  of  Goose  Lake  the  Wewe  slate,  as  has  been  said, 
appears  to  grade  up  into  the  Ajibik  qvxartzite,  in  many  places  the  boundary 
line  V)etween  the  two  being  somewhat  arbitrarily  placed. 

In  the  quartzite  range  in  sec.  29,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXII),  the  quartzite  rests  immediately  upon  the  Archean,  the  Wewe  slate 
not  appearing  between  the  two,  as  is  the  case  to  the  eastward.  This  is 
explained  by  the  fact  that  the  transgression  of  the  sea  was  from  the  east, 
but  it  is  not  impossible  that  the  lower  part  of  the  quartzite  is  really  the 
equivalent  of  the  upper  part  of  the  Wewe  slate,  sand  being  deposited  near 
shore  at  the  same  time  that  mud  was  being  deposited  offshore. 

East  of  Teal  Lake,  supposing  the  slate  belt  in  the  middle  of  the 
quartzite  to  belong  with  the  Wewe  slate,  there  is  a  transition  from  the  slate 
upward  into  the  quartzite.  West  of  Teal  Lake  it  has  been  seen  that  the 
inferior  formations  of  the  Lower  Marquette  series  were  not  deposited,  and 
therefore  that  the  quartzites  rest  directly  upon  the  Archean.  In  the  petro- 
graphical  description  it  has  been  indicated  that  here  basal  conglomerates 
occur.  North  of  the  west  end  of  Teal  Lake,  and  at  various  places  for  a 
few  miles  west,  the  actual  contacts  between  the  quartzite  and  the  green 
schists,  greenstone-conglomerates,  and  amygdaloids  of  the  Archean  are 
found.  One  of  the  best  localities  at  which  to  observe  this  contact  is  just 
north  of  the  west  end  of  Teal  Lake  (Atlas  Sheet  XXVII).  Here  the  green 
schist  strikes  approximately  east  and  west,  and  its  schistose  structure  dips  at 
a  high  angle — 75°  to  80° — to  the  south.  However,  the  contact  of  the  quartz- 
ite and  schist  dips  but  55°  to  the  south,  so  that  the  fibers  of  the  schist  abut 
against  the  contact  plane  at  an  acute  angle  (fig.  14).  Above  the  contact 
plane  is  a  genuine  basal  conglomerate,  the  pebbles  of  which  are  mainly 
derived  from  the  schist,  but  with  which  are  also  large  pebbles  of  quartz, 
some  of  them  8  or  9  inches  in  greatest  diameter.  Besides  the  green  schist 
and  quartzite  pebbles,  there  are  also  present  abundant  pebbles  of  a  more 
acid  schist  which  is  hke  the  acidic  schists  occun-ing  in  the  Northern  Com- 
plex north  of  the  stone  quarry  at  Carp  River.  There  can  be  no  doubt 
that  here  the  green  schist  had  become  foliated  and  was  deeply  truncated 
before  the  deposition  of  the  overlying  conglomerate. 


296 


THE  MARQUETTE  IRON  BEARING  DISTRICT. 


At  several  localities  for  a  half  mile  west  of  Carp  River  contacts  are 
also  found  between  the  quartzite  and  the  green  schist.  The  quartzite  near 
the  contacts  is  intensely  plicated,  but  wherever  an  opportunity  could  be 
found  to  get  at  the  junction  a  sharp  contact  between  the  two  rocks  was 
invariably  discovered.  In  only  one  place  was  the  plicated  quartzite  found 
in  any  other  position  than  on  the  south  slope  of  the  schist.  Here  it  wraps 
around  the  east  end  of  a  small  knob  of  schist,  and  is  found  on  the  north 
side  with  its  typical  characters.  This  occurrence  is  probably  explained  by 
regarding  the  green-schist  knob  as  a  headland  projecting  somewhat  diag- 
onally off  from  the  old  shore-line,  and  therefore  giving  a  bay  in  which  the 

detrital  material  could  be  deposited 
behind  the  schistose  rock.  When 
the  two  were  later  upturned  to  their 
present  inclination  the  tilting  would 
result  in  the  distribution  described. 

At  various  localities  east  of  Teal 
Lake  (Atlas  Sheet  XXX)  the  quartz- 
ite is  found  to  be  in  contact  with 
the  green  schist.  This  may  be  par- 
ticularly well  seen  just  west  of  the 
road  running  north  from  Negaunee 
and  east  of  the  gorge  of  the  Carp 
River.  The  relations  are,  however, 
essentially  the  same  as  at  the  Carp 
and  west  of  Teal  Lake,  with  the  exception  that  east  of  the  gorge  there  has 
been  such  intense  movement  near  the  contact  plane  that  the  basal  rock  has 
become  a  schist-conglomerate  which  closely  resembles  the  much  mashed 
green  schist  of  the  Northern  Complex.  It  is  difficult  to  say  exactly  where 
the  green  schist  ends  and  the  schist-conglomerate  begins.  In  discriminating 
between  the  fragmental  and  igneous  rocks  the  microscope  is  frequently 
of  considerable  assistance.  The  igneous  character  of  the  green  schist  in 
its  typical  form  is  plain,  while  the  fragmental  character  of  the  quartzite  is 
equally  evident ;  but  close  to  the  contact  even  the  microscope  fails  to  dis- 
criminate between  the  igneous  rocks  and  the  intensely  metamorphosed 


g  uDConfonnably  upon 


liELATIONS   OF  THE  AJIBIK   QUARTZITE.  297 

fragmental  rocks.  We  have,  then,  an  apparent  tranfjition  be,tveen  the 
green  schists  and  the  clastic  rocks  just  above,  as  we  have  an  apparent 
gradation  between  the  Mesnard  quartzite  and  the  granite-gneiss  south  of 
Mai'quette.  In  both  cases,  however,  the  conglomerates  along  the  contact, 
in  areas  in  wliicli  dynamic  action  was  not  so  severe,  reveal  the  true  nature 
of  the  relation,  and  show  that  the  downward  gradation  is  secondary,  and 
is  not  evidence  of  a  single  continuous  series  with  downward  progressing 
metamorphism. 

The  contacts  east  of  Teal  Lake  may  belong  ratlier  at  the  base  of  the 
Mesnard  quartzite  than  at  the  base  of  the  Ajibik  quartzite,  as  has  been 
explained  above,  but  the  connection  between  them  and  the  contacts  west 
of  Teal  Lake  is  so  close  that  their  description  Avas  deferred  to  this  place. 
Whatever  their  correlative  position,  all  of  the  contacts  along  this  belt  of 
conglomerate  mark  the  advance  of  a  shore-line,  from  the  east  toward 
the  west. 

The  intricate  structural  relations  which  obtain  between  the  quartzite 
and  granite  in  sees.  30  and  31,  T.  48  N.,  R.  28  W.,  and  in  sec.  25,  T.  48  N., 
R.  29  W.  (Atlas  Sheets  XV  and  XVIII),  have  already  been  described. 
Here,  along  the  irregular  dividing  line,  the  contacts  between  the  two  rocks 
are  found  at  numerous  localities.  In  many  cases  the  lowest  horizon  of  the 
quartzite  is  strongly  conglomeratic,  the  pebbles  of  the  conglomerate  being 
derived  mainly  from  the  immediately  subjacent  granite.  These  conglom- 
erates at  the  contacts  show  conclusively  that  the  granite  is  older  than  the 
quartzite  and  was  deeply  denuded  before  the  deposition  of  the  latter  forma- 
tion. However,  at  many  places  so  close  has  been  the  folding  and  so  gi-eat 
the  movement  along  the  contact  plane  that  the  quartzite  has  become  a 
quartz-schist,  closely  resembling  the  mashed  granite.  Further,  the  secondary 
schistose  structure  in  the  granite  and  that  in  the  quartzite  are  parallel,  and 
this  structure  is  particularly  prominent  just  at  the  contact  of  the  two  rocks. 
Here  again,  if  one  considered  only  certain  localities,  the  phenomena  might 
be  regai-ded  as  an  indication  of  the  downward  gradation  by  progressive 
metamorphism  of  the  quartzite  into  the  granite,  or  the  explanation  might 
be  given  that  the  granite  is  intrusive  within  the  quartzite.  However,  if  the 
contact  be  followed  throughout  its  various  windings,  and  the  phenomena 
carefully  studied,  the  only  conclusion  which  can  be  reached  is  that  the 


298  THE  MAEQUETTE  IRON-BBAKING  DISTRICT. 

quartzite  is  a  newer  formation  which  has  derived  its  detritus  in  largest 
measure  from  the  underlying  formation.  West  of  sec.  25  only  one  con- 
tact between  the  granite  and  quartzite  has  been  discovered,  but  the  latter 
near  the  granite  at  a  number  of  places  becomes  feldspathic  in  character, 
indicating  the  derivation  of  its  material  largely  from  the  subjacent  granite. 

The  southern  belt  of  the  Ajibik  quartzite  rests  unconformably  upon  the 
Archean  south  of  the  Cascade  range,  as  shown  by  the  presence  of  great 
basal  conglomerates,  the  bowlders  of  which  are  derived  from  the  immedi- 
ately subjacent  iron  formation.  The  only  actual  contact  here  found  is  in 
sec.  35  (see  p.  311).  As  first  observed  by  Wadsworth,  the  great  conglom- 
erate adjacent  to  the  Piatt  mine,  sec.  32,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXII),  containing  pebbles  of  granite,  basic  eruptive  rocks,  and  schists, 
each  identical  with  the  corresponding  kind  of  rock  in  the  Archean  to  the 
south,  proves  the  existence  of  this  unconformity.  Exactly  similar  phe- 
nomena are  found  in  sec.  34  (Atlas  Sheet  XXXV),  and  here  the  interval 
separating  the  basal  conglomerate  and  granite  is  but  a  few  paces.  In  sec.  28, 
T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVI),  the  movements  were  so  great  that 
the  conglomeratic  quartzite  which  here  occurs  was  changed  into  a  schist. 

The  contact  relations  of  the  Ajibik  quartzite  and  the  Archean  seen 
at  various  localities  strongly  suggest  that  in  many  cases  which  have  been 
explained  as  downward  gradation  by  metamorphism  of  a  sedimentary  into 
a  completely  crystalline  rock,  or  as  sedimentary  rock  intruded  by  granite, 
the  phenomena  ma}^  have  another  explanation.  If  the  metamorphism  in  the 
Marquette  district  had  been  so  severe  as  to  obliterate  the  conglomerates 
which  occur  at  various  places,  it  would  have  been  almost  impossible  to 
show  that  between  the  Lower  Marquette  series  and  the  Archean  there  is  a 
great  unconformity. 

For  the  southern  belt  the  overlying  formation  is  the  Negaunee,  and  the 
Ajibik  quartzite  or  conglomerate  grades  into  this  formation  by  interstratifi- 
cation,  there  being  in  some  cases  a  number  of  distinctly  interstratified  beds 
of  quartzite  or  conglomerate  and  jasper,  but  always  in  passing  to  higher 
horizons  the  jasper  becomes  predominant  and  the  fragmental  material  of  the 
Ajibik  formation  disappears.  In  the  northeast  part  of  sec.  28,  T.  47  N., 
R.  27  W.,  an  exceptional  transition  phase  is  a  ferruginous  mica-slate.     The 


RELATIONS   OF   THE    AJIBIK   QUARTZITE.  299 

character  of  the  usual  transition  may  be  well  seen  at  the  conglomerates 
occurring  south  of  the  Piatt  mine,  east  of  Cascade  Brook,  and  south  of  the 
Goodi-ich  and  Saginaw  mines,  in  sec.  19, T.  47  N.,  R.  27  W  (See  Section  VI.) 
Along  the  southern  belt  the  Ajibik  quartzite  for  much  of  the  distance  is 
narrow,  consisting  of  a  basal  conglomerate  alone,  or  of  a  basal  conglomerate 
with  a  comparatively  thin  belt  of  quartzite.  However,  the  belt  is  of  vari- 
able thickness,  this  doubtless  being  due  to  irregularities  of  the  Basement 
Complex  at  the  time  of  the  transgression  of  the  sea.  While  this  belt  of  fer- 
ruginous conglomerate  or  quartzite  is  placed  with  the  Ajibik,  it  is  probable 
that  much  of  it  really  belongs  in  time  to  the  Siamo  slate  or  to  the  Negaunee 
formation,  and  that  this  part  of  the  area  was  above  the  water  during  Ajibik 
time.  This  is  shown  to  be  highly  probable  by  the  occurrences  in  sec.  30, 
T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXII).  Here,  upon  opposite  sides  of  a 
narrow  s^mcline,  directly  below  the  Negaunee  formation  as  mapped,  are 
the  Ajibik  quartzite  and  Siamo  slate,  the  former  close  to  the  Palmer  gneiss. 
Near  a  shore  was  deposited  a  sandstone,  while  farther  from  the  shore  mud 
or  sandy  mud  was  being  deposited.  However,  as  this  southern  belt  is  litho- 
logically  like  the  Ajibik  quartzite,  and  the  part  which  belongs  in  time  with 
the  Siamo  slate  can  not  be  determined,  it  is  all  platted  as  Ajibik. 

THICKNESS. 

As  in  the  case  of  the  previous  formations,  it  is  exceedinglv  difficult  to 
give  any  accurate  estimate  of  the  thickness  of  the  Ajibik  quartzite.  As 
the  folding  is  very  complex  west  of  Goose  Lake,  where  the  most  continuous 
exposures  are,  any  computation  based  upon  the  breadth  of  outcrop  and 
average  dip  would  be  sure  to  give  conclusions  far  from  the  truth.  In  the 
belt  extending  north  and  east  from  this  area  there  are  no  continuous  expos- 
ures with  well-determined  dips  for  the  entire  breadth  of  the  formation. 
South  of  Carp  River,  in  sees  6  and  7,  T  47  N.,  R  25  W.  (Atlas  Sheet 
XXXVII),  there  is,  perhaps,  the  best  opportunity  to  make  an  approximate 
estimate.  Here  the  breadth  of  the  formation  appears  to  be  about  1,200  or 
1,300  feet  This,  with  a  dip  of  35",  would  give  a  thickness  of  about  700 
to  750  feet.  East  of  Teal  Lake  (Atlas  Sheet  XXX)  it  is  perhaps  possi- 
ble to  give  the  maximum  thickness  of  the  slate  and  (piartzite  from  tlie  base 


300  THE   MARQUETTE   IRON-BEARING  DISTRICT. 

upward,  but  how  much  of  these  exposures  belongs  with  the  lower  forma- 
tions it  is,  as  has  been  said,  impossible  to  state.  If  the  slate  included  with 
the  Wewe  slate  is  correctly  placed,  above  this  is  a  breadth  of  quartzite  of 
1,000  feet,  which,  with  an  average  dip  of  65°,  gives  a  thickness  of  about 
900  feet. 

INTERESTING   LOCALITIES. 

Michigamme  area. — Beginning  at  tlic  uortliwest,  there  are  numerous  expos- 
ures of  quartzite  just  south  of  the  Archean  from  the  west  end  of  the  area 
mapped  for  4  miles  to  the  east,  that  is,  to  sec.  21,  T.  48  N.,  R  30  W.  (Atlas 
Sheet  V).  The  rocks  hei'e  adjacent  to  the  granite  are  at  places  feldspathic 
quartzites,  but  the  oi'dinaiy  phases  are  vitreous  quartzites,  which  in  some 
cases  are  sericitic,  in  others  cherty. 

North  of  the  Michigamme  mine  the  quartzite  is  found  directly  in  con- 
tact with  a  wall  of  the  granite  and  hanging  upon  it.  The  rock  is  here  a 
coarse,  feldspathic,  gray  or  green  quartzite.  Tliree  feet  from  the  contact 
is  a  layer  of  conglomerate  10  to  15  inches  in  thickness,  which  contains 
pebbles  of  quartz,  the  larger  of  which  are  coarsely  crystalline,  but  the 
smaller  of  which  are  granulated.  In  its  upper  parts  the  Ajibik  quartzite 
is  interlaminated  with  the  Siamo  slate,  the  layers  of  quartzite  between  the 
layers  of  slate  being  1  to  6  inches  in  thickness. 

In  thin  section  the  quartzite  at  the  base  of  the  formation  has  a  finely 
crystalline,  sericitic,  kaolinic,  and  quartzose  background,  which  contains 
simple  and  complex  grains  of  quartz,  from  those  of  small  to  those  of 
large  size.  Numerous  flakes  of  biotite  and  blades  of  chloritoid  are  pres- 
ent. All  of  the  quartz  grains  show  undulatory  extinction  or  fracturing 
At  high  horizons,  between  the  grains  of  the  quartzite  there  is  abundant 
chlorite  and  garnet,  the  outlines  of  the  two  minerals  having  a  similar 
appearance.  Certain  areas  consist  in  part  of  chlorite  and  in  part  of  garnet. 
The  chlorite  is  apparently  an  alteration  product  of  the  garnet,  the  latter 
being  the  original  mineral  which  developed  within  the  rock.  In  the 
layers  of  quartzite  interstratified  with  the  slate  the  dynamic  effects  are  less 
marked  than  elsewhere  in  the  formation  of  this  part  of  the  area,  and  here 
the  quartz  grains  frequently  show  cores  and  distinct  enlargements.  The 
matrix  in  which  the  quartz  grains  are  set  is  similar  to  that  in  the  quartzite 


INTERESTING  LOCALITIES  OF   AJIBIK  QUARTZITE.  301 

adjacent  to  the  granite.  For  the  major  part  of  the  formation  in  this  general 
area  the  quartzites  are  very  crystaUine.  Distinct  cores  of  the  original  grains 
are  found  in  only  a  few  of  the  slides,  and  in  these  there  is  a  sericitic  back- 
ground. Apparently  when  the  original  sandstones  were  nearly  pure  the 
grains  were  pressed  against  one  another  so  strongly  as  to  result  in  granula- 
tion. In  the  most  crystalline  phases  there  is  a  finely  granular,  intricately 
interlocking  background  of  quartz,  in  which  is  a  small  amount  of  sericite 
and  chlorite.  In  a  less  crystalline  phase  the  original  quartz  grains  still 
exist,  but  they  have  been  mashed,  so  that  they  are  arranged  with  their 
longer  axes  in  a  common  direction.  As  usual,  the  rocks  are  cut  by  veins 
of  secondary  cherty  quartz. 

Broken  bluffs. — Tlicrc  arc  numerous  exposures  of  the  Ajibik  formation  in 
sees.  30,  31,  and  32,  T.  48  N.,  R.  28  W.  (Atlas  Sheet  XVIII).  This  is  the 
locality,  referred  to  on  pages  285-286,  where  the  peculiar  infolding  of  the 
quartzite  and  gneissoid  granite  occurs,  the  whole  series  of  folds  having 
isoclinal  dips  and  a  westward  pitch.  At  many  places  the  quartzite  formation 
is  exposed  in  contact  with  the  granite,  and  here  a  conglomerate,  bearing- 
numerous  quartz  pebbles,  many  large  feldspar  grains,  and  occasional  small 
granitic  pebbles,  is  found.  In  some  places  the  basal  rock  is  a  fine-grained 
feldspathic  quartzite,  the  granite  having  apparently  been  disintegrated  and 
broken  into  its  constituent  mineral  particles.  The  conglomeratic  parts  have 
a  feldspathic  quartzite  base  which  is  similar  to  the  nonconglomeratic  phases 
of  the  rock.  In  many  places  the  movement  during  the  folding  was  so 
great  as  to  entirely  granulate  the  quartz  pebbles,  diff"erent  specimens  show- 
ing all  gradations  between  coarse,  vitreous  vein  quartz  and  completely 
granulated,  opaque,  sugary  quartz.  In  this  mashed  phase  of  the  conglom- 
erate little  feldspar  detritus  is  seen.  If  it  was  originally  present  it  has 
become  decomposed.  However,  in  those  phases  of  the  rock  in  which  the 
pebbles  of  quartz  are  transparent  and  vitreous  the  large  detrital  feldspars 
are  abundant.  In  the  intermediate  phases  the  schist  background  contains 
numerous  roundish  but  flattened  areas  of  quartz,  the  rock  approaching  in 
its  appearance  a  fine-grained  augen-gueiss.  Examined  in  thin  section, 
the  quartzites  are  found  to  be  feldspathic.  In  the  less  mashed  phases  the 
feldspars  have  renewed  their  growth,  and  they  present  the  best  instances 


302  THE   MARQUETTE    IKON  BEARING   DISTRICT. 

of  feldspar  enlargement  in  the  Marquette  district.  The  pure  feldspathic 
quartzites  pass  into  those  which  are  micaceous  and  chloritic,  the  mica  and 
chlorite  having  largely  developed  at  the  expense  of  the  feldspar.  These 
micaceous  and  chloritic  quartzites  pass  by  interstratification  or  gradation 
into  the  mica-schists  and  chlorite- schists.  While  in  hand  specimen  these 
schists  appear  to  be  completely  crystalline,  in  thin  section  rounded  grains 
of  quartz  still  show  their  fragmental  character.  The  development  of  biotite 
and  chlorite,  with  the  separation  of  secondary  quartz  from  the  feldspar,  is 
beautifully  shown. 

Area  west  of  Teal  Lake. — Nortli  aud  wcst  of  the  west  end  of  Teal  Lake,  in 
sees.  33,  34,  and  35  (Atlas  Sheets  XXIV  and  XXVII),  there  are  very  numer- 
ous and  large  exposures  of  the  quartzite  for  a  distance  of  2i  miles.  The 
underlying  rocks  of  the  Archean  here  belong  to  the  Kitchi  formation,  this 
place  being,  in  fact,  its  typical  locality  and  here  occurring  nearly  all  of 
its  peculiar  phases.  At  a  number  of  localities  the  Ajibik  quartzite  is  found 
in  contact  with  or  close  to  the  Kitchi  rocks.  In  some  places  at  the  contact 
between  the  two  there  seems  to  be  only  a  minor  discordance,  but  a  close 
examination  shows  that  nowhere  do  the  two  formations  grade  into  each 
other.  On  the  other  hand,  there  is  always  a  perfectly  sharp  contact 
between  them,  although  Rominger  describes  the  two  formations  as  grading 
into  each  other.'  This  mistake  is  excusable,  because  the  Kitchi  formation 
is  here  a  tuff  closely  resembling  a  true  water-deposited  conglomerate. 

Near  the  north-south  quarter  line  of  sec.  34,  in  a  little  valley  between 
the  quartzite  on  the  south  and  the  green  schist  on  the  north,  a  contact  was 
found  between  the  two  formations  where  the  unconformable  relations  are 
jierfectly  clear.     This  contact  has  already  been  described  on  page  295. 

At  another  locality,  west  of  the  wagon  road  near  the  west  line  of  sec 
34,  the  quartzite  lies  on  the  south  side  of  the  green  schist  as  a  mere  skin, 
and  here  the  same  unconformable  relations  are  seen  as  in  the  center  of 
the  section.  In  one  case  the  plicated  quartzite  described  (pp.  297-298)  is 
found  wrapping  around  the  east  end  and  the  north  side  of  a  small  knob 
of  Kitchi  schist.    This  occurrence  is  believed  to  be  explained  by  a  headland 

'The  Marquette  iron  region,  by  C.  Rominger:  Geol.  Surv.  of  Michigan,  Vol.  IV,  Part  I, 
1878-1880,  pp.  37-39. 


INTERESTING   LOCALITIES   OF   AJllJIK   QUAKTZITE.  303 

projecting  somewhat  diagonally  from  the  old  shore-line  into  the  Ajibik  sea, 
thus  forming  a  bay,  and  detrital  material  was  deposited  upon  three  sides 
of  the  schist.  When  the  two  formations  were  upturned  to  the  nortli  and 
eroded,  the  rocks  assumed  their  present  relations. 

A  basal  conglomerate  grades  up  quickly  into  a  regularly  bedded, 
southward-dipping,  vitreous  quartzite,  which  shows  nearly  all  of  the  phases 
characteristic  of  the  formation,  including  ordinary  quartzite,  ferraginous 
quartzite,  veined  cherty  quartzite,  quartz-rock,  and  chloritic  quartzite.  At 
one  place,  at  the  quarry  just  west  of  the  Carp  River,  is  a  fine-grained 
conglomerate  6  or  8  inches  thick,  which  holds  very  numerous  fragments  of 
bright-red  jasper.  These  were  at  first  thought  to  have  been  derived  from 
the  Negaunee  formation  of  the  Lower  Marquette  series,  but  probably  they 
came  from  the  jasper  veins  in  the  Kitchi  schist. 

West  of  the  Carp  River,  in  passing  downward  from  the  topmost 
layers,  where  the  quartzite  is  regularly  bedded,  one  finds  them  becoming 
someAvhat  plicated,  then  more  plicated,  and  finally  closely  plicated  into  a 
series  of  minor  cross  folds,  with  axes  plunging  steeply  to  the  south.  In 
j^laces  near  the  contact  with  the  Kitchi  formation  this  plication  is  so  sharp 
that  reibungsbreccias  have  been  produced.  These  are  readily  discrimi- 
nated from  the  conglomerate,  as  no  pebbles  are  contained  in  them  other 
than  the  quartzite  pebbles,  and  because  the  brecciated  phases  grade  into  the 
nonbrecciated  phases  along  the  strike.  These  brecciated  rocks  have  been 
cemented  by  secondary  quartz,  and  by  a  large  amount  of  oxide  of  iron, 
so  that  they  have  a  strongly  ferruginous  appearance.  Because  of  their 
ferruginous  and  brecciated  character  they  have  been  thought  by  some 
geologists  to  lie  unconformably  below  the  ordinary,  regularly  bedded 
quartzite  of  other  parts  of  the  formation.  This  locality  gives,  therefore,  an 
excellent  illustration  of  the  rapid  change  from  areas  where  dynamic  effects 
are  small  to  those  where  they  are  profound.  It  is  to  be  noticed  that  the 
dynamic  effects  are  greatest  at  or  near  the  contact  with  the  underlying- 
Kitchi  schist.  This  contact  plane  was  apparently  one  of  weakness,  and 
therefore  near  it  the  major  readjustments  in  the  folding  took  place. 

In  thin  section  the  conglomerate  is  found  to  have  a  wide  variety  of 
pebbles,  derived  from  the  Kitchi  formation.     The  quartz  pebbles  in  no  case 


304  THE  MAKQUETTE   IKON-BEAKING  DISTRICT. 

prove  to  be  from  a  clastic  rock.  There  are,  however,  pebbles  of  finely 
crystalline  cherty  or  jaspery  quartz.  The  background  of  the  conglomerate 
is  slate  or  graywacke,  which  does  not  differ  in  its  character  from  the  slates 
and  graywackes  of  the  Wewe  formation  (described  on  pp.  265-269),  except 
that  a  large  amount  of  chlorite  has  developed,  and  in  some  cases  hornblende. 
The  quartzites  comprise  all  of  the  phases  described  in  the  general  descrip- 
tion (pp.  290-291),  but  the  less  mashed  and  nonbrecciated  phases  are  more 
common,  so  that  in  most  cases  the  fragmental  character  of  the  rocks  is 
evident  at  a  glance.  The  purer  quartzites  are  either  cemented  by  enlarge- 
ment or  by  enlargement  combined  with  interstitial  independent  quartz. 
These  purer  phases  vary  into  feiTuginous,  sericitic,  and  chloritic  quartzites, 
and  these,  by  an  increase  of  the  sericite  and  chlorite,  and  a  decrease  in  the 
size  of  the  quartz  grains,  into  novaculites  or  graywackes.  In  some  places 
a  small  amount  of  interstitial  hornblende  developed.  In  places  the  ledges 
are  cut  by  qviartz  veins  composed  of  intimately  intermingled  and  interlock- 
ing, finely  and  coarsely  crystalline  quartz.  The  slate  and  graywacke 
phases  are  largely  sericite-slates,  identical  with  those  of  the  Wewe  forma- 
tion. Like  them,  they  are  in  places  brecciated,  and  veined  by  secondary 
quartz  mingled  in  places  with  oxide  of  iron.  In  the  background  with  the 
chlorite  there  is,  in  some  specimens,  a  small  amount  of  liornblende. 

Area  east  of  Teal  Lake. — The  largcst  aud  uiost  coutiuuous  exposurcs  of  the 
formation  begin  north  of  the  east  end  of  Teal  Lake  and  extend  to  the  Carp 
River,  a  distance  of  abovit  3  miles  (Atlas  Sheet  XXX).  The  precipitous 
bluffs  making  up  this  area  are  known  as  the  Makwa  Hills.  For  the  central 
part  of  the  area  the  exposures  are  practically  continuous  from  the  bottom 
of  the  formation  to  the  top.  In  many  particulars  this  quartzite  is  similar 
to  that  west  of  Teal  Lake,  but  it  differs  from  that  in  being  much  thicker 
and  in  containing  many  interstratified  argillaceous  beds.  In  fact,  a  large 
portion  of  the  exposures  are  slate  and  graywacke  rather  than  quartzite. 
As  has  been  explained  in  the  previous  sections,  it  is  probable  that  the 
lower  horizons  are  really  the  time  equivalent  of  the  Wewe  slate,  the  Kona 
dolomite,  and  the  Mesnard  quartzite.  In  the  atlas  sheet  the  ridge  is  appor- 
tioned between  these  four  formations,  each  later  formation  overlapphig  the 


INTERESTING    LOCALITIES    OF   AJIBIK   QUAETZITE.  305 

preceding-,  ])ut  the  whole  constitutes  such  a  continuous  series  of  exposures 
that  it  has  been  thought  best  to  describe  them  together. 

As  west  of  Teal  Lake,  wherever  the  lowest  member  of  the  formation 
is  exposed  it  is  a  conglomerate.  The  pebbles  of  this  conglomerate  are 
mainl)'  of  white  quartz,  but  with  these  are  some  of  jasper.  One  contact  is 
found  a  short  distance  west  of  the  road  through  the  quartzite  range  running 
north  from  Negaunee,  at  about  1,425  to  1,450  steps  N.  and  450  to  500 
steps  W.  of  the  SE.  corner  of  sec.  31.  At  two  places  the  conglomerate 
was  seen  in  direct  contact  with  the  green  schist  of  the  Kitchi  formation. 
The  schistosity  of  the  schist  is  here  very  nearly  parallel  to  the  bedding  of 
the  quartzite,  and  there  is  no  apparent  unconformity.  At  the  Carj)  River 
section  the  lower  slaty  members  of  the  formation  have  been  so  strongly 
mashed  as  to  resemble  the  green  schist  below.  However,  there  always 
seems  to  l)e  a  difference.  The  schists  have  great  uniformity  in  appearance, 
their  lamiiife  being  of  the  same  character,  while  the  slates  are  comj^osed  of 
alternating  layers  of  diflPerent  characters.  Further,  the  schists  break  about 
equally  well  throughout  an  entire  zone,  parting  as  though  they  were  a 
mass  of  parallel  fibers,  rather  than  like  leaves,  as  do  the  slates.  Also 
east  of  the  Carp  River  the  lower  parts  of  the  slate  are  distinctly  conglom- 
eratic. Notwithstanding  these  differences,  at  one  place  it  is  exceedingly 
difficult  in  the  field  to  say  exactly  where  the  schists  end  and  the  slates 
begin.  However,  when  it  is  considered  that  along  this  same  horizon,  both 
at  the  State  road  conglomerate  to  the  east  and  south  of  the  Kitchi  Hills  to 
the  west,  there  is  the  clearest  sort  of  structural  break,  it  can  not  be  doubted 
that  the  same  is  true  for  this  area.  As  is  so  frequently  the  case,  the  major 
accommodation  took  place  along  the  contact  plane.  The  fragmental  rock 
and  the  Kitchi  formation  were  so  mashed  that  a  parallel  schistosity  was 
produced  in  them.  Fortunately,  while  the  matrix  has  become  crystalline  in 
the  sedimentary  rocks,  the  white  quartz  pebbles  were  sufficiently  resistant 
to  show  their  fragmental  character. 

In  a  section  at  the  widest  part  of  the  ridge,  in  passing  to  higher  hori- 
zons the  conglomerate  usually  varies  quickly  into  a  mica-slate,  and  this 
passes  into  the  typical  quartzites  of  the  formation.     South  of  this  quartzite 

MON  XXVIII 20 


306  THE  MARQUETTE   lEON-BEARmG  DISTRICT. 

is  a  belt  of  red  graywacke.  In  it  slatiness  and  schistosity  have  develo^Dsd. 
The  slate  is  folded  into  minute  crinkles,  and  in  places  cross -folded  by 
east-west  pressure.  It  is  also  fractured,  and  the  cracks  and  veins  are  filled 
with  cherty  or  vein  quartz.  This  belt,  on  account  of  its  uniform  character 
and  schistosity,  macroscopically  resembles  closely  the  schists  of  the  Kitclii 
formation.  South  of  the  belt  of  graywacke  is  a  broad  belt  of  reddish  and 
greenish  slates,  interstratified  with  occasional  beds  of  quartzite  and  cherty- 
looking  quartz.  In  this  part  of  the  formation,  in  a  single  ledge,  black  slate, 
red  slate,  novaculite,  fine-grained  red  quartzite,  and  cherty  quartzite  may 
be  seen  regularly  interstratified.  As  a  result  of  the  movements,  the  slates 
in  many  places  take  on  a  rather  crystalline  aspect.  The  whole  is  usually 
veined  with  white  quartz  and  cherty  quartz,  and  altogether  the  rocks  have 
a  very  crystalline  aspect.  At  one  place  a  stratum  of  slate  abuts  directly 
against  the  quartzite  to  the  west,  showing  that  there  is  here  a  minor  trans- 
verse fault. 

The  southernmost  exposures  of  the  formation  are  in  sec.  32,  where 
the  belt  is  the  broadest,  and  they  are  vitreous  quartzites;  and  here  occur 
peculiar  rocks,  which  at  first  sight  were  taken  for  conglomerates,  having 
a  quartzite  matrix  and  quartzite  pebbles,  the  matrix  being  stained  by  oxide 
of  iron.  When  this  belt  was  closely  examined  the  peculiar  conglomerate 
was  found  to  be  dynamic.  Under  the  stress  to  which  the  rock  was  sub- 
jected it  fractured  in  a  spheroidal  manner,  each  of  the  spheroids  at  first 
sight  appearing  to  be  a  pebble,  but  close  examination  shows  that  many  of 
them  are  attached  at  some  place  to  the  matrix.  This  conglomeratic  rock, 
when  traced  along  the  strike,  is  found  to  become  less  and  less  fractured, 
and  to  grade  into  the  ordinary  quartzite. 

The  whole  set  of  beds  making  up  the  bluff's  has  a  rather  uniform  dip 
to  the  south,  the  dip  perhaps  being  somewhat  higher  on  the  northern  side, 
near  the  contact  with  the  green  schists,  than  farther  south.  The  dips 
observed  vary  between  57°  and  70°  to  the  south.  It  is  perhaps  possible 
that  the  lower  bed  of  conglomerate  and  quartzite  represents  the  Mesnard 
formation;  the  red  gray wacke,  the  Kona  dolomite ;  the  interstratified  slates, 
gi-aywackes,  and  quartzites,  the  Wewe  slate;  and  the  upper  quartzite,  the 


INTERESTING   LOCALITIES   OF   AJIBIK   QUARTZITE.  307 

Ajibik.  If  this  be  true,  the  three  lower  formations  must  one  by  one  die 
out  to  the  west,  each  higher  formation  overlapping  the  one  next  lower. 

In  thin  section  nearly  all  of  the  more  altered  phases  of  the  Ajibik 
quartzite,  the  Wewe  slate,  and  the  Mesnard  quartzite  are  found.  Their 
descriptions  will  not  be  here  repeated. 

Eastern  area. — The  ucxt  promiuent  cxposures  to  the  east  are  in  the  north- 
ern  part  of  sec.  6  and  the  western  part  of  sec  5,  T.  47  N.,  R.  25  W.  (Atlas 
Sheet  XXXVII).  The  numerous  ledges  are  very  nearly  pure  quartzites^ 
or  ferruginous  quartzites.  None  of  them  are  changed  into  sericitic  quartz- 
schist  or  into  cherty  quartzite.  Tlie  folding  to  which  they  have  been 
subjected  has  merely  cracked  the  rocks,  and  along  these  cracks  small,  sec- 
ondary quartz  and  iron  oxide  veins  have  formed. 

In  the  southeastern  part  of  sec.  6  and  the  northern  part  of  sec.  7  there 
are  large  exposures  of  quartzites,  which  in  most  respects  are  similar  to 
those  east  of  Teal  Lake.  However,  in  the  exposure  just  south  of  the 
Carp  River  the  quartzite  is  found  to  rest  upon  the  Wewe  slate  and  to 
bear  fragments  of  it.  Apparently  there  is  a  very  slight  discordance 
between  them.  The  basal  conglomerate  is  only  a  few  feet  in  thickness^ 
and  quickly  passes  up  into  a  gray  slate,  which  bears  several  thin  layers 
of  conglomerate.  The  interstratified  slate  and  conglomerate  in  turn  pass 
up  into  interlaminated  slate  and  ferruginous  quartzite,  and  this  into  the 
ordinary  quartzite.  So  far  as  the  structural  evidence  is  concerned,  the  dis- 
cordance is  so  slight  as  to  have  little  significance.  The  phenomena  could' 
be  explained  by  the  mud  rising  above  the  water  for  a  short  time,  becoming- 
slightly  compacted,  and  then,  when  buried  beneath  the  water,  furnish- 
ing fragments  to  the  overlying  formation.  Such  an  occuri'ence  might  be 
extremely  local.  On  the  west  side  of  this  exposure  the  Wewe  slate  is  found 
to  be  faulted  against  the  quartzite.  This  fault,  or  another  ruiming  north- 
west and  southeast,  has  displaced  the  quartzite  and  a  small  part  of  the 
underlying  Wewe  slate  to  the  southward  for  a  distance  of  about  an  eighth 
of  a  mile,  thus  making  the  quartzites  of  the  south  side  of  the  river  stand 
directly  opposite  large  exposures  of  the  Wewe  slates  of  the  north  side  of 
the  river.     Apparently  the  river  follows  approximately  the  fault  line.     An 


308  THE    MARQUETTE    IRON  BEARING   DISTRICT. 

.examination  of  the  thin  sections  of  the  basal  conglomerates,  described  on 
page  292,  confirms  the  conclusion  that  the  mud  was  but  slightly  compacted 
;at  the  time  it  yielded  fragments  to  the  Ajibik  quartzite.  The  outlines 
'of  the  pebbles  are  minutely  irregular,  the  projections  filling  the  spaces 
^between  adjacent  fragmental  quartz  grains  and  thus  contrasting  with  the 
clean-cut  forms  of  well-indurated  waterworn  pebbles. 

Large  exposures  of  the  Ajibik  formation  occur  in  sees.  11  and  12, 
T.  47  N.,  R.  26  W.,  and  on  both  sides  of  the  northwest  arm  of  Goose  Lake 
'(Atlas  Sheet  XXXIV).  These  ledges  are  all  rather  pure,  fresh-looking 
.quartzites. 

The  foregoing  series  of  ledges  connecting  Goose  Lake  and  Teal  Lake 
are  all  in  the  eastern  part  of  the  great  westward-plunging  syncline,  where 
the  minor  plications  are  slight.  Corresponding  with  this  in  thin  section,  we 
find  the  normal  phase  of  quartzite  to  be  the  pure  enlargement  kind,  although 
secondary  independent  quartz  was  also  deposited  in  the  interstices.  While 
the  dynamic  effects  are  slight,  nearly  all  of  the  qiiartz  grains  show  undula- 
tory  extinction,  and  many  of  them  are  distinctly  fractured. 

wewe  Hills. — Tlic  ncxt  large  bunch  of  exposures  is  in  sees.  22  and  23, 
T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXV).  The  northernmost  of  these  ledges 
-are  similar  to  those  just  described.  The  large  exposures  in  the  southeastern 
part  of  this  area  constitute  a  westward-plunging,  isoclinal,  synclinal  fold, 
.  and,  as  a  consequence  of  this,  many  of  the  quartzites  become  sericitic,  cherty, 
.  and  vitreous,  and  some  of  them  schistose.  At  certain  places  along  the  south 
border  of  this  set  of  ledges  the  rock  is  a  distinct  reibungsbreccia  which 
•  closely  simulates  a  conglomerate.  Indeed,  this  breccia  was  at  first  taken 
for  a  basal  conglomerate  resting  upon  an  older  rock.  A  closer  examination, 
however,  showed  that  while  many  of  the  fragments  have  been  shattered  in  a 
spheroidal  manner,  many  others  are  angular  or  subangular,  and  all  are  similar 
to  the  adjacent  phases  of  quartzite.  As  usual,  these  breccias  are  cemented 
with  vein  quartz,  cherty  quartz,  and  the  oxides  of  iron.  The  latter  are 
naturally  more  abundant  in  the  matrix  than  in  the  fragments,  thus  gi\'ing 
to  the  former  a  dark  color  in  which  the  fragments  stand  out  sharply. 

Ajibik  Hills. — The  next  great  ledges,  mainly  in  sees.  27,  28,  and  29, 
'T.47  N.,  R.  26  W.  (Atlas  Sheets  XXXII  and  XXXV),  are  the  Ajibik  Hills, 


INTERESTING    LOCALITIES    OF   AJIBIK   QUAKTZITE.  3U9 

u})on  wliicli  occur  tlie  tyjjical  exi)osures  of  tlie  formation.  Tliese  are  exceed- 
dingly  i)recipitous  ridges,  very  rough  in  detail,  the  different  ledges  l)reaking 
off"  with  vertical  cliffs  or  with  A-ery  steep  slopes,  anil  each  large  ridge  is 
made  up  of  many  smaller  ones.  The  roughness  would  hardly  be  exceeded 
if  the  ridges  were  made  by  piling  up  at  random  a  vast  number  of  gigantic 
blocks,  except  that  the  bluffs  are  somewhat  rounded  by  glacial  abrasion. 
It  is  difficult  to  find  ridges  more  fatiguing  to  cross  than  these.  One  is  not 
able  to  keep  his  elevation,  but  after  climbing  one  ridge  he  is  obliged  to 
descend  into  a  steep  ravine,  only  to  climb  another  precipitous  slope  which 
rises  somewhat  higher  than  the  first,  to  again  descend  a  sharp  declivit}'. 

As  the  formation  is  directly  in  contact  with  the  Archean  in  sec.  29  and 
rests  upon  the  Wewe  slates  in  sees.  27  and  28,  and  the  folding  was  locally 
severe,  nearly  all  phases  of  the  formation  are  found.  In  sec.  29  the  basal 
conglomerate  is  made  up  of  Archean  debris.  In  sees.  27  and  28  Ave  have 
the  transitional  variety  between  the  Wewe  slate  and  the  Ajibik  quartzite. 
Here  in  the  quartzite  are  interstratified  novaculites,  slates,  and  graywackes. 
Certain  of  the  quartzites  in  areas  of  relief  were  but  little  affected  by 
dynamic  forces,  being  ordinary  fresh  quartzites.  Others  were  fractured 
extensively  in  both  a  major  and  a  minor  way,  thus  producing  the  veined 
cherty  quartzites.  In  other  places  the  fracturing  Avent  so  far  as  to  pro- 
duce a  dynamic  breccia  exactly  similar  to  breccias  in  sec.  22  (p.  308). 
In  a  number  of  places  also  the  fracturing  resulted  in  the  production  of 
spheroidal-looking  fragments,  Avhich  are  set  in  an  iron-stained  matrix, 
thus  giving  a  very  conglomeratic  appearance.  At  numerous  places  in  the 
grayAvacke-like  phases  a  schistosity  developed  as  a  result  of  the  mashing,, 
while  in  the  overlying  beds  of  purer  quartzite  the  pseudo-conglomerates  or 
breccias  were  produced.  We  thus  have  at  first  sight  a  vertical  schistose 
rock  overlain  by  a  conglomerate  which  occasionally  bears  fragments  of  the- 
schist.  The  a^jpearance  of  a  stnictural  break  was  so  great  that  at  a  first, 
and  even  a  second,  examination  it  was  confidently  believed  that  there  was 
here  a  great  unconformity  between  a  schist  series  and  a  quartzite-conglom- 
erate  series;  but  a  detailed  and  close  examination  left  no  doubt  that  the 
peculiar  phenomena  were  the  different  effects  of  dynamic  forces  in  an  argil- 
laceous and  a  nonargillaceous  rock — in  the  first,  floAvage  and  schistosity 


310  THE  MARQUETTE  IRON-BEARING  DISTRICT. 

resulting;  in  the  second,  sliattering.  These  phenomena  are  best  seen  in 
the  NW.  J  sec.  28,  especially  along  the  southwestern  slopes  of  the  IjlufiFs; 
and  a  still  further  complication  is  here  found,  since  apparently  the  true 
Archean  schist  does  appear  at  one  place  below  the  quartzite-conglomerate. 

It  is  possible  that  in  the  Ajibik  Hills  there  are  small  areas  of  the 
Groodrich  quartzite  which,  as  a  consequence  of  the  removal  by  erosion  of 
the  Negaunee  formation,  were  deposited  upon  the  Ajibik  quartzite.  This  is 
suggested  by  certain  little-mashed  quartzite  exposures,  which  contain  jasper 
pebbles.  But  in  no  case  could  such  suspected  later  quartzite  be  certainly 
discriminated  from  tlie  Ajibik  quartzite ;  so  all  are  mapped  as  belonging 
to  the  older  formation. 

To  give  the  microscopical  characters  of  the  different  phases  of  rocks 
on  the  Ajibik  Hills  would  be  practically  to  repeat  the  general  description 
{pp.  289-294),  as  nearly  all  phases  of  the  formation  ax'e  here  found. 

Goose  Lake. — Eastward  along  this  belt  the  next  bunch  of  ledges  found  is 
south  of  the  southeast  arm  of  Goose  Lake,  in  sees.  23  and  24  (Atlas  Sheet 
XXXV).  The  quartzites  are  underlain  by  the  We  we  slates,  and  between 
iire  the  transition  forms.  The  quartzites  suffered  great  deformation,  and 
consequently  little-altered  quartzites  are  rare,  and  the  cherty  quartzites, 
quartz-rocks,  and  quartzite-breccias  are  particularly  abundant.  The  veins 
in  this  area  are  iinusually  large  and  numerous,  and  they  are  filled  to  moi'e 
than  a  usual  degree  by  secondary  hematite  and  magnetite.  The  larger 
of  these  veins  and  the  most  brecciated  phases  of  the  quartzites  simulate 
ferruginous  chert  or  jasper,  and  the  abundance  of  iron  oxide  has  led  to 
prospecting  in  a  number  of  localities. 

In  the  northeast  part  of  sec.  24,  surrounded  on  tln-ee  sides  by  the 
Wewe  slates,  is  a  great  ledge  of  quartzite,  precisely  similar  to  the  ledges 
•southwest  of  Goose  Lake.  Here,  however,  are  particularly  well  seen  the 
interstratifications  of  the  slate  and  quartzite  and  the  different  maimers  in 
which  the  folding  affected  the  vitreous  rock  and  the  slates.  The  folded 
Wewe  rocks  passed  into  mica-slates,  with  a  nearly  vertical  cleavage,  which 
stops  abruptly  upon  reaching  the  quartzite  layers.  These  quartzites  were 
shattered  and  cemented  by  quartz  and  iron  oxide. 

Cascade  area. — Passiug  to  thc  southem  belt  of  the  Ajibik  quartzite,  we 
find   numerous   exposures   at  various  points  from  the  northwest  jjart  of 


INTERESTING  LOCALITIES   OF   AJIBIK   QUAKTZITE.  311 

sec.  36  to  the  west  part  of  sec  32,  T.  47  N.,  R.  26  ^V.  (Atlas  Sheets  XXXII 
and  XXXV). 

Hanging  on  the  west  face  of  the  great  ledges  in  the  NW.  ^  sec.  35 
is  a  coarse  conglomerate.  This  conglomerate  passes  up  into  the  pure 
vitreous  quartzite.  The  Palmer  formation  is  here  a  white  sericite-schist, 
cut  by  granite  veins,  brecciated,  and  cemented  with  coarse  vein  quartz, 
cherty  quartz,  and  ferruginous  chert  or  jasper.  All  of  these  materials 
are  abundantly  found  in  the  conglomerate,  and  that  they  are  derived 
from  the  underlying  formation  can  not  be  doubted.  Farther  west  along 
the  contact  between  the  Palmer  gneiss  and  the  Ajibik  formation,  in  the 
northeast  part  of  sec.  34  is  a  great  bluff  of  conglomerate,  containing  huge 
bowlders  of  the  white  sericitic  quartz-schist  and  chert,  and  also  irregular 
masses  of  jasper.  A  short  distance  to  the  south  are  found  liuge  ledges 
of  the  Palmer  gneiss.  This  conglomerate  appears  clearly  to  be  the  basal 
conglomerate  of  the  Ajibik  quartzite.  However,  it  is  not  positively  certain 
that  the  conglomerate  in  sec.  35  may  not  be  the  Goodrich  quartzite  resting 
upon  the  Basement  Complex,  as  a  consequence  of  the  removal  of  the 
Lower  Marquette  series  in  the  erosion  interval  l^etween  Lower  and  Upper 
Marquette  time. 

Near  the  line  between  sees.  32  and  33  a  conglomerate  again  occurs, 
which  grades  up  into  quartzite.  South  and  a  little  west  of  the  Piatt  mine, 
in  sec.  32,  is  found  a  great  conglomerate  near  the  base  of  the  formation, 
which  has  been  described  by  Wadsworth.  This  conglomerate  occurs  in  a 
precipitous  southward-facing  bluff,  and  is  a  mass  of  well-rounded  pebbles 
and  bowlders  cemented  by  a  sparse  matrix.  About  50  steps  south  of  the 
conglomerate  is  a  dense,  nearly  massive,  green  rock,  which  is  cut  through 
and  through  by  granite  veins.  In  the  vicinity  other  varieties  of  the  Palmer 
gneiss  occur.  In  the  conglomerate  the  predominant  pebbles  and  bowlders 
are  of  rocks  identical  in  character  with  those  found  in  the  Palmer  formation 
just  to  the  south.  In  the  conglomerate  there  are  also  quartz  pebbles,  and 
its  upper  parts  alternate  with  layers  which  approach  jasper.  In  passing  to 
higher  horizons  the  jasper  layers  become  more  and  more  prevalent,  luitil 
they  are  predominant.  In  the  conglomerate  are  no  undoubted  jasper 
pebbles,  but  there  are  roundish  areas  of  jasper  or  chert  which  appear  to 
be  secondary  concentrations. 


312  THE   MARQCTETTE   lEOX-BEAEING  DISTRICT. 

A  short  distance  east  of  the  Cascade  Brook,  in  the  valley  between  the 
Negaunee  formation  and  the  Palmer  gneiss,  is  a  conglomerate  which  con- 
tains pebbles  of  many  kinds  from  the  Basement  Complex,  including  green 
schist  and  sericite-schist,  precisely  similar  to  rocks  in  the  Palmer  formation 
at  the  Brook  section.  This  conglomerate  quickly  grades  up  into  jasper, 
there  being,  however,  at  many  places  several  alternations  of  conglomerate 
or  quartzite  and  jasper  before  the  typical  banded  jasper  is  reached.  On  the 
west  side  of  the  old  open  pit  of  the  Volunteer  or  old  Cascade  mine  is  a 
conglomerate  which  again  contains  detritus  from  the  Palmer  gneiss,  but  it 
is  rather  probable  that  this  conglomerate  belongs  to  the  Ishpeming  for- 
mation, and  that  the  entire  Lower  Marquette  series  has  been  removed 
by  erosion.  Adjacent  to  the  center  of  sec.  30  and  to  the  westward  are 
large  exposures  of  the  Ajibik  quartzite,  and  at  one  place  this  becomes 
conglomeratic. 

Sees.  27  and  28,  T.  47  N.,  R.  27  w. — A  sliort  distance  northwest  of  the  center  of  sec. 
28,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVI),  on  the  north  side  of  the  expos- 
ures, is  a  knob  of  quartzite-conglomerate,  and  south  of  this,  a  little  farther  up 
the  hill,  is  a  white  mashed  gneiss,  which  at  so  many  places  is  the  uppermost 
member  of  the  Palmer  formation.  In  the  NW.  ^  sec.  27  and  in  the  NE.  | 
sec.  28  are  ferruginous  mica-slates  interstratified  with  amygdaloids.  As 
platted  on  the  ground,  there  are  three  belts  of  amygdaloid.  These  slates 
when  first  examined  were  thought  to  have  a  regular  east-west  strike  and  a 
uniform  northern  dip.  When  the  ledges  were  closely  examined  the  slates 
were  seen  to  be  intricately  folded  into  a  series  of  northern-dipping  isoclinal 
folds,  the  axes  of  which  plunge  steeply  to  the  east.  A  specimen  from  one 
of  the  more  open  of  these  folds  shows  a  difference  of  dip  of  only  32° 
between  the  two  legs  (PI.  XXXV,  fig.  1).  While  there  is  everywhere  a 
northern  dip  and  an  east-west  strike,  the  same  belt  of  slate  is  repeated 
many  times,  and  the  question  arises  whether  the  three  apparent  belts  of 
amygdaloid  are  not  really  one,  being  repeated  by  isoclinal  folding. 

When  studied  in  thin  section,  these  slates  are  found  to  be  exactly  like 
the  heavily  hematitic  and  magnetitic  slates  which  occur  at  the  transition 
horizon  of  the  Siamo  slate  and  the  Negaunee  formation.  These  are 
described  on  a  subsequent  page. 


INTERESTING   LOCALITIES  OF   AJIBIK   QUARTZITE.  313 

Republic  and  Southwest  tongues. — TliG  ratliei'  uuusual  pliascs  of  the  Ajibilc 
quartzite  occurring-  in  the  southwestern  part  of  the  area,  along  the  Republic 
and  Southwest  tongues,  are  described  in  the  general  description  (pp.  287, 
289,  293)  and  in  Chapter  VI,  upon  the  Republic  trough.  Near  Republic 
occurs  one  of  the  best  basal  conglomerates  in  contact  with  the  Archean  in 
the  district.  Also  the  coarse  quartzite  just  above  this  conglomerate  is  worthy 
of  mention.  It  is  gray,  massive,  crystalline-looking,  and  appears  in  hand 
specimen  to  contain  griinerite.  When  examined  in  thin  section  it  is  found 
to  be  composed  mainly  of  coarsely  crystalline,  interlocking  quartz.  The 
particles  show  undulatory  extinction,  but  no  granulation.  Some  of  them 
have  a  i-ouudish  appearance,  but  no  distinct  cores  are  observable.  Between 
these  grains  and  included  in  some  of  them  are  clusters  of  griinerite,  garnet, 
chlorite,  and  magnetite  crystals.  Some  of  the  garnet  crystals  are  so  large 
as  to  inclose  a  number  of  grains  of  quartz.  The  griinerite  in  radiating 
blades  penetrates  the  quartz  grains  in  all  directions.  Apparently  the  chlo- 
rite is  a  secondary  product,  which  has  developed  in  part  from  the  garnet 
and  in  part  from  the  griinerite.  The  relations  of  these  minerals,  and  par- 
ticularly the  griinerite,  to  the  quartz  strongly  suggest  that  the  rock  has 
largely  recrystallized. 

Sees.  29  and  30,  T.  48  N.,  R.  27  w. — Moi'o  tliau  a  mile  north  of  the  main  northern 
belt  of  the  Ajibik  quartzite  is  an  isolated  quartzite  ridge  about  a  half  mile 
long,  bounded  on  the  north  by  peridotite  and  on  the  south  by  the  Kitchi 
schist.  The  rock  strikes  northeast-southwest  and  dips  to  the  southeast 
at  an  angle  of  25°.  This  ridge  in  places  is  conglomeratic.  Both  the 
quartzite  and  quartzite-conglomerate  are  similar  macroscopically  and 
microscopically  to  the  rocks  west  of  Teal  Lake,  and  are  placed  with  the 
Ajibik  formation  on  lithological  grounds.  The  exceptional  position  of  the 
area  is  probably  due  either  to  overfolding  or  to  faulting. 

SECTION  v.— THE  SIAMO  SLATE. 

The  Siamo  slate  is  so  called  because  abundant  exposures  occur  between 
the  Ajibik  quartzite  and  the  Negaimee  formation  on  the  Siarao  Hills,  just 
south  of  the  west  part  of  Teal  Lake  (Atlas  Sheet  XXVII),  and  because  the 
most  typical  rock  of  the  formation  is  a  slate,  although  locally  it  passes  into 
a  graywacke,  or  into  a  rock  approaching  a  quartzite. 


314  THE  MAKQUETTE   IRON  BEARING  DISTRICT. 

DISTRIBUTION,    EXPOSURES,  AND    TOPOGRAPHY. 

Beginning  at  the  north  and  west  (Atlas  Sheet  IV),  the  first  exjjosures 
of  this  rock  occur  between  the  Ajibik  quartzite  and  the  Negaunee  formation 
north  of  the  Michigamme  mine.  From  this  place  the  formation  stretches  in 
a  general  easterly  course  for  a  number  of  miles  to  sec.  33,  T.  48  N.,  R.  26  W., 
east  of  Teal  Lake.  East  of  sec.  33  is  a  broad  area  of  the  formation, 
extending  to  sec.  5,  T.  47  N.,  R.  25  W.,  the  slate  being  the  tojimost  member 
of  the  great  westward-plunging  syncline.  On  the  south  side  of  this  area 
the  formation  divides  into  two  parts.  The  eastern  arm  swings  to  the  south 
and  southwest,  past  the  northwest  arm  of  Goose  Lake ;  thence  west,  south- 
west, and  south  to  sec.  29,  T.  47  N.,  R.  26  W.;  thence  south  about  the  west 
end  of  the  anticline  made  by  the  Ajibik  quartzite;  and  thence  east  to  the 
sand  plain  in  sec.  23,  T.  47  N.,  R.  26  W.  The  western  area  extends  west 
through  sec.  3,  and  in  sees.  4,  5,  8,  and  9,  T.  47  N.,  R.  26  W.,  constitutes  a 
broad  dome  with  minor  folds.  From  sec.  9  an  arm  extends  southwest, 
terminating  as  a  plunging  anticlinal  dome  in  sec.  20.  Farther  southwest,  in 
sees.  19  and  30,  is  another  area,  which  was  probably  originally  continuous 
with  the  area  terminating  in  sec.  20. 

The  slate,  being  a  soft  formation,  is  not  well  exposed  tlnoughout,  but 
at  various  localities  where  it  is  a  mica-slate  or  a  coarse  graywacke  the 
ledges  are  numerous.  Upon  the  whole,  however,  the  formation  occupies 
the  lowlands  between  the  more  resistant  Ajibik  quartzite  and  Negaunee 
formation.  North  of  the  Michigamme  mine  there  are  a  number  of  expos- 
ures; from  this  place  to  Teal  Lake  there  are  few.  However,  south  of  the 
west  arm  of  Teal  Lake  are  the  Siamo  Hills,  which  give  the  name  to  the  for- 
mation and  upon  which  outcrops  are  abundant.  The  iron  formation  is  here 
soft,  and  occupies  even  lower  ground  than  the  slate.  East  of  Teal  Lake  there 
are  many  exposures  of  the  formation  south  of  the  quartzite  range.  The 
exposures  become  more  and  more  sparse  in  passing  east,  but  they  are  still 
frequent  to  sec.  35,  T.  48  N.,  R.  26  W.  From  this  place  southward  there  are 
no  more  natural  outcrops  until  the  Carp  River  is  reached,  south  of  which 
a  number  of  exposures  occur,  but  with  long  intervals  between  them.  As 
the  belt  swings  to  the  west,  following  the  western  arm,  the  outcrops  in 


FOLDING    OF   THE    SIAMO   SLATE. 


315 


sees.  3,  4,  .5,  8,  and  9,  T.  47  N.,  R.  26  W.,  are  very  numerous.  The  formation 
here  contains  much  graywacke,  and  is  therefore  resistant,  and  we  have  a 
rough  and  elevated  area  surrounded  by  the  less  resistant  iron  formation. 
In  the  southwesterly  extension  of  the  belt  prominent  exposures  occur  in 
the  center  of  sec.  20.  In  the  arm  which  swings  southwesterly  from  sec.  2, 
around  the  Ajibik  quartzite,  to  the  sand  plain,  the  outcrops  are  few,  and 
the  land  occupied  by  the  belt  is  low.  In  sees.  19  and  30  there  are  numerous 
exposures,  and  the  area,  as  a  whole,  is  one  of  elevation. 

FOLDING. 

Beginning  at  the  west,  the  northern  belt  of  the  formation  has,  upon 
the  whole,  a  southern  dip.     However,  when  the  ledges  are  examined  in 


'^<^ik\\{\^ 


detail,  it  is  found  that  the  rocks  are  in  a  set  of  minor  rolls,  the  dips  some- 
times being  to  the  north,  sometimes  to  the  south.  The  latter  are  more 
persistent  because  of  the  general  south  dip  of  the  formation,  and  therefore 
more  conspicuous  (fig.  15).  Also,  in  places  where  the  folds  are  overturned, 
the  horizontal  or  northern  dips  iipon  the  tops  of  the  anticlines  and  the 


Fig.  16.— Eolations  of  schistosity  and  bedding  in  SUmo  slate. 

bottoms  of  the  synclines  turn  so  quickly  to  the  general  southwest  direction 
as  to  be  easily  overlooked.  This  is  especially  true  where  there  is  a  schis- 
tosity parallel  to  the  prevalent  dip  (fig.  16).  To  the  subordinate  folding 
is  doubtless  due  the  very  greatly  varying  width  of  the  formation.  These 
minor  rolls  may  be  particularly  well  seen  south  of  the  west  end  of  Teal 


316  THE  MARQUETTE  IRON  BEARING  DISTRICT. 

Lake  (Atlas  Sheet  XXVII).  The  broad  eastern  area  is  a  gently  westward- 
plunging  syncline  with  minor  folds.  In  tlie  eastern  arm,  which  swings  to 
the  south  in  sec.  2,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXIV),  the  dips  are 
always  away  from  the  Ajibik  quartzite  and  luider  the  Negaunee  formation. 
Following  the  main  belt  from  sec.  3,  T.  47  N.,  R.  26  W.,  into  sees.  4,  5,  8, 
and  9  (Atlas  Sheet  XXXI),  the  slates,  upon  the  whole,  constitute  a  great 
anticlinal  dome.  There  the  folding  is  complex.  The  pressure  was  more 
severe  in  a  north-south  than  in  an  east-west  direction,  so  that  on  the  north- 
ern side  of  the  area  the  dips  are,  in  general,  to  the  south,  and  upon  the 
southern  side  to  the  north.  This,  however,  is  by  no  means  a  simple  fold, 
but  an  anticlinorium  with  a  large  number  of  minor  rolls  with  east- west  axes. 
The  north-south  major  cross  fold  causes  these  minor  plications  to  plunge 
under  the  iron  formation  to  the  west,  and  the  contact  line  between  the  for- 
mations curves  outward  and  inward  in  a  number  of  reentrants  and  salients. 
The  salients  correspond  to  anticlines  in  the  slates,  the  reentrants  to  synclines. 
The  same  irregularity  is  probable  upon  the  east  side  of  the  area,  but  here 
a  swamp  prevents  a  close  delimitation  of  the  Siamo  slate  and  the  Negaunee 
formation.  Following  the  belt  to  the  southwest,  the  southwestern  termi- 
nation of  the  fold  occurs  in  sec.  20  (Atlas  Sheet  XXXII),  where  the  iron 
formation  appears  in  a  semicircular  belt  about  the  plunging  anticline. 
The  Siamo  slate,  thus  plunging  beneath  the  iron  formation,  reappears  as 
an  anticlinal  dome  in  sees.  19  and  30,  T.  47  N.,  R.  26  W.  On  a  smaller 
scale,  the  phenomena  of  folding  are  here  the  same  as  in  the  large  exposures 
of  this  formation  to  the  northeast. 

PETROGRAPHICAL   CHARACTER. 

Macroscopicai. — The  Siamo  slate  varies  from  a  coarse-grained  feldspathic 
graywacke  approaching  a  quartzite,  through  typical,  massive  gray wacke,  to 
a  very  fine  grained,  slaty  rock.  The  slate  and  fine-grained  gray  wackes  are 
more  abundant  than  the  coarse,  feldspathic  graywackes. 

The  finer-grained  phases  are  very  generally  aff"ected  by  a  slaty  cleav- 
age, which  in  places  approximately  corresponds  with  the  bedding,  but  which 
also  at  other  places  cuts  across  the  bedding  at  various  angles.  As  explained 
on  a  previous  page,  the  slate  in  many  places  is  folded  into  a  series  of  minor, 


PETROGRArHICAL  CHARACTER   OF   SIAMO   SLATE.  317 

isoclinal  folds.  Usuall}'  the  slaty  cleavage  nearly  corresponds  with  the 
longer  limbs  of  these  folds,  and  cuts  across  the  bedding  of  the  shorter  limbs. 
Nowhere  is  the  slaty  cleavage  so  regular  as  to  furnish  roofing  slates.  At 
some  places  when  the  rocks  were  in  the  zone  of  fracture  there  was  so 
much  movement  along  the  cleavage  planes  and  between  the  beds  as  to 
develop  distinct  slickensides,  the  rock  parting  into  irregular  blocks  with 
sides  parallel  to  the  bedding  and  to  the  cleavage.  Each  block  was 
smoothed  by  movement  along  two  sets  of  shearing  planes.  The  cleavage 
therefore  passes  into  a  fissility.'  In  the  most  extreme  stage  of  alteration 
the  rock  is  a  crystalline  mica-schist,  with  well-developed  mica  folia.  In 
proportion  as  the  rocks  are  coarse-grained,  slaty  cleavage  is  not  devel- 
oped in  them,  and  it  is  entirely  absent  in  the  coarser-grained  graywackes. 
In  general,  the  rocks  of  the  formation  have  yielded  to  the  forces  to 
which  they  have  been  subjected  by  folding  and  mashing,  but  occasion- 
ally the  coarser  phases  are  brecciated,  and  rarely  they  become  reibungs- 
breccias.  This  indicates  that  the  formation  is  more  plastic  than  the 
other  Lower  Marquette  formations,  in  which  autoclastic  rocks  are  very 
common. 

The  normal  varieties  of  the  formation  are  not  heavily  ferruginous,  but 
at  the  upper  and  lower  horizons  the  slates  contain  a  great  deal  of  iron 
oxide  and,  locally,  interlaminated  layers  of  chert  and  feiTUginous  chert,  or 
even  gruneritic  schist.  The  contact  plane  between  the  Siamo  slate  and 
the  Ajibik  formation  seems  to  have  been  one  of  the  major  planes  of  differ- 
ential movement,  and  thus  numerous  cracks  and  crevices  have  formed, 
which  have  been  taken  advantage  of  by  iron-bearing  solutions  from  above. 
The  concentration  of  feri-uginous  masses  at  this  horizon,  although  occur- 
rino-  on  a  comparatively  small  scale,  is  analogous  to  the  concentration  of 
the  ore  bodies  on  impervious  basements  in  pitching  troughs,  as  explained 
in  Section  VI.  At  the  upper  horizon  the  slate  changes  by  gradation  or  by 
interlamination  into  rocks  belonging  to  the  Negaunee  formation.  The 
ferruginous  phases  are  usually  hematitic  or  magnetitic  slates,  but  occasion- 
ally interlaminated  or  intermingled  with  the  slates  are  layers  of  chert  or 

'Principles  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise:  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Part  1, 1896,  pp.  654-656. 


318  THE   MAKQUETTE   IRON-BEAKING  DISTRICT. 

ferruginous  chert  which  are  identical  with  the  similar  rocks  of  the  iron 
formation.  In  color  the  nonferruginous  varieties  of  the  rocks  are  usually 
dark-gray  or  greenish-gray,  but  some  of  the  coarser  kinds  are  light-gray. 
In  these  the  naked  eye  distinctly  sees  the  well-rounded  grains  of  quartz 
and  feldspar.  Also,  in  many  of  them  there  appear  to  be  large  fragmental 
grains  of  mica.  In  general,  the  iron  oxide  staining  the  slates  is  hematite, 
but  in  some  cases  it  is  magnetite.  The  fine  and  coarse  varieties  of  the 
rock  are  interlaminated  at  many  places,  a  layer  of  coarse  graywacke  being 
between  two  fine-grained,  slaty  layers,  and  these  bands  being  composed  of 
still  finer  bands  of  different  degrees  of  coarseness. 

Microscopical. — Tlio  Icast  altered  and  coarsest  graywackes  are  composed 
mainly  of  large,  well-rounded  grains  of  quartz,  a  few  of  them  finely  com- 
plex and  cherty-looking,  and  of  grains  of  feldspar,  between  which  is  a, 
sparse  matrix  consisting  of  chlorite,  biotite,  muscovite,  finely  crystalline 
quartz,  and  more  or  less  ferrite.  Usually  the  chlorite  is  predominant,  but 
in  some  cases  the  biotite  and  muscovite  are  equally  abundant.  Frequently 
the  quartz  grains  are  distinctly  enlarged.  In  most  cases  they  show  pressure 
effects  by  undulatory  extinction  and  fracturing,  the  latter  sometimes  being 
in  a  rectangular  system.  The  feldspar  grains  comprise  orthoclase,  micro- 
cline,  and  plagioclase.  They  show  beautifully  their  metasomatic  change 
into  chlorite  and  quartz,  biotite  and  quartz,  or  muscovite  and  quartz.  In  any 
one  case  the  alteration  of  individual  grains  may  result  in  only  one  of  the 
micaceous  minerals,  more  often  chlorite  than  any  other;  very  frequently 
the  alteration  is  into  chlorite  and  biotite,  or  into  biotite  and  sericite, 
although  chlorite  may  also  be  a  simultaneous  product.  All  stages  of  the 
change  may  be  seen,  from  those  cases  where  the  outer  borders  of  the  feld- 
spar grains  are  surrounded  by  a  film  of  the  chlorite  and  mica,  through 
those  in  which  the  grains  are  interlocking  masses  of  the  chlorite,  mica, 
quartz,  and  feldspar,  to  those  where  the  feldspar  grains  have  entirely  dis- 
appeared, their  places  being  taken  by  a  roundish,  complex  mass  of  the 
secondary  materials.  This  alteration  of  the  large  feldspar  grains  is  so 
general  that  it  strongly  suggests  that  the  most  of  the  chlorite,  biotite,  and 
sericite  in  the  matrix  developed  from  a  feldspathic  background. 


PETROGEAPHICAL   CHAKACTEK   OF   SIAMO   SLATE.  319 

In  the  least  mashed  phases  of  the  graywackes  there  appears  to  be  no 
arrangement  of  the  secondary  leaflets  of  chlorite,  muscovite,  and  sericite  in 
any  definite  direction.  Where  the  dynamic  action  was  somewhat  greater 
there  is  a  suggestion  of  the  arrangement  of  the  leaflets  of  these  minei-als 
in  a  parallel  direction;  also  the  original  grains  of  quartz  and  feldspar  are 
mashed  or  somewhat  rotated,  so  as  to  have  a  similar  arrangement.  Further, 
finely  crystalline  secondary  quartz  begins  to  appear  prominently  in  the 
background,  and  chlorite,  which  was  predominant  in  the  less  mashed  phases, 
becomes  less  prominent,  being  replaced  by  biotite  and  sericite  or  muscovite. 
Where  the  dynamic  action  was  somewhat  more  severe  the  slides  show 
distinct  evidence  of  minor  fault-slipping  along  two  sets  of  diagonal  planes, 
the  somewhat  irregular,  connecting,  and  mesh-like  slip-planes  being  marked 
by  continuous  bands  of  chlorite  and  mica,  mingled  with  oxide  of  iron. 

The  chloritic  and  micaceous  slates  difffer  from  the  graywackes  only  in 
that  the  distinctly  recognizable  fragmental  quartz  and  feldspar  are  much 
less  abundant  and  the  inatrix  much  more  abundant.  As  tlie  quartz  and 
feldspar  grains  become  of  very  small  size  they  are  less  rounded,  apparently 
being  below  the  limit  of  magnitude  aff"ected  by  water  action.  Out  of  the 
feldspar  there  formed  chlorite,  biotite,  sericite,  muscovite,  and  quartz,  exactly 
as  in  the  graywackes,  and  the  same  minerals  also  developed  in  the  matrix. 
On  account  of  the  more  plastic  character  of  these  rocks  the  evidence  of 
interior  movement  is  much  greater  than  in  the  graywackes,  the  inter- 
secting slip-planes  being  more  numerous  and  approximately  parallel,  like  a 
drawn-out  net. 

In  a  more  advanced  stage  of  alteration  the  slip-planes  increase  in 
number  and  are  more  nearly  parallel,  until  there  are  several  or  many 
in  the  breadth  of  a  single  millimeter,  and  here  we  have  typical  fissility. 
The  chlorite  and  mica  developed  or  were  arranged  parallel  to  the  fissil- 
ity. The  slip-cleavage  very  often  corresponds  with  bedding.  It  appears  as 
though  diff"erent  layers  had  been  pushed  forward  over  one  another,  some- 
what as  are  particles  of  dough  under  the  roller,  the  elongation  being  greater 
in  the  direction  of  the  movement  of  the  roller  and  less  at  riglit  angles  to 
this  in  the  plane  of  movement.     Sometimes  there  are  present  large  flakes 


320  THE  MARQUETTE   IRONBEARIIJJG  DISTRICT. 

of  mica  or  of  chlorite,  whicli  are  often  bent  or  contorted,  but  these  appear 
to  be  fragmental. 

In  a  still  more  advanced  stage  of  metamorphism  the  larger  quartz 
grains  are  partly  granulated,  secondary  quartz  is  present,  the  whole  of  the 
feldspar  is  decomposed,  and  we  have  a  fine-grained  mica-slate.  In  many 
places  these  mica-slates  are  interlaminated  with  coarser-grained  layers, 
which  distinctly  show  the  clastic  origin  of  the  rock.  In  a  single  section 
there  may  be  a  number  of  alterations  of  mica-slate  and  micaceous  gray- 
Avacke.  Sometimes  the  fissility  is  well  developed  in  the  mica-slate,  and 
abuts  diagonally  against  the  laminae  of  graywacke,  in  which  it  is  less 
prominent  or  absent  altogether.  In  the  most  extreme  stage  of  metamor- 
phism the  coarse,  fragmental  grains  of  quartz,  if  there  were  any,  were 
granulated,  and  the  secondary  quartz  is  as  coarsely  crystalline  as  this 
original  quartz.  The  grains  of  varying  sizes  fit  closely  or  interlock.  The 
mica  and  chlorite  are  in  well-developed  parallel  blades  of  considerable  size, 
and  thus  the  rock  is  a  mica-schist.  In  one  ])hase  of  the  mica-schist  are 
numerous  large  crystals  of  chlorite,  which  have  their  cleavage  transverse 
to  the  schistosity.  They  include  numerous  grains  of  quartz.  These  have 
probably  developed  under  static  conditions  after  the  dynamic  action  had 
ceased.  In  some  of  the  mica-schists  is  a  considerable  amount  of  clear 
feldspar,  which  looks  as  though  it  were  in  part  a  secondar}^  development, 
and  thus  the  rock  approaches  a  mica-gneiss.  Although  the  Siamo  forma- 
tion thus  locally  becomes  a  completely  crystalline  schist,  in  that  it  no  longer 
shows  in  itself  any  distinct  evidence  of  original  fragmental  character,  the 
gradation  phases  enable  one  to  determine  its  manner  of  development  as 
above  given. 

Where  the  slates  and  graywackes  pass  into  the  ferruginous  varieties 
there  appears  in  the  matrix  more  and  more  of  iron  oxide,  generally  hematite, 
but  in  many  cases  magnetite  also.  These  increase  in  quantity  until  there 
are  more  or  less  continuous,  nearly  solid  layers  of  iron,  oxide,  and  in  the 
extreme  stage  of  fen-ugination  the  matrix  is  so  heavily  iron-stained  that 
little  else  can  be  discriminated.  Where  the  iron  oxide  is  magnetite,  this  is 
apt  to  take  definite  crystal  outlines.  In  most  cases  it  is  plain  that  the 
oxides  of  iron  are  secondary  infiltrations,  being  in  part  in  crystals,  and  in 


PETEOGRAPHICAL   CHARACTER  OF  THE   SIAMO  SLATE.       321 

part  included  in  the  enlarg-ed  borders  of  the  quartz  grains.  These  ferrugi- 
nous slates  have  interlaminated  layers  of  material  which  in  all  respects, 
except  that  an  occasional  fragmental  grain  of  quartz  may  be  seen,  are  like 
the  ferruginous  and  sideritic  slates  and  cherts  and  griinerite-magnetite- 
schists  of  the  Negaunee  iron  formation.  These  are  subsequently  described 
in  connection  with  that  formation.  In  many  instances  the  ferniginous  chert 
belts  cut  across  the  layers  in  a  minor  way,  and  thus  show  that  they  are 
certainly  a  secondary  product  which  formed  by  the  alteration  or  replacement 
of  some  original  constituent.  In  other  instances  they  are  along  cracks 
which  formed  as  a  consequence  of  movement.  Many  of  these  belts  are 
probably  replacements  of  original  sideritic  layers,  which  were  interlaminated 
with  the  fragmental  sediments  at  the  basal  and  topmost  horizons.  From 
the  siderite  the  other  minerals  developed,  just  as  in  the  Negaunee  formation. 
In  other  cases  the  ferruginous  and  cherty  materials  which  fill  the  cracks  are 
probably  from  an  extraneous  source. 

RELATIONS   TO   AD.TACENT   FORMATIONS. 

It  has  already  been  said  that  the  Ajibik  quartzite  grades  upward  into 
the  Siamo  slate.  The  transition  rocks  are  usually  feldspathic  quartzites  and 
graywackes.  The  best  locality  at  which  to  observe  all  the  phases  of  this 
gradation  is  east  of  Teal  Lake,  in  sec.  32,  T.  48  N.,  R  26  W.  (Atlas  Sheet 
XXX).  For  the  most  part  along  the  contact  there  are  no  conspicuous 
exposures  which  show  the  exact  manner  of  transition. 

Aliove,  the  slate  is  overlain  conformably  by  the  Negaunee  iron  forma- 
tion. In  many  places  the  transition  is  gradual;  in  others,  rather  abrupt. 
Near  the  center  of  sec.  20,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXII),  the 
formation  is  a  coarse  graywacke,  and  even  approaches  a  quartzite,  which 
grades  upward  into  the  iron-bearing  formation.  There  are  various  inter- 
larainations  of  fragmental  and  nonfragmental  material,  until  finally  the 
latter  becomes  predominant.  Within  this  gradation  zone  the  slate  contains 
more  or  less  of  nonfragmental  material,  and  after  the  iron  formation 
becomes  practically  continuous  it  includes  some  fragmental  material.  The 
interlaminated  beds  were  closely  infolded,  and  consequently  brecciation  and 

MON  xxviii 21 


322  THE  MARQUETTE   IRON-BEARING  DISTRICT. 

minor  faulting  occurred.  As  a  result,  blocks  of  Negaunee  jasper  are  found 
in  the  graywacke.  At  one  place  the  lamination  of  the  jasper  abuts  against 
the  bedding  of  the  graywacke.  In  sec.  35,  T.  48  N.,  R.  27  W.  (Atlas  Sheet 
XXVII),  south  of  the  west  end  of  Teal  Lake,  the  change  is  somewhat 
al)rupt.  Here  the  top  of  the  slate  seems  to  have  been  a  shear  zone,  and  the 
iron-stained  slates  are  semicrystalline.  Resting  iipon  these,  with  scarcely 
any  gradation  zone,  are  the  iron-ore  deposits.  The  above  localities  are  the 
best  found  for  showing  the  transition  zone  between  the  Siamo  slate  and  the 
Negaunee  iron  formation.  For  most  of  the  district  exposures  are  not  found 
along  the  contact  zone. 

THICKNESS. 

To  give  an  estimate  of  the  thickness  of  the  underlying  formations  has 
Ijeen  difficult,  and  to  determine  the  thickness  of  the  Siamo  slate  is  even 
more  difficult,  because  of  its  close  minor  plications.  In  the  broad  area  of 
Siamo  slate  in  sees.  3,  4,  5,  8,  and  9,  T.  47  N.,  R.  26  W.  (Atlas  Sheets 
XXXI  and  XXXIV),  the  folding  is  so  complicated  that  it  is  impossible 
to  make  any  estimate  of  the  thickness.  The  area  perhaps  most  favorable 
is  that  west  and  east  of  Teal  Lake  (Atlas  Sheets  XXVII  and  XXX), 
where  the  belt  has  a  width  varying-  from  a  quarter  of  a  mile  to  a  half  mile, 
or  even  more.  This  great  variation  in  width  is  undoubtedly  due  to  minor 
rolls  in  the  formation,  and  taking  the  smallest  width,  1,300  feet,  with  a  dip 
of  75°,  we  would  have  a  thickness  of  about  1,250  feet.  However,  it  is 
known  that  slaty  cleavage  and  subordinate  rolls  are  here  developed,  so 
that  it  is  probable  that  this  thickness  should  be  reduced  by  one-half,  and 
perhaps  by  more. 

INTERESTING   LOCALITIES. 

Michigamme  area. — Beginning  at  tlie  north  and  west,  the  first  numerous 
exposures  of  the  Siamo  slate  are  north  of  the  Michigamme  mine,  in  sees. 
19  and  20,  T.  48  N.,  R.  30  W.  (Atlas  Sheet  V),  occupying  for  the  most  part 
a  valley  between  the  greenstone  range  and  the  granites.  This  is  the 
locality  in  which  all  of  the  mica-schists  are  found.  As  examined  in  the 
field,  they  vary  from  a  biotitic  and  chloritic  quartz-schist  to  a  finely  lami- 
nated, chloritic  biotite-schist  which  often  contains  large  crystals  of  chlorite 
and  sometimes  large   crystals  of  hornblende.     The  rock,  while  distinctly 


INTERESTING   LOCALITIES   OF   THE   SIAMO   SLATE.  323 

schistose,  is  not  strong'ly  foliated.  In  general  the  schistosity  dips  to  the 
south,  but  there  are  in  many  cases  minor  crinkles,  and  for  short  distances  a 
northern  dip.  Just  north  of  the  main  ridge  of  greenstones  the  schists  are 
cut  at  a  number  of  places  by  dikes  of  greenstone,  varying  in  width  from 
minute  ones  to  those  1.5  feet  across.  The  larger  dikes  usually  cut  across 
the  schistosity,  but  some  of  the  smaller  ones  were  intruded  almost  exactly 
parallel  to  the  schistosity.  At  one  place  an  earlier,  coarse  greenstone  is 
cut  by  a  later  dike.  At  one  exposure  the  schistosity  has  a  dome  structure, 
apparently  occasioned  by  intrusive  greenstone,  which  just  reaches  the  sur- 
face of  the  ground.  Adjacent  to  the  dikes  the  minor  crinklings  of  the 
schist  are  often  jirominent.  At  various  jDlaces  the  greenstone  dikes  and  the 
schists  are  so  firmly  welded  that  the  rock  breaks  elsewhere "  rather  than 
along  the  contact,  and  there  appears  to  be  a  gradation  zone  a  fraction  of  an 
inch  across  between  the  two,  but  in  general  the  contact  is  rather  sharp.  In 
and  upon  the  main  ridge  of  greenstone  to  the  south  are  considerable  areas 
of  the  Siamo  slate,  which  have  apparently  been  caught  in  the  eruptive  mass. 
Near  the  base  of  the  formation  there  is  seen  at  one  place  a  continuous  expo- 
sure from  the  Ajibik  quartzite  to  the  typical  mica-slate,  and  in  the  passage 
from  one  to  the  other  there  are  a  number  of  interlaminations  of  the  quartzite 
and  mica-slate,  some  of  the  beds  being  so  thin  that  a  hand  specimen  shows 
several  of  each  of  the  two  formations.  Here  the  schistosity  developed 
parallel  to  the  bedding,  as  .shown  by  its  relation  to  the  coarse  and  tine 
layers.  At  one  or  two  places  near  the  base  of  the  formation  there  are  also 
thin  interlaminated  belts  of  garnetiferous  griinerite-magnetite-schist. 

As  examined  in  thin  section,  the  most  altered  phases  of  the  rocks 
are  typical,  regularly  laminated,  chloritic  and  sericitic  biotite-schists,  but, 
as  explained  in  the  general  description,  there  are  less  altered  phases  in 
this  area  which  distinctly  show  the  fragmental  character  of  the  rock  and 
the  manner  of  transition  from  one  to  the  other.  The  mica-schists  are  found 
adjacent  to  the  intrusive  greenstone,  but  the  mica -slate  also  has  as  close 
relations.  The  contact  between  the  two  rocks,  as  seen  in  thin  section,  is 
very  sharp,  both  in  the  case  of  slate  or  schist  cut  by  dikes  and  in  the  case  of 
included  blocks  in  the  main  mass  of  greetistone.  Near  the  contact,  within 
the  biotite-schist  there  are  found  large  crystals  of  hornblende,  which  include 


324  THE   MAEQUETTE   IKON-BEARlNG  DISTRICT. 

numerous  granules  of  quartz.  Large  crystals  of  chlorite,  including  gi-ains 
of  quartz,  witli  folia  transverse  to  the  schistosity,  are  plentiful  in  the 
coarser  schists.  Tliese  and  the  hornblende  crystals  appear  to  be  the  latest 
minerals  of  the  rock.  Probably  they  developed  under  static  conditions 
after  the  folding  and  after  the  intrusion  of  the  greenstone.  It  appears 
highly  probable  that  the  unusually  crystalline  character  of  the  Siamo  slate 
in  this  neighborhood  is  partly  due  to  the  later  intrusives. 

The  garnetiferous  griinerite-magnetite-schists  near  the  base  of  the 
Siamo  formation  af  Michigamme  are  identical  with  similar  transition  rocks 
near  the  base  of  the  Negaunee,  and  are  described  in  connection  with  that 
formation. 

Nonpareil  mine. — Tlie  uext  cxposures  of  iutercst  are  at  and  south  of  the 
Nonpareil  mine,  west  of  Lake  Cooper  (Atlas  Sheet  XXV).  Here  the  rock 
is  a  regularly  laminated  ferraginous  slate,  which  contains  layers  of  fer- 
ruginous chert  and  graywacke.  In  the  less  ferruginous  layers  is  seen 
a  fissility  Avhich  dips  to  tlie  south  70°  or  7.5°,  but  the  more  ferruginous 
layers  on  the  south  or  hanging  wall  of  an  open  pit  dip  south  about  45°, 
and  these  probably  follow  the  true  bedding.  Farther  south  the  exposures 
of  the  formation  are  ordinary  ferruginous  slate.  Wlien  examined  in  thin 
section  the  ferraginous  chert  differs  from  that  of  the  Negaunee  formation 
only  in  that  it  contains  scattered,  distinctly  fragmental  grains  of  quartz 
and  layers  which  contain  a  great  deal  of  fragmental  material.  In  some  of 
these  cherty  phases  there  is  present  a  small  amount  of  siderite.  This  occur- 
rence, taken  in  connection  with  the  known  origin  of  the  similar  rocks  in 
the  iron  formation,  suggests  that  at  the  Nonpareil  mine  the  lowest  horizon 
of  the  Siamo  slate  contained  interlaminated  sideritic  phases. 

Siamo  Hills. — The  uext  important  set  of  exposures  is  southwest  of  Teal 
Lake,  on  the  Siamo  Hills  (Atlas  Sheet  XXVII),  and  these  are  taken  as  the 
type  outcrops  of  the  formation.  The  exposures,  besides  being  numerous 
and  large,  give  a  nearly  complete  section  from  the  Ajibik  quartzite  below 
to  the  Negaunee  formation  above.  At  the  base  of  the  formation,  as  at 
the  Nonpareil  mine,  as  shown  by  test-pitting,  there  is  ferruginous  chert. 
The  central  large  exposures  of  the  formation  comprise  all  varieties  of  slate 


INTERESTING   LOCALITIES   OF   THE    SIAMO   SLATE.  325 

and  graywucke,  both  ferruginous  and  nonferruginous,  from  the  finest- 
grained  phases  to  coarse  rocks  which  approach  a  quartzite.  The  uppermost 
horizons,  by  interlamination  or  gradation,  pass  into  or  are  overlain  by  the 
typical  rocks  of  the  Negaunee  formation  (PI.  X,  fig.  2).  The  Siamo  forma- 
tion here  constitutes  the  foot-wall  of  the  iron-ore  deposits.  In  some  places 
it  is  a  feiTuginous  quartzite,  but  at  most  places  it  is  a  slate,  the  alternate 
beds  of  which  are  ferruginous  and  nonferruginous.  These  have  a  southern 
dip  at  an  angle  of  about  45°.  These  beds  are  cut  at  many  places  by  a  cleav- 
age which  at  xiwu  >us  points  passes  into  fissility,  and  which  dips  at  a  steeper 
angle  to  the  south,  and  hence  cuts  diagonally  across  the  bedding.  It 
appears  probable,  therefore,  that  the  ferruginous  layers  were  originally  of  a 
different  character  from  the  nonferruginous  ones.  The  central  parts  of  the 
formation  show  that  the  slate  is  folded  in  a  series  of  minor  rolls.  There 
is  a  uniform  secondary  structure,  with  a  high  dip  to  the  south,  which 
corresponds  in  a  general  way  to  one  set  of  legs  of  the  series  of  folds. 

As  examined  in  thin  section,  the  basal  ferruginous  chert  is  again  found 
to  contain  a  great  deal  of  fragmental  quartz,  and  also  a  large  amount  of 
siderite,  out  of  which  the  hematite  plainly  developed.  Many  of  the  larger 
areas  of  siderite  are  surrounded  by  zones  of  hematite.  The  hematite 
decreases  in  amount,  and  the  siderite  increases,  in  passing  inward.  In 
other  cases  through  the  siderite  everywhere  are  crystals  of  hematite  and 
magnetite.  In  the  upper  part  of  the  formation  the  Negaunee  ferruginous 
chert  in  some  cases  appears  somewhat  suddenly  upon  the  ferruginous  Siamo 
slate;  and  in  other  cases  there  are  interlaminations  of  the  two,  and  it  is 
apparent  in  these  latter  that  the  ferruginous  chert  is  secondary  material, 
as  it  does  not  follow  the  fragmental  layers  closely,  but  cuts  across  them 
minutely  and  irregularly.  One  of  the  slides  from  the  central  part  of  the 
belt  shows  beautifully  the  development  of  the  finer-grained  rock  into  a 
mica-slate,  the  fissility  of  which  is  very  uniformly  parallel,  and  which  comes 
abruptly  against  a  graywacke  layer  at  an  angle  of  about  30°.  Here  the 
planes  of  fissility  die  out  or  become  extremely  irregular,  but  reappear 
upon  the  other  side  of  the  narrow  graywacke  band.  In  some  of  the 
graywacke  belts  irregular  fragments  of  slate  are  found  which  are  plainly 


326  THE  MAKQUETTE  IBON-BEAEING  DISTRICT. 

autoclastic,  having  been  rent  from  the  slate  in  the  folding.  In  these  phases 
the  rock  approaches  a  breccia,  but  the  breccias  oc^iur  only  adjacent  to  the 
contact  plane  between  the  slate  and  graywacke. 

Area  east  of  Teal  Lake — East  of  Tcal  Lakc,  in  sccs.  31,  32,  33,  and  34  (Atlas 
Sheets  XXX  and  XXXIII),  there  are  again  very  large  exposures  of  slate 
and  graywacke,  but  these  need  not  be  especially  described,  as  in  all  par- 
ticulars they  are  similar  to  the  central  mass  of  slate  and  graywacke  of  the 
Siamo  Hills. 

In  the  southern  part  of  sec.  35  (Atlas  Sheet  XXXIII),  about  three- 
fourths  of  a  mile  southeast  of  Eagle  Mills,  are  numerous  exposures  of  slate 
and  graywacke,  which  are  in  most  respects  similar  to  those  of  the  Siamo 
Hills.  At  one  place  the  slates  are  sharply  folded  into  a  minor  anticline, 
which  plunges  to  the  west  at  an  angle  of  15°.  At  the  middle  of  the  ledge 
is  a  band  of  reibungsbreccia,  about  4  or  5  feet  broad,  composed  mainly 
of  cemented  slate  fragments,  but  containing  areas  of  quartz  and  ferruginous 
chert.  These  latter  are  apparently  secondary.  Iron  oxide  is  one  of  the 
abundant  cementing  materials,  and  many  of  the  slate  fragments  are  heavily 
impregnated  with  this  material. 

Eastern  area. — Tlie  uext  important  exposures  are  at  the  east  end  of  the 
gi-eat  Avestward-plunging  syncline  in  sec.  31,  T.  48  N.,  R.  25  W.,  in  sec. 
1,  T.  47  N.,  R.  26  W.,  and  in  sec.  6,  T.  47  N.,  R.  25  W.  (Atlas  Sheets 
XXXVI  and  XXXVII).  Here,  at  the  bottom  of  the  formation,  especially 
in  sees.  1  and  6,  the  slate  and  graywackes  are  very  ferruginous,  and  they 
contain  considerable  belts  of  material  which  approaches  very  closely  to  a 
ferruginous  chei't.  In  some  cases,  for  narrow  zones,  this  chert  is  in  all 
respects  similar  to  the  ferruginous  chert  of  the  Negaunee  formation.  As 
in  the  previous  localities,  the  bands  of  pure  chert  or  ferruginous  chert  are 
minutely  interlaminated  with  belts  Which  are  largely  fragmental.  The 
unusual  abundance  and  persistence  of  the  ferruginous  slates  at  this  localitj- 
have  already  been  explained  as  due  to  the  fact  that  they  are  at  the  bottom 
of  a  westward-plunging  syncline  and  rest  upon  a  quartzite ;  that  is,  they 
are  at  a  place  where  there  has  been  extensive  readjustment  between  the 
two  formations,  and  also  where  percolating  waters  would  be  converged. 
In  passing  to  higher  horizons  these  ferruginous  slates  grade  into  the 
ordinary  slates  and  graywackes  of  the  formation. 


INTEKESTINCx  LOCALITIES  OP  THE  SIAMO  SLATE.  327 

When  examined  in  thin  section,  the  purest  pliases  of  the  ferruginous 
chert  are  found  to  contain  a  certain  amount  of  plainly  fragmental  quartz. 
Siderite  is  also  found.  This  suggests,  as  at  the  Nonpareil  mine  and 
Siamo  Hills,  that  this  mineral  was  a  partial  source,  at  least,  of  the  iron 
oxides. 

In  the  NW.  i  sec.  2  and  in  sec.  3,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXTV),  there  are  large  exposures  of  typical  slate  and  graywacke.  These 
show  minor  rolls  and  transition  phases  into  the  Negaunee  formation. 

westhaif  of  T.47N.,  R.:!6w. — The  most  exteusive  area  of  the  exposures  of  the 
formation  occurs  in  sees.  4,  5,  8,  and  9,  T.  47  N.,  R.  26  W.  (Atlas  Sheet 
XXXI).  Here,  as  on  the  Siarao  Hills,  are  nearly  all  phases  of  the 
formation.  On  the  irregular  west  side  of  the  area  the  trajisition  phases 
between  the  Siamo  slate  and  the  Negaunee  formation  are  well  shown.  In 
going  east  from  the  Butfalo  mine  embayment,  one  sees  at  various  places  in 
the  slates  and  graywackes  unusually  ferruginous  phases  and  often  fer- 
ruginous chert.  These  are  probably  near  the  top  of  the  formation.  As 
examined  in  thin  section,  the  rocks  of  this  area  are  less  modified  than  those 
of  the  Siamo  Hills  area,  and  therefore  show  particularly  well  the  feldspar 
decomposition  into  chlorite,  biotite,  and  muscovite,  especially  the  first. 

About  the  center  and  north  of  the  center  of  sec.  20  of  the  same  town- 
ship (Atlas  Sheet  XXXII)  are  other  large  exposures  of  the  Siamo  formation. 
These  are  chiefly  of  the  feldspathic  graywacke  phases,  which  approach  a 
quartzite.  On  the  west  side  of  the  exposures  the  interlaminations  and 
gradations  between  the  Siamo  slate  and  the  Negaunee  formation  are  par- 
ticularly well  seen.  In  some  cases  there  are  numerous  interlaminations  of 
the  fragmental  and  nonfragmental  rock  before  the  pure  iron  formation 
material  is  reached.  The  relations  are  still  further  complicated  by  the  fact 
that  both  the  graywacke  and  the  jasper  are  rolled  into  a  series  of  minor 
isoclinal,  westward-dipping  folds,  and  these  subordinate  folds  are  not  simple 
but  are  each  composed  of  several  anticlines  and  syncliues  of  the  third 
order.  Moreover,  the  axes  of  these  folds  are  inclined.  Thus,  the  apparent 
number  of  interstratifications  of  graywacke  and  jasper  is  far  greater  than  is 
really  the  case,  as  the  same  beds  reappear  at  the  surface  several  times.  lu 
places  the  folding  went  to  such  an  extreme  as  to  brecciate  the  rocks,  so  that 


328  THE   MAKQUETTE   IRON  BEARING   DISTRICT. 

great  blocks  of  tlie  jasper  were  broken  off  and  are  contained  in  the  gray- 
wacke.  At  one  place  minor  faulting  occurred,  and  as  a  result  of  this  the 
lamination  of  the  jasper  abuts  directly  against  that  of  the  graywacke. 
These  phenomena  at  first  sight  led  to  the  belief  that  there  is  here  an 
unconformable  contact  between  the  jasper  and  the  graywacke,  and  that  the 
whole  series  is  overturned;  but  a  closer  study  showed  this  conclusion  to  be 
erroneous,  and  that  the  phenomena  are  explained,  as  above,  by  interstratifi- 
cation  of  the  graywacke  and  jasper,  by  the  isoclinal  folding,  brecciation, 
and  minor  faulting.  As  would  be  expected,  there  results  an  extremely 
irregular  boundary  line  between  the  two  formations,  which  is  placed  so 
that  upon  the  whole  the  jasper  is  predominant  upon  one  side  and  the 
quartzite  upon  the  other. 

The  last  important  set  of  exposures  of  the  Siamo  formation  is  in  sees. 
19  and  30  of  the  same  atlas  sheet.  Here  the  rock  is  a  coarse  graywacke, 
which  at  times  contains  very  large  grains  of  quartz  and  feldspar,  becoming 
almost  conglomeratic  in  narrow  belts.  These  are,  however,  minutely  inter- 
stratified  with  finer  material.  In  the  northeastern  part  of  the  area,  near 
the  top  of  the  formation,  the  usual  interlaminations  of  the  graywacke  and 
iron-bearing  formation  occur. 

As  examined  in  thin  section,  these  rocks  again  beautifully  exhibit  all 
stages  of  the  alteration  of  the  fragmental  feldspar  into  biotite,  chlorite, 
sericite,  and  quartz.  In  many  cases  in  an  intermediate  stage  we  have  these 
different  minerals  interlocking,  and  residual  feldspar  also  present.  Such  a 
section,  viewed  with  a  high  power,  looks  like  a  crystalline  schist,  but  with 
a  lower  power  the  fragmental  character  of  the  whole  is  at  once  apparent, 
and  so  many  transition  phases  are  found  between  such  areas  and  those  in 
which  the  feldspar  is  but  slightly  affected  as  to  show  conclusively  that  the 
complex  areas  were  originally  feldspar. 

SECTIOX  TI.— THE   NEGAUNEB  FORMATIOK. 

The  Lower  Marquette  iron-bearing  formation  is  given  the  distinctive 
name  Negaunee  because  in  the  town  of  that  name  and  to  the  southward 
are  extensive  typical  exposures  of  the  formation  (Atlas  Sheets  XXVIII  and 


EELATIOXS   OF   NECxAUNEE   FORMATION  TO  ERUFTIVES.      329 

XXXI).     It  is  called  the  iron-bearing-  formation  because  within  it  occur 
many  of  the  Marquette  iron-ore  deposits. 

RELATIONS   TO   EKXJPTIVES. 

Vast  quantities  of  greenstone  are  associated  with  the  iron-bearing 
formation.  This  greenstone  includes  both  intrusives  and  extrusives,  the 
former  being  much  the  more  abundant.  The  intrusive  rocks  are  diabases 
and  their  altered  equivalents.  The  most  conspicuous  of  these  intrusives 
are  in  the  form  of  bosses,  varying  from  those  of  small  size  to  those  2 
miles  or  more  long  and  a  half  mile  wide.  The  bosses  are  of  exceed- 
ingly irregular  shapes,  and  from  them  radiate  numerous  dikes,  varying 
from  small  ones  to  those  many  feet  in  diameter.  These  dikes  usually 
do  not  outcrop,  liut  mining  shows  that  they  frequently  connect  one  boss 
with  another,  and  thus  unite  into  one  mass  several  apparently  detached 
areas  of  greenstone.  In  many  cases  the  gi-eeustone  intruded  the  .sedi- 
mentary series  in  a  laccolitic  fashion,  so  that  the  iron  formation  has  a 
quaquaversal  dip  about  the  greenstone  masses  (PI.  XI).  In  some  places 
fragments  of  the  Negaunee  formation  are  included  in  the  intrusives 
(PI.  XII.)  In  other  places  the  greenstone  breaks  across  the  iron  formation, 
and  at  these  the  latter  beds  may  dip  against  the  greenstone,  although  in 
many  cases  the  dip  of  Negaunee  beds  may  be  locally  modified  (figs.  17, 
24,  and  25). 

The  intrusives  particularly  affect  the  iron  formation,  the  bosses  of  this 
rock  found  in  the  underlying  and  overlying  formations  being  relatively  few 
and  of  small  size.  This  is  illustrated  by  the  fact  that  a  map  including 
the  greenstone  areas  about  Ishpeming  and  Negaunee  would  approximately 
cover  the  distribution  of  the  iron-bearing  formation.  Large  and  abundant 
masses  of  intrusives  are  also  found  in  the  central-eastern  arm  of  the  iron 
formation,  are  very  conspicuous  in  the  masses  of  griinerite-magnetite-schist 
constituting  Mount  Humboldt  (fig.  27),  and  are  abundant  in  the  great  out- 
crops of  iron  formation  at  Republic  and  at  Michigamme.  At  the  latter  place 
fragments  of  the  Negaunee  formation  are  included  within  the  intrusives 
(PI.  XII).  While  this  general  relation  is  very  marked,  the  greenstone  not 
infrequently  penetrates  the  superior  formation  (PI.  XXX),  and  is  also  found 
in  the  inferior  formation.     A  possible  explanation  of  this  relation  between 


330 


THE   MAEQUETTE   IRON-BEARING   DISTRICT, 


the  intrusives  and  the  iron  formation  may  he  in  the  exceeding  brittleness 
of  the  latter.  When  the  series  was  folded  this  formation  was  fractured  at 
innumerable  places,  thus  allowing  the  wedges  of  igneous  material  to  enter. 

At  a  few  places  the  tuffaceous  igneous  rocks  occur,  giving  evidence  of 
contemporaneous  volcanic  activity. 

In  the  mapping  only  those  areas  are  colored  as  greenstone  Avhich  are 
shown  by  visible  exjjosure  or  b)'  underground  working  to  be  igneous. 
There  can  be  no  doubt  that  greenstone,  in  the  forms  of  bosses  and  dikes, 
occupies  a  considerable  area  which  is  given  the  color  of  the  Negaunee  iron 


I  griinerite-raagnetite-schist,  from  near 


sec.  12,T.47N.,R.; 


formation,  liut  the  positions  of  such  greenstones  are  undetermined.  There- 
fore the  iron-formation  color  covers  both  the  iron  formation  proper  and 
unknown  areas  of  included  greenstones. 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

The  largest  area  of  the  iron  formation  (Atlas  Sheet  IV)  occupies  the 
major  part  of  the  E.  i  of  T.  47  N.,  R.  27  W.,  and  the  W.  ^  of  T.  47  N., 
R.  26  W.,  extending  from  near  Teal  Lake  on  the  north  to  the  village  ot 
Palmer  and  to  Summit  Mountain  on  the  south.  From  the  southern  part 
of  this  broad  central  area  two  arms  project  to  the  northeast  and  east. 
The  first  arm  runs  in  a  northeast  direction  from  Palmer,  spreads  out  into  a 
broad  area  in  sees.  10  and  15,  T.  47  N.,  R.  26  W.,  and  terminates  in  sec.  3; 
the  second  arm,  a  half  to  three-quarters  of  a  mile  Avide,  extends  east  from 
Palmer  to  the  sand  plain  in  sec.  27,  T.  47  N.,  R.  26  W.  Its  course  after 
reaching  the  sand  plain  is  undetermined.  From  the  broad  Ishpeming- 
Negaunee  area  two  arms  pass  to  the  west,  one  near  the  south  side  of  the 
Marquette  series  and  the  other  near  the  north  side.     The  southern  belt  has 


EXPOSURES  OF  THE  NEGAUNEE  FORMATION.       331 

a  considerable  width  in  sees.  20  and  21,  T.  47  N.,  R.  27  W.,  but  farther 
west,  as  a  consequence  of  the  inter-Marquette  erosion,  it  occupies  but  a 
narrow  zone  until  Humboldt  is  reached,  and  it  is  not  even  certain  that  for 
a  part  of  this  distance  the  entire  formation  is  not  cut  out.  However  this 
may  be,  in  sec.  18,  T.  47  N.,  R.  28  W.,  the  formation  reappears  with  a 
considerable  width,  and  has  a  breadth  of  half  a  mile  south  of  Humboldt. 
West  of  Humboldt  for  some  distance  the  fomnation  may  be  entirely  cut 
off  b)^  the  Upper  Marquette  transgression,  but  exposures  reappear  at 
Chamjiion.  From  Champion  to  the  eastern  side  of  the  Republic  tongue 
the  formation  is  cut  out.  At  the  southeast  end  of  the  Republic  tongue  it 
swings  to  the  south,  west,  and  northwest,  to  the  western  side  of  the  trough, 
being  again  cut  out  at  intervals.  Thus  in  the  Republic  tongue  the  two 
belts  are  in  a  syncline  which  is  independent  of  the  main  Marquette  area. 
West  of  Republic  is  another  similar  tongue. 

From  the  main  area  the  northern  belt  extends  west  from  Ishpeming, 
with  frequent  exposures,  to  sec.  G,  T.  47  N.,  R.  27  W.  West  of  this  place 
it  is  known  only  by  occasional  outcrops  to  near  Michigamme.  At  Michi- 
gamme  and  vSpurr  the  iron  formation  has  a  considerable  width,  and  from 
the  latter  place  it  extends  to  the  west  for  an  undetermined  distance. 
It  is  wholly  possible  that  in  the  area  between  Michigamme  and  sec.  4, 
T.  47  N.,  R.  28  W.,  the  Upper  Marquette  transgression  entirely  cut  out  the 
Negaunee  formation  for  a  greater  or  less  part  of  the  distance,  but  in  the 
absence  of  evidence  of  this  it  is  mapped  as  continuous. 

As  has  been  seen,  throughout  much  of  the  extent  of  the  Negaunee 
formation  there  are  abundant  masses  of  intrusives,  and  these,  rather  than 
the  iron-liearing  formation  itself,  usually  give  the  prominent  topographic 
features.  In  the  broad  Ishpeming-Negaunee  area  this  is  particularly 
the  case,  nearly  all  of  the  bluffs  being  composed  of  greenstone,  the  iron 
formation  occupying  the  valleys  between  the  numerous  greenstone  knobs 
and  ridges  (Pis.  XIII  and  XIV).  For  much  of  this  part  of  the  district 
the  1,400-foot  contour  is  approximately  the  boundary  line  between  the 
greenstone  and  the  iron  formation.  However,  where  the  Negaunee  forma- 
tion is  a  jasper  or  a  griinerite-magnetite-schist,  it  is  likely  to  be  hard  and 
resistant,  and  so  to  make  important  topographic  features.     Large  outcrops 


332  THE  MARQUETTE   IltOXBEAKING  DISTKICT. 

of  the  jasper  may  be  seen  about  Ishpeniing  and  Negaunee,  southt 

east  of  Palmer,  and  at  Republic.     The  magnetite-griinerite-schis- 

makes  prominent  exposures  southeast  of  the  Goodi'ich  mine,  at 

I    Mount  Humboldt,  at  Champion,  and  at  Republic.     In  general, 

:§    where  the  Goodrich  quartzite  is  in  contact  with  the  iron-bearing 

I    formation  the  former  is  the  more  resistant  rock.     The  same  is  true 

g 

^    of  the  Ajibik  quai-tzite  along   the   Cascade  range,   and  where 

I  graywackes  are  abundant  at  the  upper  part  of  the  Siamo  slate 

I  this  occupies  the  higher  lands.     Hence,  upon  the  whole,  the  iron 

I  formation  is  not  well  exposed,  and  occupies  depressions,  either 

I  between  intrusives   within  the   iron   formation  or  between   the 

"Mis    I  underlying  and  overlying  formations. 

m 


i 


I  Beginning  at  the  east,  the  two  long  arms  of  the  iron  formation 
I  constitute  two  synclinal  troughs.  As  a  result  of  the  general  west- 
s' ward  pitch  of  the  series,  the  northern  tongue  is  known  to  termi- 
I  nate  to  the  east,  but  the  termination  of  the  southern  trough  is 
5  undetermined  because  of  its  disappearance  below  the  Pleistocene 
I  sands.  As  another  consequence  of  the  westward  pitch,  these  two 
I  tongues  and  other  shorter  ones  unite  into  the  broad  Ishpeming 
I  and  Negaunee  synclinorium.  The  continued  westerly  pitch  of 
I  the  series  brings  the  quartzite  of  the  Upper  Marquette  to  the 
g  surface  at  Ishpeming,  and  this  divides  the  Negaunee  formation 
I  into  two  arms,  one  of  which  extends  along  the  south  side  of  the 
■2  Marquette  district  and  the  other  along  the  north  side.  Therefore, 
I  west  of  Ishpeming  the  formation  appears  in  two  belts  on  opposite 
I  sides  of  the  great  synclinorium.  At  Lake  Michigamme  an  intei-- 
I  mediate  anticline  becomes  prominent,  and  as  a  result  of  it  a 
•^  synclinal  arm  extending  southeast,  terminating  at  Republic,  is 
^  produced.  West  of  the  Republic  fold  is  another  very  similar 
one.  In  the  large  Ishpeming-Negaunee  area  the  secondary 
folding  of  the  formation,  combined  with  the  distortions  of  the 
intrusions,  produces  extremely  complicated  contact  lines,  both 


FOLDING  OF  THE  NEGAUNBE  FORMATION.        333 

with  the  underlying  Siamo  slate  and  the  overlying  Goodi-icli  quartzite. 
By  studying  these  lines  it  is  seen  that  the  formation  is  in  a  number  of  east- 
west  secondary  folds,  which  produce  several  large  reentrants  and  salients, 
each  of  which  is  composed  of  smaller  reentrants  and  salients,  due  to  folds 
of  the  third  order  (PI.  XV).  "The  eastern  swings  of  the  contact  lines  mark 
synclines,  and  the  western  swings  anticlines.  Putting  it  in  another  way, 
in  going  west  the  iron  formation  first  appears  above  the  Siamo  slate  in 
several  fingers,  each  being  a  syncline.  These  to  the  west  unite  to  form  the 
broad  area.  Farther  to  the  west  the  Goodrich  quartzite  appears,  and  hides 
the  iron  formation  in  a  manner  exactly  similar.  The  secondary  folds  are 
still  further  modified  and  complicated  by  the  intrusion  of  the  igneous 
masses,  about  which  the  iron  formation  in  some  places  has  a  quaquaversal 
dip.  At  other  places  the  dip  is  but  Httle  modified  by  the  intrusives  (fig.  17). 
The  western  arms  of  the  iron  formation  also  have  minor  overfolds,  which 
are  more  easily  discernible  when  infolded  with  the  Goodrich  quartzite, 
but  for  the  most  part  the  belts  are  not  sufficiently  well  exposed  to  indicate 
the  minor  folding. 

A  few  localities  in  which  such  subordinate  folds  appear  may,  however, 
be  mentioned.  East  of  Palmer  the  general  syncline  of  the  iron  formation 
has  near  its  center  a  subordinate  anticline,  which  causes  the  belt  of  Good- 
rich quartzite  at  Volunteer  to  split  just  south  of  Palmer  into  two  arms 
(Atlas  Sheet  XXXII).  As  a  result  of  this  anticline  the  lower  members  of 
the  formation  are  exposed  near  the  railroad  track  east  of  Palmer,  in  the 
center  of  the  iron  belt.  At  Humboldt  the  griinerite-magnetite-schist  has  a 
subordinate  anticline,  which  causes  the  Goodrich  quartzite  to  be  distributed 
about  the  great  mass  of  griinerite-magnetite-schist  in  a  quaquaversal 
fashion. 

Upon  the  secondary  folds  are  superimposed  those  of  the  third  order 
(Pis.  XV  and  XVI  and  figs.  18  and  19),  and  on  these  those  of  a  still  higher 
order,  and  so  on  to  microscopic  plications. 

RELATIONS  TO  UNDERLYING  AND  OVERLYING  FORMATIONS. 

The  iron-bearing  formation  rests  conformably  upon  the  Siamo  slate  or 
the  Ajibik  quartzite,  and  grades  downward  into  one  or  the  other  of  these 


334 


THE   MARQUETTE   IRON-BEARING   DISTRICT. 


formations.  In  passing  upward  within  the  fx-agmeutal  formation  nonfrag- 
mental  material  begins  to  appear  and  the  slate  or  quartzite  becomes  more 
or  less  ferruginous,  and  by  an  increase  of  the  ferruginous  constituent  it 
grades  up  into  the  iron-bearing  formation.  This  gradation  may  occur  within 
a  comparatively  few  feet,  or  it  may  require  a  thickness  of  100  or  more  feet. 
More  often  than  not  the  gradation  is  not  a  regular  transition,  but  is  accom- 
plished by  interlaminations  of  material  which  is  mainly  fi-agmental  and 


Fig.  19 I'oldcd  ferruginous  chert  of  Starwest  m; 


material  Avhich  is  mainly  nonfragmental.  These  interstratifications  are 
particularly  well  shown  at  the  top  of  the  Ajibik  quartzite  south  of  Palmer 
and  at  the  top  of  the  Siamo  slate  east  of  Negaunee.  In  different  places 
the  lowest  horizon  of  the  Negaunee  formation  may  be  the  sideritic  slate, 
the  griinerite-magnetic-schist,  the  ferruginous  chert,  or  the  jasper. 

The  overlying  formation  is  the  Ishpeming  formation  of  the  Upper 
Marquette  series.  The  relations  between  the  two  are  those  of  imconformity, 
there  having  been  considerable  orogenic  movement  and  deep  erosion  after 


EELATIONS  OF  THE  NEGAUNEE  FORMATION. 


335 


the  deposition  of  the  Negaunee  formation,  before  the  Ishpeming-  formation 
began  to  be  deposited.  The  degree  of  fokling  and  the  amount  of  erosion 
are  different  in  different  parts  of  the  district.  At  most  jjUices  the  discordance 
is  not  more  than  5°  to  15°,  but  locally,  us  at  the  Goodrich   mine  (Atlas 


Sheet  XXVI),  the  Goodrich  quartzite  cuts  vertically  across  the  plicated 
jasper  (figs.  20  and  21).  In  some  places  the  erosion  has  cut  so  deep  as 
ti>  have  entirely  removed  the  Negaunee  formation,  and  in  other  i)laces  the 


--~:^  .^':^»-j 


FiH.  21 Crosa  section  of  contact  of  Goodrich  quartzite 

on  plicateil  Negaunee  jaspilite. 

formation  has  a  verj^  considerable  thickness.  It  thus  follows  that  the 
contact  line  between  the  two  formations  is  now  at  one  horizon  of  the  iron- 
bearing  formation  and  now  at  another,  varying  fr(.)m  the  highest  known 
liorizon  of  the  formation  to  its  lowest. 


THE   MAKQUETTE  lEON-BEAEING  DISTRICT. 


THICKNESS. 


The  average  original  thickness  of  the  Negaunee  iron  formation  may 
have  been  greater  than  its  present  maximum  thickness,  for  we  have  no 
means  of  ascertaining  what  part  of  it  and  of  overlying  formations  was 
removed  by  erosion. 

If  subordinate  foldings  are  not  considered,  the  interstratified  eruptives 
are  neglected,  and  the  maximum  breadth  of  outcrop  is  multiplied  by  the  sine 
of  the  angle  of  dip,  this  gives  a  thickness  of  about  1,500  feet;  but  the  sub- 
ordinate folding  and  eruptives  certainly  reduce  this  thickness  somewhat — 
probably  as  much  as  one-third.  In  the  broad  area  of  Ishpeming  and 
Negaunee  it  is  impossible  to  determine  the  thickness,  for  nowhere  have 
diamond  drills  penetrated  the  underlying  Siamo  slate.  The  folding  is 
here  so  complicated  that  an  accurate  estimate  of  the  thickness  can  not  be 
given,  even  of  the  part  of  the  formation  which  is  exposed  and  explored.  It 
is,  however,  certain  that  the  thickness  is  considerable,  and  it  may  be  more 
than  1,000  feet.  From  what  has  been  said  in  reference  to  erosion  it  is  e>n- 
dent  that  the  formation  varies  from  its  maximum  thickness  to  disappearance. 


PETROGRAPHICAL   CHARACTER. 


Macroscopicai. — Pctrographically  the  iron-bearing  formation  comprises  sid- 
eritic  slates,  which  may  be  griineritic,  magnetitic,  hematitic,  or  limonitic; 
griinerite-magnetite-schists;  ferruginous  slates;  ferruginous  cherts;  jas- 
pilite;  and  iron  ores.  The  ferruginous  cherts  and  jaspilite  are  frequently 
brecciated;  the  other  kinds  less  frequently. 

The  sideritic  slates  are  most  abundantly  found  in  the  valleys  between 
the  greenstone  masses  in  the  large  area  south  of  Ishpeming  and  Negaunee, 
although  they  occur  at  other  localities.  These  rocks  are  regularly  lami- 
nated, fine-grained,  and  when  unaltered  are  of  a  dull-gray  color  (PI.  XVII). 
The  purest  phases  of  them  are  approximately  cherty  iron  carbonate,  as 
shown  by  two  analyses  made  by  George  Steiger  in  the  laboratory  of 
the  Survey. 


rETKOGRAPHIOAL  CHARACTER  OF  NEGAUNEE  FORxMATiOiJ.    337 

Anali/neH  of  eherty  sideriten. 


Per  cent. 

FIHST  ANALYSIS. 

SiO^ 

42.37 
1.09 

F62O3 

FeO 

CaO 

.50 
2.48 
21.80 

99.65 

26.67 
.12 
.16 
.10 

MgO  . . . 

CO.2 

Total  

SECOND  ANALYSIS. 

Insoluble  iuHCl: 

SiO. 

Al.O, 

Fe-Os 

MgO 

Soluble  in  HCl: 

AuC:::::::::::::::::::::;:: 

FeO 

MnO """■ 

CaO 

MgO 

(KNa).,0 

P.Os 

CO2 

.03 

Water  below  100°  C 

Water  above  100°  C. '..... 

Total 

100. 17 

'  The  above  determinationa  of  water  were  made  on  the  original  sample. 

It  is  unusual  to  find  exposures  of  the  clierty  siderite-slates  which  have 
not  been  more  or  less  afifected  by  deep-seated  alteration  or  by  weathering 
processes.  As  a  consequence,  the  iron  carbonates  pass  by  gradations,  on 
the  one  hand  into  griinerite-magnetite-schists,  and  on  the  other  into  ferrugi- 
nous slates,  ferruginous  chert,  jasper,  or  iron  ore. 

The  grunerite-magnetite-scMsts  consist  of  alternating  bands  composed 
of  varying  proportions  of  the  minerals  griinerite  and  magnetite  and 
quartz  (PI.  XVIII).  Where  least  modified  they  have  a  structui-e  precisely 
MON  xxviii 22 


338 


THE  MARQUETTE  lEON-BEAEING   DISTRICT. 


like  the  sideritic  slates  from  which  they  grade,  the  griinerite-magnetite  belts 
having  taken  the  place  of  the  carbonate  bands.  In  some  places  the  griinerite- 
magnetite-schists  are  minutely  banded,  the  alternate  bands  consisting  of 
dense,  green  griinerite  and  of  white  or  gray  chert,  with  but  a  small  quan- 
tity of  magnetite.  Certain  important  kinds  appear  to  be  composed  almost 
altogether  of  griinerite,  with  a  little  magnetite.  In  general,  the  griinerite- 
magnetite-schists  are  found  at  low  horizons,  below  the  ferruginous  chert  and 
jaspilite,  i.  e.,  at  or  near  the  same  horizon  as  the  sideritic  slates.  Frequently, 
also,  they  are  below  intnisive  masses  of  greenstone. 

Analyses  of  four  of  the  typical  griinerite-magnetite-schists  were  made 
in  the  chemical  laboratory  of  the  Geological  Survey,  the  first  by  George 
Steiger,  the  second  and  third  by  W.  H.  Melville,  and  the  fourth  by  H.  M. 
Stokes.  The  material  for  the  first  analysis  (specimen  21146,  1,200  steps  N., 
985  W.,  sec.  12,  T.  47  N.,  R.  27  W.)  was  from  the  broad  area  of  kon  formation 
southeast  of  Ishpeming;  thait  for  the  second  (specimen  16149,  1,475  steps 
N.,  150  W.,  sec.  11,  T.  47  N.,  K  27  W.),  from  Humboldt;  that  for  the  third 
(specimen  16566,  1,555  steps  N.,  1,375  W.,  sec.  18,  T.  47  N.,  R.  -28  W.), 
from  a  mile  or  two  east  of  Humboldt  Mountain ;  and  that  for  the  fourth 
(specimen  18938,  900  steps  N.,  50  W.,  sec.  20,  T.  46  N.,  R.  30  W.),  from 
the  Magnetic  mine,  at  the  northwest  end  of  the  Republic  trough. 

Analyses  of  griinerite-magnetUe-schisls. 


Loss 

SiOj 

AI2O3 

Fe,03 

FeO 

MnO 

CaO 

MgO 

CuO 

Na^O 

P^Os 

CO, 

H.2O  (above  110°)  . 


2.42 
26.49 


0.67 
46.94 
.66 
4.51 
33.72 
.31 


.16 
.07 
2.79 


1.40 
49.70 
1.35 
3.10 
37.19 


Trace. 
.12 


30.62 
16.92 
1.01 
1.69 
2.13 
Trace. 


A- -  )  ^^^-mti- 


PLATE    XVII 


Plate  XVII.— BANDED,  CHEETY  SIDERITE. 

Fig.  1.  Cherty  siderite  from  sec.  19,  T.  47  N.,  E.  27  W.  (Atlas  Sheet  XXVI).  This  is  one  of  the  purest 
cherty  siderites  found  in  the  Marquette  district.  The  gray  material  consists  almost  wholly 
of  very  finely  crystalline  and  oiJaliue  silica  and  of  siderite.  The  bluish-gray  layers  contain 
some  silica,  the  greenish  layers  some  siderite.  On  the  weathered  surface  the  siderite  is 
entirely  decomposed  and  in  place  of  it  is  hematite  and  limonite.  The  beginning  of  the 
same  kind  of  alteration  may  be  seen  to  affect  some  of  the  siderite  belts  quite  to  the  center 
of  the  specimen.  As  examined  in  thin  section  the  secondary  limonite  is  found  to  be  in 
pseudomorijhoua  areas  after  the  siderite.  Between  the  unaltered  siderite  and  that  which  is 
completely  decomposed  there  is  every  gradation,  different  granules  showing  all  stages  of 
the  transformation.     Natural  size. 

Fig.  2.  Cherty  siderite  from  the  Peuokee  district,  sec.  13,  T.  47  N.,  R.  46  W.  (See  PI.  XXI,  fig.  4, 
Mon.  U.  S.  Geol.  Survey,  Vol.  XIX.)  The  original  cherty  siderite  of  the  Penokee  district 
is  represented  perfectly  by  the  grayish-green  material.  Its  very  close  similarity  to  that  of 
the  Marquette  siderite  represented  in  the  previous  figure  is  noticeable.  The  beginning 
of  the  transformation  of  the  siderite  to  limonite  and  hematite  is  beautifully  shown.  The 
transitions  between  the  two  are  clearer  than  in  the  previous  figure.  The  processes  of 
change  begin  along  the  bedding  planes  and  along  intersecting  veins.  These  two  together 
make  two  sets  of  nearly  right-angle  planes,  which  doubtless  are  shearing  planes.  The 
veins  are  entirely  filled  with  limonite  and  hematite,  and  therefore  are  minute  layers  of  ore. 
The  changes  along  the  bedding  illustrate  the  beginning  of  the  process  which  results  in  the 
formation  of  the  iron-ore  deposits.  It  is  noticeable  that,  as  a  result  of  the  alterations, 
the  original  banding  of  the  rock  is  emphasized,  although  the  emphasizing  bands  are  not 
so  regular  as  the  original  sedimentary  laminie.  This  emphasizing  of  the  original  banding 
of  the  iron-bearing  rocks  by  metasomatic  changes  is  a  general  law  for  the  iron  formations  of 
the  entire  Lake  Superior  region.  Natural  size. 
340 


SGEOLOGICAL  SURVE 


h'lC.  1    CITERTY  SIDKIIITK, 
FIG  li.  CHERTY  SIDEKITE, 


OM  TlIK  .\IAJi()LK-|TE  DISTRICT 
;0M  THEPENOIvEE  nJSTRR'T. 


PLATE    XVIII. 


Plate  XVIII.-MAGNETITE-GRUNERITE-SCHISTS. 

Fig.  1.  Magnetite-griinerite-schist  from  Republic  mine  (Atlas  Sheet  XI).  This  is  one  of  the  coarsest 
varieties  of  the  griinerite-magnetite-schists.  In  place  of  the  siderite  of  Plate  XVII  we  have 
griinente  and  to  some  extent  iimonite,  hematite,  and  magnetite.  The  griinerite  is  caused 
by  the  decomposition  of  siderite  into  iron  protoxide  and  carbon  dioxide  and  the  union  of 
the  former  with  silica.  The  iron  oxides,  and  especially  the  magnetite  associated  with  the 
griinerite,  are  in  part  the  direct  results  of  the  oxidation  of  the  original  siderite.  Some  of 
the  Iimonite  and  hematite  are  due  to  the  decomposition  of  the  griinerite.  Finally,  a  part 
of  each  of  the  iron  oxides  is  a  secondary  concentration.  This  is  shown  by  their  appearance 
in  veins  cutting  the  bedding.     Natural  size. 

Fig.  2.  Sideritic  magnetite-griinerite-schist  from  sec.  13,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVIII). 
The  different  bands  consist  mainly  of  griinerite,  hematite,  magnetite,  and  quartz,  in 
varying  proportions.  The  darker-colored  bands  contain  much  of  the  iron  oxide.  In  the 
lighter  bands  griinerite  is  abundant.  In  all  of  the  layers  there  is  a  sufficient  amount  of 
residual  siderite  to  show  that  from  this  mineral  and  silica  the  griinerite  formed,  and  from 
it,  with  partial  or  complete  oxidation,  the  magnetite  and  hematite  developed.  The  most 
of  the  hematite  is  of  the  specular  variety,  but  in  places  blood-red  flecks  of  hematite  may 
be  seen,  and  parts  of  the  specimens  are  stained  by  Iimonite.  This  is  doubtless  the  result 
of  weathering.  Natural  size. 
342 


MONOGRAPH    XXVIII    PL 


KIC,  1     MAr.XKTlTE  -  GHUNERITK    SCHIST 

VIC.  2.  SIDKHITIC  .\L-\OXETITE- GRUNERITK '  SCHIST. 


PLATE    XIX. 


Plate  XIX.— GRUNERITIC  MAGNETITE-SCHIST  AND  FERRUGINOUS  SLATE. 

Fig.  1.  Griineritic  magnetite-schist  from  Republic  mine.  Ttie  lighter-colored  bands  are  strongly 
quartzose.  The  darker  bands  are  heavily  ferruginous,  but  contain  a  great  deal  of  quartz. 
The  iron  oxide  is  largely  magnetite,  but  with  this  is  much  hematite.  The  griinerite  is 
scattered  throughout  the  rock,  but  is  more  prevalent  in  the  heavily  ferruginous  bauds. 
In  its  regular  banding  the  rock  is  very  similar  to  the  original  cherty  sideritic  slates 
represented  by  PI.  XVII,  tig.  2. 

Fig.  2.  Ferruginous  slate  from  sec.  7,  T.  47  N.,  E.  26  W.  (Atlas  Sheet  XXXI).  The  bluish-gray  bauds 
are  largely  chert,  but  in  them  iron  oxide  is  contained.  The  reddish-brown  bands  are  largely 
limonite  and  hematite,  but  contain  much  chert.  This  rock  is  evidently  exactly  what  would 
be  produced  by  the  complete  oxidation  of  the  cherty  slderite  shown  in  fig.  1,  PI.  XVII. 
The  chert  bands  of  the  two  are  almost  absolutely  of  the  same  color  and  composition.  In 
place  of  the  siderite  bands  of  the  latter  are  the  limonite  and  hematite  bands.  The  change 
emphasizes  the  structure  as  indicated  in  the  description  of  fig.  2,  PI.  XVII.  Also,  as  in  that 
figure,  the  ferruginous  layers  are  not  so  regular  as  the  original  siderite  layers.  In  the 
rearrangement  the  iron-bearing  solutions  have  penetrated  to  a  greater  or  less  degree  into 
the  cherty  layers.  At  a  number  of  places  the  rock  was  fractured  across  the  layers.  At 
such  places  the  iron  oxide  has  been  leached  out  to  some  extent,  and  the  belts  of  chert 
connect  difi'erent  layers  of  that  material.  Last  of  all,  along  one  vein  secondary  iron  oxide 
has  formed.    Natural  size. 

Fig.  3.  Ferruginous  slate  or  jasper  from  sec.  7,T.  47  N.,  R.  26  W.  (Atlas  sheet  XXXI).  This  figure  rep- 
resents a  somewhat  more  advanced  stage  of  alteration.  The  irou  oxide  is  largely  concen- 
trated in  the  red  and  black  bands  .lud  the  silica  is  largely  concentrated  in  the  yellowish-red 
layers.  The  illustration  might  perhaps  as  well  have  been  placed  with  the  jaspers  as  with 
the  ferruginous  slates.  It  is,  in  fact,  a  trausition  variety.  If  the  chert  were  somewhat 
more  stained  with  brilliant-red  hematite  it  would  be  called  jasper. 

The  specimen  beautifully  illustrates  deformation  in  the  zone  of  combined  fracture  and 
flowage.  The  rigid  cherty  layer  is  fractured  and  faulted.  The  fault  is  normal.  The  more 
plastic  ferruginous  layers  accommodated  themselves  to  the  changed  position  of  the  siliceous 
layer  by  flexure.  The  specimen  looks  as  though  black  hematite  material  had  flowed  in 
between  the  broken  siliceous  bands,  like  dough.  The  specimen  illustrates  in  miniature  how 
a  fault  may  pass  into  a  flexure  either  above  or  below.  Natural  size. 
344 


S  GEOLOGICAL  SURVE 


{■■ic,  1  GHiNKrjTic  MAr.M-rrrri';   sriiisi 

FIG.  Z.FEHlUIC.INt)l'S  SLATK. 
FIG,  o.  FEHHIKUNOFS  SLAFK, 


PLATE    XX 


Plate  XX.— FERRUGINOUS  CHERT. 

Fig.  1.  Ferruginous  chert  from  Taylor  mine  sec.  9,  T.  49  N.,  R.  33  W.  This  specimen  illustrates  a 
somewhat  different  stage  of  alteration  from  PI.  XIX.  The  silica  has  been  almost  perfectly- 
concentrated  into  hands.  The  same  is  true  of  iron  oxides.  Movement  has  fractured  the 
siliceous  bands,  and  along  these  the  removal  of  silica  has  begun.  If  nearly  all  of  the  silica 
were  replaced  by  hematite,  iron  ore  would  be  formed.  In  fig.  1  of  PI.  XIX  the  reverse 
process  is  seen — that  is,  the  solution  of  ore  and  the  deposition  of  silica.  It  is  a  general 
law  of  the  Lake  Superior  region  that  the  solution  of  silica  and  the  deposition  of  iron  oxides 
occur  at  places  where  abuudant  percolating  waters  are  concentrated.  It  will  be  shown 
later  (see  Pis.  XXVIII  .and  XXIX)  that  these  favorable  conditions  are  just  above  impervious 
formations  which  occur  in  pitching  troughs.  This  figure,  from  a  specimen  obtained  from 
the  Upper  Marquette  series  outside  of  the  district  mapped,  is  here  iuserted  for  comparison 
with  the  ferruginous  cherts  of  the  Negaunee  formation.     Natural  size. 

Fig.  2.  Ferruginous  chert  from  south  of  Jackson  mine,  sec.  1,  T.  47  N.,  E.  27  W.  (Atlas  Sheet  XXVIII). 
The  iron  oxide  and  chert  were  largely  concentrated  into  bands  before  the  last  folding.  At 
the  time  of  the  folding  radial  cracks  were  formed,  especially  in  the  chert  layers,  due  to  the 
position  of  the  rock  on  the  crowu  of  an  anticline.  Along  these  cracks  the  silica  has  to 
some  extent  been  leached  out  and  iron  oxide  introduced.  One  light-colored  area  of  chert 
appears  to  be  a  secondary  infiltration,  but  it  was  apparently  present  before  the  last  folding, 
as  it  is  fractured  the  same  as  the  other  layers.  Natural  size. 
346 


ONOGRAPH    XXVIII,  PL 


SGEOLOGICAL  SURVE 


ViV. 
FIG 


■■K.i;iUT,lNurS   CUKIIT 

•■i-:hiuh;in()CS  ciikim' 


PLATE    XXI 


Plate  XXI.— HEMATITIC  CHERT  FROM  NEGAUNEE. 


The  plate  shows  a  somewhat  dififerent  stage  of  alteration  from  PI.  XX.  The  bands  of  chert  are  so 
broken  by  movement  that  they  are  in  some  places  difficult  to  follow.  Many  of  the  fragments 
have  roundish  outlines,  due  to  their  partial  .solution  and  replacement  by  iron  oxide.  In  the  field 
there  may  be  found  every  phase  of  transition  between  the  rock  represented  by  fig.  1  of  PI.  XVII, 
through  the  rocks  represented  by  the  figures  of  PI.  XIX,  to  the  rock  represented  by  fig.  2  of 
PI.  XX  and  to  the  varieties  represented  by  PI.  XXI.  Fig.  2  represents  a  somewhat  more 
advanced  stage  of  alteration  than  fig.  1.  The  material  illustrated  is  frequently  found  very 
close  to  the  ore  bodies.  If  a  portion  of  the  remaining  silica  were  removed  and  iron  oxides 
introduced  in  its  place,  it  Would  become  iron  ore.  The  hematite  is  soft,  and  the  material 
ilhistrated  is  therefore  called  soft-ore  jasper  by  the  miners. 
348 


US  GEOLOGICAL  SU 

RVEY 

MONOGRAPH    X 

1 

k'J^     4 

« 

Ir 

iif^^^^ife 

MBp '  '^^^^r 

^>^ 

:sL> 

mS^^'iii^ 

S5B55^ 

, 

k 

5^j^J^^JqJl 

^ 

L.  a 

iir^- 

KIO  1 

"  ^>  .ifi^^^^^^SI 

-— , .- -^ 

:»iw^ 

V              *N 

s. 

Pll**^^!' 

•^ 

^- 

if^S 

l"^ 

::i^ 

^ 

^ 

•^ 

^ 

mar^^ 

.%?" 

!iw^ 

£^ 

HMH 

^^ 

■1 

BIS 

s? 

-^0/t 

r/  -" 

Ml'.    1 

^^_-K-" 

^ 

r^ 

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--'» 

k 

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^^^ 

t.- ».. 

^ 

iii£.\i.vrrric  chkr'i 


PLATE    XXII 


Plate  XXII.— HEMATITIC  CHERT  FROM  NEGAUNEE. 

This  plate  represents  the  same  phenomena  as  PI.  XXI,  but  in  a  more  satisfactory  way.  The  folding 
has  shattered  the  chert  layers  throughout.  Along  all  the  openings  between  the  chert  fragments 
hematite  has  formed  or  has  been  forced  in  by  pressure.  A  later  folding  has  slightly  shattered 
the  rock,  and  in  the  cracks  minute  veins  of  magnetite  have  formed.  The  specimen  beautifully 
illustrates  the  action  of  material  when  folded  in  the  zone  of  combined  fracture  and  flowage. 
The  regularity  of  the  fracturing  of  the  chert  layers  in  a  direction  almost  transverse  to  their 
length  is  noticeable.  This  suggests  that  the  cracks  formed  in  tensile  planes  when  the  chert 
belts  were  being  bent. 


HKMATITIC  ClIKKT.   KKOM    Nl'X.AlJNKE. 


PLATE    XXIII. 


Plate  XXIII.— MAGNETITIC  CHERT  FROM  THE  MICHIGAMME  MINE. 

)liii8h-gray  bands  are  rather  coarsely  crystalline  quartz.  Tliis  kind  of  quartz  is  characteristic  of 
the  west  end  of  the  district.  The  dark  material  is  hematite,  and  the  lustrous  material  is  mag- 
netite. The  hematite  -was  present  before  the  last  folding,  and  is  in  brilliant  flecks,  due  to 
accommodations  along  the  beds.  The  magnetite  entered  after  the  last  folding.  In  fig.  1  the 
abundance  of  the  magnetite  is  seen  to  be  in  direct  ratio  to  the  fracturing.  On  the  left-hand 
side  of  the  figure,  where  the  rock  is  much  broken,  there  is  little  quartz.  The  peculiar  magnetitic 
chert  represented  by  this  plate  is  found  closely  associated  with  the  ore  at  the  Michigamme  and 
Spurr  mines. 


-S  GEOLOGICAL  SURVE 


ONOGRAPH    XXVIII    PL  ; 


MACNKTITIC   CHKRT. 


PLATE    XXIV. 


MON    XXVIII- 


Plate  XXIV.— JASriLlTE. 

This  rock  is  from  tlie  Grand  Rapids  mine,  Negaunee  (Atlas  Sheet  XXVIII).  The  dark  bands  are 
mainly  hematite.  The  brown  bauds  consist  mainly  of  minute  grains  of  quartz,  but  each  grain 
is  stained  with  hematite.  The  red  jasper  belts  do  not  continue  indefinitely,  but  layers  die  out 
after  extending  a  greater  or  less  distance,  with  oval  terminations.  This  is  beautifully  shown  in 
one  of  the  bands.  The  rock  illustrated  by  this  plate  is  allied  to  that  of  fig.  3,  PI.  XIX.  In  the 
change  from  the  original  rock  there  was  almost  complete  oxidation  of  the  siderite,  little  or  no 
hydration  of  iron  oxide,  and  but  little  of  the  iron  oxide  united  with  silica  to  form  griinerite.  In 
the  rock  of  fig.  2  no  griinerite  is  present,  but  some  occurs  in  the  upper  part  of  fig.  1. 


JASPILITK   KHOM   GllVM)   IJAI'IDS    MiXK .  XEC.ArXKK. 


PLATE    XXV 


Plate  XXV.— JASPILITE  FROM  JASPER  BLUFF,  ISHPEMING. 

This  is  a  representation  of  a  typical  piece  of  the  brilliant  jaspilite  which  occurs  associated  with  the 
hard  ores  of  the  Marquette  district.  The  rock  was  folded  in  the  zone  of  combined  fracture 
and  flowaKe.  The  jasper  bands  bent  for  a  certain  time  without  macroscopic  fracture,  but  later 
were  broken  through  and  through.  During  the  time  of  folding  the  rock  may  have  been  more 
deeply  buried  thau  during  the  time  of  fracturing.  At  both  periods  the  hematite  accommodated 
itself  to  its  new  position  without  apparent  fracture.  However,  the  lamiuin  moved  over  one 
another,  giving  them  a  brilliant  specular  appearance.  To  some  extent  it  flowed  in  between 
the  broken  jasper  fragments.  Between  the  leaflets  of  hematite  there  were  minute  spaces.  The 
spaces,  large  and  small,  were  occupied  by  subsequently  infiltrated  hematite  and  magnetite,  which 
in  thin  section  may  be  discriminated  by  its  crystal  outlines  from  the  hematite  present  before 
the  folding. 


PLATE    XXVI. 


Plate  XXVI.— JASPILITE. 

Fig.  1.  Folded  jaspilite,  from  Jasper  Bluff,  Ishpeming.  The  illustration  beautifully  shows  the  second- 
ary infiltration  of  iron  oxide  and  deformation  by  combined  fracture  and  flow.  By  close 
observation  iron  oxide  of  three  dift'erent  ages  may  be  seen.  The  oldest  is  the  dark-gray 
hematite.  Intersecting  this  is  the  more  brilliant  steel-gray  hematite  and  magnetite,  and 
cutting  both  of  the  former  are  other  veins  of  brilliant  hematite  and  magnetite.  The  history 
of  the  rock  seems  to  be  briefly  as  follows:  Banded  hematite  and  jasper  was  bent  by  folding, 
probably  while  the  rock  was  deep-seated.  During  this  folding  the  hematite  was  mashed. 
In  a  later  stage,  when  the  rock  was  more  rapidly  deformed  near  the  surface,  fracturing 
occurred.  This  gave  the  conditions  for  the  first  infiltration  of  iron  oxide,  and  later,  when 
the  rook  was  perhaps  still  nearer  the  surface,  further  deformation  resulted  in  new  fractures. 
Finally,  the  crevices  thus  formed  were  filled  with  the  latest  iron  oxide. 

Fig.  2.  Brecciated  jaspilite,  from  Jasper  Bhiff,  Ishpeming.  The  illustration  gives  evidence  of  the 
history  as  shown  by  fig.  1.  However,  during  the  final  process  the  layers  of  jasper,  which 
were  bent  at  the  earlier  stage,  were  broken  through  and  through,  producing  a  breccia.  The 
same  evidences  are  seen  of  three  stages  of  iron  oxide  as  in  fig.  1.  The  less  brilliant  gray  is 
the  earliest-mashed  hematite;  the  intermediate  gray  represents  a  first  infiltration;  after 
this  there  was  shattering,  and  finally  the  breccia  was  cemented  by  brilliant  steel-gray 
hematite  and  magnetite. 


PLATE    XXVII. 


Plate  XXVII.— JASPILITE,  AND  ORE  AND  JASPER  CONGLOMERATE. 


Fig.  1.  Jaspilite  from  the  Jackson  mine,  Negaunee  (Atlas  Sheet  XXVIIl).  This  figure  represents  a 
typical  piece  of  the  regularly  banded  jaspilite.  The  blui.sh-gray  bands  are  brilliant  specular 
hematite.  In  the  red  bands  each  granule  of  quartz  contains  innumerable  particles  of  trans- 
lucent blood-red  hematite.  The  lenticular  character  of  the  jasper  bands  is  well  illustrated, 
the  specimen  being  selected  especially  to  show  this.  The  transverse  fracturing  of  the  jasper 
and  other  liands  and  the  secondary  infiltration  of  iron  oxides  are  shown. 

Fig.  2.  Ore  and  jasper  conglomerate  from  Saginaw  range  (Atlas  Sheet  XXVI).  This  is  a  typical 
basal  conglomerate  of  the  Goodrich  quartzite  of  the  Upper  Marquette  series.  The  detritus 
consists  almost  wholly  of  various  materials  derived  from  the  Negaunee  formation,  including 
jasper,  chert,  and  ore.  There  is  present,  however,  some  quartz  derived  from  the  Archean. 
A  close  examination  of  the  illustration  shows  that  secondary  hematite  and  magnetite  have 
largely  formed  in  the  spaces  between  the  grains  about  many  of  the  jasper  fragments,  and, 
indeed,  have  partly  replaced  the  jasper  fragments  themselves.  This  is  beautifully  shown 
at  the  lower  left-hand  corner  of  the  figure.  In  those  places  where  the  basal  conglomerate 
is  fine-grained  these  replacements  by  iron  oxide  may  be  almost  complete,  in  which  case  an 
iron-ore  deposit  is  formed.  Of  such  an  origin  is  the  iron  ore  of  the  Volunteer  and  some 
other  mines. 


FU,   1      .JASI'lLlTlv 

FIC   2    ORK  AM)  JASPKH  COXCLOMKHATE. 


PETEOGBAPHICAL   CHARACTER  OF  NEGAUNEE   FORMATION.      361 

The  analyses  on  page  338  show  that  the  rocks  are  composed  mainly  of 
impure  griinerite,  magnetite,  and  quartz.  The  analyses  indicate  that  the 
amphibole  is  intermediate  between  griinerite  and  actinolite,  but  upon  the 
whole  is  much  nearer  the  former  than  the  latter. 

By  oxidation  of  the  iron  carbonate  the  sideritic  slates  pass  into  the 
ferruginous  slates,  the  iron  oxide  being  hematite  or  limonite,  or  both. 
These  rocks,  in  regularity  of  lamination  and  in  structure,  are  similar  to  the 
sideritic  slates,  differing  from  them  mainly  in  the  fact  that  the  iron  is  present 
in  another  combination  (PI.  XIX).  In  the  different  ledges  may  be  seen 
every  possible  stage  of  change  from  the  sideritic  slates  to  the  ferruginous 
slates.  The  only  necessary  change  is  a  loss  of  carbon  dioxide  and  per- 
oxidation of  the  iron.  In  PI.  XVII  the  beginning  of  the  jjrocess  is  beauti- 
fully shown.  On  weathered  surfaces,  along  veins,  and  along  some  of 
the  bedding  planes  the  transformation  is  complete.  Between  this  trans- 
formed material  and  the  original  rock  there  is  complete  gradation. 
PI.  XIX  illustrates  different  ferruginous  slates  in  which  the  siderite  is 
partly  or  wholly  decomposed. 

The  ferruginous  cherts  are  rocks  consisting  mainly  of  alternating  layers 
of  chert  and  iron  oxide,  although  in  the  iron-oxide  bands  chert  is  contained, 
and  also  in  the  chert  bands  iron  oxide  is  found  (Pis.  XX-XXII).  This 
iron  oxide  is  mainly  hematite,  but  both  limonite  and  magnetite  are  some- 
times present.  Rarely  magnetite  is  the  predominant  oxide  of  iron  (PL 
XXIII).  In  such  cases  the  silica  is  usually  coarsely  crystallized.  In  the 
field  the  ferruginous  slates  are  found  to  grade  step  by  step  into  the  ferru- 
ginous cherts,  and  it  is  manifest  that  they  were  produced  from  them  by  a 
rearrangement  of  the  iron  oxide  and  silica,  with  a  possible  introduction  of 
extraneous  silica  and  iron  oxide.  The  rocks  are  folded  in  a  complicated 
fashion,  as  a  result  of  wliich  the  layers  present  an  extremely  contorted 
appearance.  The  folded  layers  frequently  show  minor  faulting.  On  account 
of  the  exceedingly  brittle  character  of  these  rocks,  they  are  very  often 
broken  through  and  through,  and  sometimes  they  pass  into  reibungs- 
breccias.  Sometimes  the  shearing  of  the  fragments  over  one  another  has 
been  so  severe  as  to  produce  a  conglomeratic  aspect.  The  ferruginous 
cherts  are  particularly  abundant  at  the  middle  and  lower  parts  of  the  iron- 
bearing  formation,  just  above  or  in  contact  with  the  greenstone  masses. 


362  THE   MAKQUETTE   IKON-BEARING   DISTRICT. 

They  thus  occupy  a  horizon  within  the  iron-bearing  formation,  and  in  a 
number  of  cases  they  are  between  the  griinerite-magnetite-schists  or  sideritic 
slates  below  and  the  jaspilite  above.  The  rocks  here  named  ferruginous 
chert  are  called  by  the  miners  "soft-ore  jasper,"  discriminating  them  from 
the  hard-ore  jasper,  or  jaspilite.  This  material  is  so  called  because  within 
or  associated  with  it  are  found  the  soft  ores  of  the  district. 


of  chert,  in  the  red  jasper. 


Fig.  22.— Jaspilite  of  Republic  mine.,  showing  w 

The  jaspilites  are  rocks  consisting  of  alternate  bands  composed  mainly 
of  finely  crystalline,  iron-stained  quartz  and  iron  oxide  (Pis.  XXIV-XXVII, 
and  fig.  22).  The  exposures  present  a  brilliant  appearance,  due  to  the  inter- 
lamination  of  the  bright-red  jasper  and  the  dark-red  or  black  iron  oxides. 
The  iron  oxide  is  mainly  hematite,  and  includes  both  red  and  specular  varie- 
ties, but  magnetite  is  frequently  present.  The  jasper  bands  often  have  oval 
terminations,  or  die  out  in  an  irregular  manner.  The  folding,  faulting,  and 
brecciation  of  the  jaspilites  are  precisely  like  those  of  the  ferruginous  chert, 
except  that  in  the  jaspilite  they  are  more  severe.  The  interstices  produced 
by  the  dynamic  action  are  largely  cemented  with  crystalline  hematite,  but 
magnetite  is  present  in  subordinate  quantity. 


PETROGRAPHIGAL  CHARACTEK   OF  XEGAUNEE   FORMATION.       363 

In  the  folding'  of  the  rock  the  readjustment  has  occurred  mainl}'  in  the 
iron  oxide  between  the  jasper  bands.  As  a  result  of  this,  the  iron  oxide  has 
been  sheai'ed,  and  when  a  specimen  is  cleaved  along  a  layer,  it  presents  a 
brilliant  micaceous  appearance,  and  such  ore  has  been  called  micaceous 
hematite.  This  sheared  lustrous  hematite,  present  as  some  forui  of  iron 
oxide  before  the  dynamic  movement,  is  discriminated  with  the  naked  eye  or 
with  the  lens  from  the  later  crj^stal-outlined  hematite  and  magnetite  which 
fills  the  cracks  in  the  jasper  bands  and  the  spaces  between  the  sheared 
laminae  of  hematite.  The  jaspiHte  differs  mainly  from  the  ferruginous  chert, 
with  which  it  is  closely  associated,  in  that  the  siliceous  bands  of  the  former 
are  stained  a  bright  red  by  hematite,  and  the  bands  of  ore  between  them 
are  mainly  specular  hematite,  while  in  the  cherts  the  iron  oxide  is  earthy 
hematite.  The  jaspilite  in  its  tj'pical  form,  whenever  present,  always  occu- 
pies one  horizon — the  present  top  of  the  iron-bearing  formation,  just  below 
the  Goodrich  quartzite.  In  different  parts  of  the  district  it  has  a  varying 
thickness.  With  this  jasper,  or  just  above  it,  are  the  hard  iron  ores  of  the 
district;  hence  it  has  been  called  ''hard-ore  jasper"  Ij}-  the  miners,  to 
discriminate  it  from  the  ferruginous  chert,  or  "soft-ore  jasper." 

An  analysis  of  one  of  the  typical  jaspilites,  made  by  George  Steiger, 
in  the  laboratory  of  the  Survey,  is  as  follows: 

Analyais  of  jaspilite. 


Soluble  matter,  chiefly  iron  oxide. 
Insoluble  matter 


62.36 
37.64 


The  insoluble  matter  contains — 


SiOi 

AI2O3 

FeoO;, 

MgO 

Alkali  oxides. 
Total  .. 


364  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

This  analysis  shows  that  the  rock  is  composed  ahnost  wholly  of  silica 
and  iron  oxides. 

The  iron  ores  in  the  Marquette  district  comprise  many  varieties,  among 
which  the  more  prominent  are  hematitic,  granular  magnetite;  magnetitie, 
specular  hematite,  and  soft,  red  hematite,  which  is  very  often  limonitic;  and 
all  gradation  phases.  The  magnetites  and  specular  hematites  are  called 
hard  ores  by  the  miners,  and  the  red  hematites  are  called  soft  ores. 

The  hematitic  magnetites  vary  from  very  coarsely  granular  magnetite 
to  finely  granular  magnetite.  With  the  magnetite  there  is  always  more 
or  less  of  hematite,  in  many  cases  a  large  part  of  this  resulting  from  the 
alteration  of  the  magnetite.  The  hematite  varies  from  a  subordinate  to  an 
important  amount.  Also,  at  many  places,  with  the  magnetite  are  varying 
quantities  of  pyrite  and  garnet,  and  the  alteration  product  of  the  latter, 
chlorite.     The  magnetites  vary  in  color  from  pure  black  to  gray. 

By  an  increase  in  the  quantity  of  the  hematite  the  magnetites  pass 
into  specular  hematites.  The  specular  hematites  vary  in  texture  from  very 
coarse  to  aphanitic.  In  the  coarse  hematites  each  individual  of  hematite 
resembles  a  flake  of  mica.  Such  ores  are  frequently  called  micaceous 
hematites.  The  flakes  are  largely  arranged  with  their  greater  dimensions 
parallel,  thus  giving  the  ore  a  marked  schistosity  or  rift.  The  coarse 
hematites  are  usually  strongly  magnetitie,  there  being  between  the  flakes 
of  hematite  many  crystals  of  magnetite.  In  the  finer-grained  specular 
hematites  the  particles  are  so  small  that  the  eye  does  not  easily  discriminate 
them.  In  many  cases  such  ore  cleaves  like  slate  or  schist,  giving  a  smooth, 
brilliant  surface.  These  ores  are  frequently  called  slate  ores.  "While  mag- 
netitie, they  are  usually  less  strongly  so  than  the  so-called  micaceous  ores, 
although  in  them  in  many  cases  may  be  seen  numerous  small  crystals 
of  magnetite.  Another  variety  of  the  fine-grained,  specular  henaatite  is 
steel-gray,  dense,  very  hard,  and  breaks  with  a  conchoidal  fracture. 

Many  of  the  specular  ores  contain  a  greater  or  less  quantity  of  red 
hematite,  which  gives  them  a  mottled  appearance.  This  class  of  ores, 
which  is  abundant,  is  either  very  slightly  magnetitie  or  not  at  all  so.  These 
ores  are  gradation  phases  between  the  pure  specular  hard  ores  and  the  soft 
red  hematites. 


PETEOGKAPHICAL  CHARACTER  OF   NEGAUNEE   FORxMATlON.      3G5 

The  reddish  specular  ores  pass  gradually  into  the  soft  hematites.  The 
majority  of  the  soft  ores  have  a  distinct  lamination.  Many  of  them  on 
freshly  broken  surfaces  have  a  finely  crystalline  appearance.  All  give  a 
brilliant  cherry-red  streak.  In  hand  specimen  the  pure  hematites  are  bright- 
red.  The  limonitic  hematites  differ  from  the  red  hematites  in  having  a 
brown  color,  due  to  the  presence  of  hydrated  iron  oxide. 

While  a  laminated  or  banded  appearance  of  the  ores  is  very  general, 
locally  they  are  entirely  without  any  such  structure.  Oftentimes  the 
massive  varieties  contain  numerous  cavities,  varying  from  those  of  minute 
size  to  those  of  considerable  magnitude.  These  cavities  are  commonly 
lined  with  crystals  of  hematite  or  magnetite,  or  by  radiating  needles  of 
hematite  or  the  hydrated  oxides  of  iron.  Very  often  the  interiors  of  these 
cavities  have  a  botryoidal  appearance.  In  a  few  of  the  soft  hematite  mines 
oxides  of  manganese  occur  in  various  forms,  and  in  some  places  this 
material  is  so  concentrated  as  to  furnish  manganese  ore. 

The  magnetites  and  coarse  specular  hematites  are  confined  to  the 
upper  horizon  of  the  Negaunee  formation  or  to  the  basal  horizon  of 
the  overlying  Ishpeming  formation,  and  largely  to  the  western  part  of  the 
district.  The  most  common  rock  associated  with  these  ores  is  jaspilite, 
although  in  places  it  is  a  coarse,  white,  ferruginous  chert.  The  fine-grained 
specular  ores  are  confined  to  the  upper  horizon  of  the  iron  formation,  and 
mainly  to  the  eastern  part  of  the  district.  The  soft  hematites  are  found  at 
middle  or  lower  horizons  of  the  Negaunee  formation,  associated  with  the 
ferruginous  slates  or  ferruginous  cherts. 

It  appears  that  the  ferruginous  slates,  ferruginous  cherts,  jasper,  and 
ore  form  in  the  zone  of  weathering,  and  that  the  griinerite-magnetite-schists 
develop  in  the  zone  of  deep-seated  alteration.  The  difi"erent  characters 
are  then  due  to  original  position  within  the  formation  and  to  subsequent 
environment,  rather  than  to  difference  in  the  original  rock. 

In  fullest  section  the  Negaunee  formation  exhibits,  therefore,  the  fol- 
lowing stratigraphy:  At  the  bottom  are  the  sideritic  slate  and  griinerite- 
magnetite-slate;  above  these,  ferruginous  slate;  above  this,  ferruginous 
chert;  and  at  the  top  of  the  formation,  jaspilite.  The  iron  ore  may  occur 
at  any  horizon.  While  this  is  the  common  order,  in  a  given  locality  one  or 
more  of  these  members  may  be  absent. 


366  THE  MARQUETTE   IKOX-BEARING   DISTRICT. 

For  instance,  at  Republic  (Atlas  Sheet  XI)  only  the  griinerite- 
magnetite-schists,  the  ferruginous  chert,  and  the  jaspilite  are  found.  South 
of  the  Saginaw  mine  (Atlas  Sheet  XXVI),  at  tlie  base,  is  the  griinerite- 
magnetite-scliist;  at  the  intermediate  horizons,  the  ferruginous  chert;  and  at 
the  top,  the  jaspilite.  South  of  Palmer  (Atlas  Sheet  XXXII)  the  jaspilite 
occupies  the  whole  breadth  of  the  formation  between  the  Goodrich  quartz- 
ite  and  the  Ajibik  quartzite.  Farther  to  the  east,  however,  where  the 
formation  has  a  greater  thickness,  the  ferruginous  chert  occurs  below  the 
jaspilite.  At  and  south  of  Negaunee  (Atlas  Sheets  XXVIII  and  XXXI) 
the  full  succession  is  found.  Beginning  at  the  Jackson  mine  and  passing 
southward,  we  find  at  the  top  of  the  formation  magnificent  exposures  of 
jaspilite;  below  this  are  numerous  open  pits,  which  give  typical  exposures 
of  the  ferruginous  chert.  This  grades  down  into  the  ferruginous  slate  of 
the  Grand  Rapids  mine,  and  continuing  southward,  we  find  within  the 
valleys  between  the  greenstones  the  griinerite-magnetite-schists  and  the 
very  little  altered  sideritic  slate. 

Microscopical. — Thc  Iduds  of  rocks  found  in  the  iron-bearing  member  of 
the  Lower  Marquette  series  and  their  relations  to  one  another  are  very 
similar  to  those  of  the  iron-bearing  member  of  the  Penokee  and  Auimikie 
series,  which  have  been  described  in  great  detail.  Also,  the  microscopical 
characters  of  the  different  phases  of  rocks  are  similar  to  those  of  the  Peno- 
kee series.  In  fact,  so  remarkable  is  the  likeness  that,  with  a  restatement  of 
localities,  what  has  been  written  in  reference  to  the  Penokee  and  Animikie 
iron  formations  might  be  applied  almost  verbatim  to  the  Marquette  iron- 
bearing  formation.  Therefore,  for  a  detailed  description  of  the  different 
phases  of  the  iron-bearing  formation  and  the  manner  in  which  the  original 
rock  grades  into  the  other  phases,  reference  is  made  to  Mon.  U.  S.  Geol. 
Survey,  Vol.  XIX,  Chapter  V,  pages  182-268. 

A  very  brief  description  will,  however,  be  given  of  the  general  char- 
acter of  the  different  phases  of  the  iron-bearing  formation,  and  a  more 
detailed  statement  will  be  made  in  reference  to  those  points  in  which  there 
are  differences  between  the  Lower  Marquette  and  the  Penokee  iron-bearing 
formations. 

In  the  purest  phases  of  cherUj  sUIerite-slate  (PI.  XVII)  there  is  a  contin- 
uous mass  of  siderite,  which  contains  sei)arate  granules  or  irregular,  complex 


PETEOGEAPHICAL  CHARACTEE  OF  NEGAUNEE   FOEMATIOX.      367 

areas  of  cherty  silica,  small  crystals  of  magnetite,  and  needles  of  actinolite 
or  griinerite.  The  silica  is  rarely  partly  amorj^hous,  being  in  minute  opa- 
line droplets,  but  is  more  commonly  completely  individualized  quartz,  the 
grains  varying  in  the  different  slides  from  0.01  to  0.03  mm.  in  diameter. 
The  siderite  is  in  closely  packed,  small  rliombohedra.  Upon  the  weathered 
surfaces  the  siderite  is  entirely  oxidized,  being  changed  into  hematite  or 
limonite,  with  pseudomorphous  forms.  In  this  iron  oxide  is  contained 
cherty  silica,  identical  with  that  in  the  unaltered  part  of  the  rocks. 
Between  the  two  there  is  a  transition  zone,  in  which  are  seen  the  various 
stages  of  alteration  from  the  unchanged  siderite  to  the  secondary  hematite. 
In  one  of  the  finest  instances  the  transition  band  is  broad,  and  there  are 
seen  many  rhombohedi-a  of  siderite  surrounded  by  bands  of  beautiful, 
blood-red,  translucent  hematite.  These  borders  vary  from  mere  films  to 
those  so  broad  that  but  a  minute  speck  of  the  siderite  remains.  If  the 
oxidized  portion  of  the  slide  were  seen  by  itself  it  would  be  regarded  as  a 
ferruginous  slate,  with  which  it  is  in  every  respect  identical ;  but  in  this  case 
it  can  not  be  doubted  that  the  siderite  is  the  original  source  of  the  hema- 
tite. Where  the  siderite  is  less  abundant  and  the  chert  more  plentiful,  the 
rliombohedra  of  siderite  are  set  in  a  matrix  of  chert,  which  may  consist 
wholly  of  individualized  quartz,  but  which  sometimes  apparently  contains 
some  opaline  silica.  Oftentimes  bands  consisting  largely  of  silica  alternate 
with  bands  consisting  largely  of  siderite.  In  the  less  pure  phases,  near 
the  base  of  the  Negaunee  formation,  the  cherty  siderite  in  some  cases 
alternates  with  strata  of  an  impure  clayey  rock,  approaching  the  Siamo 
ferruginous  clay -slates;  in  other  cases,  mingled  with  the  siderite  itself 
is  fragmental  material,  including  both  quartz  and  feldspar,  and  their 
alteration  products.  Not  infrequently  within  these  semifragmental  rocks, 
along  cracks  and  joints,  all  transitions  between  the  impure  siderite  and  a 
ferriferous  or  cherty  slate,  partly  fragmental  and  partly  nonfraginental, 
may  be  seen. 

Where  the  sideritic  slates  are  altered,  not  by  weathering,  but  by  deep- 
seated  metasomatic  action,  there  develoj)  abundant  magnetite  and  a  light 
amphibole,  nonpleochroic  in  thin  section,  which  will  be  called  griinerite. 
There  is  thus  produced  a  magnetite-griinerite-siderite-slate,  intermediate 
between  the  sideritic  slates  and  the  typical  magnetite-griinerite-schists.     It 


368  THE   MAKQUETTE   lEOXBEAEING   DISTEICT. 

appears,  as  in  the  cases  of  the  Penokee  and  Animikie  series,  that  when  the 
siderite  decomposed  there  was  an  abundance  of  siHca  present,  and  conditions 
not  favorable  to  oxidation,  so  that  the  siHca  united  with  the  iron  oxide 
alone,  producing  griiuerite,  or  with  the  iron,  calcium,  and  magnesium 
oxide,  producing  a  mineral  intermediate  between  griinerite  and  actinolite. 
The  iron  which  does  not  combine  with  the  silica,  not  being  completely 
oxidized,  is  in  the  form  of  magnetite.  This  is  the  first  stage  of  the 
development  of  the  magnetite-griinerite-schists  from  the  sideritic  slates. 

The  gruneritic  and  magnetitic  schists  (PI.  XVIII)  may  vary  from  nearly 
pure  griinerite-schists  to  nearly  pure  magnetite-schists.  However,  the 
more  common  phase  is  the  griinerite-magnetite-schist.  Griinerite,  magnet- 
ite, and  quartz  are  the  three  important  constituents,  but  in  some  areas 
quartz  is  in  subordinate  quantity.  The  minerals  are  usually  concentrated 
to  some  extent  into  bands,  although  a  layer  composed  chiefly  of  any  one 
of  the  three  always  includes  a  greater  or  less  quantity  of  the  other  two. 
In  many  cases  within  a  felted  mass  of  griinerite  or  magnetite  are  found 
many  rhombohedra  of  siderite,  and  this  siderite  has  such  relations  to  the 
griinerite  and  magnetite  as  to  suggest  that  these  minerals  developed  from 
the  siderite.  We  thus  have  evidence  of  the  transition  of  the  sideritic  slates 
into  a  griinerite-magnetite-schist.  When  the  transformation  is  complete 
there  remains  no  evidence  of  the  change,  as  the  rock  then  consists  of  a 
completely  interlocking  crystalline  mass  of  the  three  minerals,  griinerite, 
magnetite,  and  quartz. 

Not  infrequently  with  the  magnetite  is  a  varial^le  quantity  of  hema- 
t  .  In  some  cases  this  appeal's  to  have  been  an  early  development,  simul- 
taneous with  the  magnetite,  and  in  other  cases  it  has  resulted  from  the 
weathering  of  the  rock,  developing  either  from  the  magnetite  or  from  the 
griinerite.  Less  frequently  limonite  is  found  in  similar  relations.  A  common 
hornblende  appears  in  some  cases  to  be  separable  from  the  griinerite  by  a 
decided  pleochroism,  and  the  two  often  occur  in  the  same  section  or  inter- 
grown  in  the  same  individual.  Not  infrequently  the  quartz  grains  have  a 
peculiar  parallel  arrangement,  with  their  longer  axes  in  a  common  direction, 
and  with  this  an  undulatory  extinction.  This  is  taken  as  indicating  that 
these  rocks  have  been  subjected  to  stress  during  or  subsequent  to  the  time  the 


PETROGKAl'niGAL   (inARACTEK   OF    NEGAUNEB   FORMATION.       369 

quartz  developed.  The  gTiiuerite  and  magnetite  are  closely  associated, 
often  penetrating  eacli  other,  and  are  also  fovind  within  and  penetrating  the 
quartz,  showing  that  the  minerals  developed  to  some  extent  simultaneously, 
although  the  quartz  appears  on  the  whole  to  be  somewhat  later  than  the 
griinerite  and  magnetite.  In  the  finer-grained  phases  opaline  silica  is  also 
present.  As  in  the  case  of  the  sideritic  slates,  some  of  the  griineritic 
schists  contain  interstratified  or  intermingled  fragmental  material,  and  the 
rock  by  transition  passes  downward  into  the  fragmental  Siamo  slate.  In 
these  kinds  ordinary  hornblende  has  abundantly  developed,  and  chlorite 
and  biotite  are  important  secondary  products.  Sometimes  associated  or 
included  in  the  magnetite-griinerite-schists  is  a  great  deal  of  secondary 
garnet,  and  this  is  particularly  abundant  adjacent  to  greenstones,  showing 
that  its  development  is  related  to  the  intrusives  (see  pp.  513-514). 

The  development  of  the  griinerite-magnetite-schists,  in  contrast  with 
the  ferruginous  slates,  cherts,  and  jaspers,  seems  to  have  been  favored  by 
deep-seated  metasomatic  changes,  rather  than  by  weathering  processes. 
This  is  indicated  bj^  the  following  facts:  Where  weathering  has  been  active, 
the  ferruginous  slates  and  cherts  are  found  rather  than  the  griinerite-mag- 
netite-schists; the  griinerite-magnetite-schists  where  weathered  have  been 
partly  transformed  into  the  ferruginous  slates  or  cherts;  the  go-iinerite- 
magnetite-schists  are  usually  closely  associated  with  the  greenstones.  This 
suggests  that  the  heat  of  these  intrusives  increased  the  activity  of  percolat- 
ing wators;  possibly  also  the  heat  helped  to  decompose  the  iron  carbonate; 
and  the  greenstones  may  also  have  furnished  alkalis  to  assist  in  the  solu- 
tion of  silica.  The  silica  in  solution  united  with  the  protoxides  presenl  ^r., 
produce  the  griinerite  and  other  amphiboles,  the  excess  of  iron  oxide,  not 
coTupletely  oxidized,  remaining  as  magnetite. 

The  ferruginous  slates  (PI.  XIX)  consist  of  cherty  silica,  like  that  of 
the  sideritic  slates,  and  of  hematite  and  limonite,  the  latter  minerals  occu- 
])ying  the  place  of  the  siderite  in  the  sideritic  slates.  Where  the  iron 
oxides  are  abundant  the  slides  consist  of  a  continuous,  ramifying  mass  of 
hematite  and  limonite,  within  Avhicli  the  numerous  patches  or  particles 
of  cherty  silica  are  set.  Where  the  silica  is  aljundant  the  reverse  relations 
MON  xxviii 24 


370  THE   MARQUETTE   IROX-BEARING   DISTRICT. 

obtain.  The  maimer  in  which  this  phase  of  rock  developed  from  the 
sideritic  slate  has  already  been  indicated. 

The  ferruginous  cherts  (Pis.  XX— XXIII)  difter  from  the  ferruginous 
slates  chiefly  in  that  the  silica  has  been  more  extensively  rearranged.  As  a 
consequence  of  this  the  chert  and  iron  oxides  are  more  or  less  concentrated 
in  alternating  bands,  instead  of  being  uniformly  mingled  in  a  mass,  as  in 
the  ferruginous  slates.  However,  the  chert  bands  are  never  free  from  the 
iron  oxide,  nor  are  the  iron-oxide  bands  ever  free  from  the  chert.  Between 
almost  pure  iron  oxide  and  almost  pure  chert  bands  there  are  all  grada- 
tions. The  silica  of  the  chert  is  usually  completely  individualized,  but 
in  different  sections  varies  from  partly  amorphous  tlu-ough  finely  crys- 
talline to  rather  coarsely  crystalline.  The  quartz  which  does  not  show 
evidence  of  much  rearrangement  is  very  like  in  size  of  granules  and  in 
appearance  to  the  quartz  of  the  sideritic  slates,  but  that  in  veins  and  filled 
areas  is  much  more  coarsely  crystalline.  In  arrangement  the  particles  of 
iron  oxide  appear  to  be  wholly  independent  of  the  quartz.  There  is  no 
apparent  concentration  of  the  iron  oxides  between  the  quartz  grains,  but 
they  occur  concentrated  in  laminse  or  as  separate  flecks  included  in  the 
grains  of  quartz,  just  as  though  they  were  all  in  their  present  positions 
before  the  silica  began  to  crystallize.  In  the  ferruginous  cherts  which 
are  near  the  ore  bodies  cavities  are  very  common,  due  to  the  solution  of 
the  quartz.  These  cavities  have  often  been  subsequently  partly  or  wholly 
filled  by  hematite.  In  all  these  particulars  these  ferruginous  cherts  are 
similar  to  those  from  the  Penokee  and  Animikie  districts,  but  they  differ 
from  them  in  not  showing  extensively  the  somewhat  remarkable  concre- 
tionary structure  characteristic  of  those  districts,  although  in  a  few  places 
this  is  well  developed. 

The  Marquette  ferruginous  cherts  have  been  subjected  to  profound 
dynamic  action,  and  the  brittle  rock  has  become  shattered  through  and 
through,  producing  innumerable  cracks  and  fissures,  and  not  infrequently 
reibungsbreccias  (Pis.  XXI-XXIII).  Within  the  spaces  thus  produced  sec- 
ondary hematite  and"  magnetite  in  well-defined  crystals  have  formed.  By  the 
crystal  outlines  the  secondary  iron  oxide  can  frequently  be  discriminated 
from  that  present   before  the    mashing    occurred.      The  metamorphosing 


PETEOGKArHICAL  CHAKAOTEE  OF  NEGAUNEE  FOEMATIOK      371 

processes  were  so  long  continued  within  the  ferruginous  cherts  that  it  could 
not  be  expected  that  residual  siclerite  should  occur,  yet  in  one  or  two  cases  a 
small  quantity  is  found.  However,  in  the  field  the  gradations  are  so  com- 
plete that  one  can  not  doubt  that  these  rocks  were  produced  by  the  altera- 
tion of  an  original  sideritic  chert,  combined  with  secondary  infiltration.  It 
is  highly  probable  that  much  of  the  iron  oxide  and  much  of  the  silica  now 
present  were  derived  from  an  iron  carbonate  once  above  the  ferruginous 
cherts,  but  now  removed  by  erosion.  The  ferruginous  slates  represent 
the  kinds  of  rock  produced  by  the  simple  oxidation  in  place  of  the  orig- 
inal sideritic  slates,  and  from  them,  by  the  secondary  actions  described, 
are  produced  the  ferruginous  cherts  As  in  the  case  of  the  sideritic  slates 
and  the'  griinerite- magnetite -schists,  fragmental  material  is  occasionally 
recognized. 

In  thin  section  the  jaspilites  (Pis.  XXIV-XXVII)  have  a  minutely 
laminated  character,  each  of  the  coai'ser  bands,  as  seen  in  hand  specimen, 
being  composed  of  many  laminae,  due  to  the  irregular  concentration  of 
the  iron  oxide.  These  laminae  are  of  greatly  varying  width.  They  unite 
and  part  in  a  most  irregular  fashion,  producing  a  mesh-like  appearance,  and 
frequently  laminae  disappear,  as  do  the  coarser  bands.  The  complex,  bright- 
red  jasper  bands  are  composed  mainly  of  finely  crystalline  cherty  quartz, 
but  they  are  everywhere  stained  with  minute  particles  of  blood-red  hema- 
tite. The  particles  of  quartz  average  less  rather  than  more  than  0.01  mm. 
in  diameter,  and  each  of  these  ininute  grains  contains  one  or  more  particles 
of  hematite.  These  are  concentrated  in  laminae  or  are  separate  flecks 
included  in  the  quartz  grains.  In  some  cases  the  hematite  appears  to  be 
somewhat  concentrated  between  the  grains,  but  in  general  it  is  arranged  in 
entire  independence  of  them,  as  though  it  were  present  before  the  silica  had 
crystallized.  The  most  ferruginous  bands  contain  a  predominant  amount 
of  iron  oxide,  but  in  them  is  included  much  quartz,  exactly  similar  to  that  of 
the  jasper  bands.  The  original,  translucent,  red,  mashed  hematite  is  easily 
discrinnnated  from  the  secondary,  crystal-outlined  hematite  and  magnetite. 

The  folding,  faulting,  fracturing,  and  brecciation,  spoken  of  in  hand 
specimen  (Pis.  XXV  and  XXVI),  are  beautifully  shown  under  the  micro- 
scope.    The  resultant  cracks  and  crevices  are  filled  with  secondary  quartz 


372  THE  MAKQUETTE  IRON-BEAEmG   DISTRICT. 

and  crystalline  hematite  and  maguetite.  This  quartz  is  much  more  coarsely 
crystalline  than  the  older  quartz,  the  grains  oftentimes  averaging  from  0.05 
to  0.1  mm.  in  diameter.  While  much  of  this  secondary  quartz  occurs  in 
veins  which  cut  across  the  original  lamination  of  the  rock,  a  great  deal  of  it 
was  deposited  parallel  to  the  lamination.  Its  likeness  to  the  vein  quartz  and 
its  coarseness  readily  discriminate  it  from  the  earlier  quartz.  The  crystal - 
outlined  hematite  and  magnetite  also  help  to  fill  the  veins  and  the  spaces 
between  the  micaceous  hematite  laminse  between  which  accommodation 
took  place.  The  secondary  material  usually  fills  the  spaces  entirely,  thus 
completely  healing  the  rock,  and  because  much  of  the  material  is  arranged 
parallel  to  the  original  lamination  the  structure  is  emphasized  by  the  sec- 
ondary impregnations. 

It  has  been  noted  that  the  jaspilite  is  characteristic  of  the  uppermost 
horizon  of  the  iron-bearing  formation — that  is,  it  is  innnediately  below  the 
next  overlying  series.  This  contact  zone  was  one  of  the  great  planes  of 
accommodation,  and  thus  the  d3niamic  effects  upon  the  jasper  are  explained. 
Between  the  jasper  horizon  and  that  at  which  the  ferruginous  cherts  occur 
is  a  transition  zone.  In  this  the  layers  of  siliceous  material  sometimes 
have  borders  of  red,  iron-stained  quartz.  It  has  been  explained  that  the 
chief  differences  between  the  jaspilites  and  feiTuginous  cherts  are  the  blood- 
red  character  of  the  minute  hematite  particles  and  the  micaceous  character 
of  the  ferruginous  layers  of  the  former.  It  appears  highly  probable,  there- 
fore, that  dynamic  action  transformed  the  ferruginous  chert  into  the  jasper, 
the  layers  of  earthy  hematite  being  sheared  into  micaceous  hematite,  and  the 
inclusions  of  earthy  hematite  being  changed  into  the  blood-red  variety. 

The  foregoing  general  description  is  of  the  rocks  as  they  occur  in  the 
eastern  pai't  of  the  district.  At  the  west  end  of  the  district  the  predominant 
varieties  of  the  Negaunee  formation  are  the  grlinerite-magnetite-schists  and 
the  jasper.  There  are  also  subordinate  amounts  of  ferruginous  chert.  In 
this  part  of  the  area  the  rocks  are  much  more  coarsely  crystalline  than  in 
the  eastern  part  of  the  district  CPl.  XXIII).  The  quartz  grains  in  the 
extreme  western  end  of  the  district  have  diameters  averaging  from  0.10  to 
0.15  mm.,  and  in 'the  southwest  arm  they  average  about  0.20  to  0.40  mm., 
and  run  as  high  as  1  mm.     It  will  be  seen  that  the  size  of  the  grains  is 


rETEOdllAPIIICAL   CHAItACTEll   OF   XEGAUNEE   FORMATION.       373 

many  times  greater  than  in  the  Islipemiiig--NegainK'e  area,  wliere  the 
average  diameters  vary  from  less  tlian  0.01  mm.  to  0.03  mm.  The  (|uartz 
grains  of  the  western  area  are  of  sutHcient  size  to  show  distinctly  undu- 
latory  extinction  and  fracturing,  the  latter  rarely  in  a  rectangular  manner. 
In  the  more  mashed  varieties  they  are  arranged,  to  some  extent,  with  then- 
axes  in  a  common  direction.  The  griinerite  is  also  coarsely  crystallized. 
Exact  comparison  with  the  griinerite  of  the  Ishpeming  area  is,  however, 
difficult.  The  jaspers  of  the  western  end  of  the  district  afford  a  good 
opportunity  to  observe  the  relations  of  the  included  particles  of  hematite 
and  the  grains  of  quartz.  ,  The  former  appear  just  as  if  they  were  in  their 
present  positions  before  the  silica  had  taken  the  remaining  space  and  crys- 
tallized. There  is  no  tendency  to  concentration  of  this  hematite  at  the 
borders  of  the  quartz  grains  or  in  the  cracks  formed  by  their  fractur- 
ing. In  the  jaspers  and  in  some  of  the  more  quartzose  griinerite  -  mag- 
netite-schists is  also  a  beautiful  concretionary  structure,  exactly  similar 
to  that  of  the  ferruginous  cherts  of  the  Penokee  district.  The  concen- 
tric zones  of  red  hematite,  separated  by  a  greater  or  less  distance,  appear 
as  if  painted  upon  the  quartzose  background,  the  grains  of  which  seem 
in  no  Avay  to  be  affected  by  the  hematite.  The  crystals  of  hematite 
and  magnetite  formed  still  earlier,  or  else  developed  where  the  red 
liematite  and  the  quartz  have  been  dissolved,  for  they  are  scattered  at 
random  through  the  section,  interrupting  the  concentric  zones  of  hematite 
at  many  places.  In  some  slides  the  concretions  are  decidedly  flattened 
by  pressure. 

The  foregoing  facts  show  that  in  these  jaspers  the  minerals,  witli  the 
possible  exception  of  the  crystals  of  hematite  and  magnetite,  had  assumed 
their  present  relations  before  the  last  orogenic  movement.  The  concre- 
tions, the  coarsely  crystalline  character  of  the  rocks,  and  the  absence  of 
the  sideritic  and  ferruginous  slates  imply  a  much  more  nearly  complete 
recrystallization  of  the  entire  formation  than  has  taken  place  in  the  eastern 
part  of  the  district.  If  the  original  rocks  in  the  western  part  of  the  district 
were  of  the  same  character  as  about  Ishpeming  and  Negaunee,  the  silica 
must  have  entirely  recrystallized.  It  is  to  be  noted  that  in  this  part  of  the 
district  the  other  formations  of  the  Marquette  series  are  also  much  more 


374  THE  MARQUETTE   lEOX-BEARING  DISTRICT. 

jjrofoundly  metamorphosed  than  they  are  farther  east.  Therefore  the 
unusually  modified  character  of  the  rocks  of  the  Negaunee  formation 
accords  with  what  would  be  expected  from  a  study  of  the  other  formations. 

On  account  of  the  opacity  of  the  iron  ores,  comparatively  little  is 
learned  by  a  study  of  their  thin  sections. 

The  magnetites  are  perfectly  opaque  in  transmitted  light,  and  in 
reflected  light  give  the  characteristic  spotty  appearance  of  that  mineral. 
Where  not  pure  the  usual  minerals  contained  in  the  iron  formation  appear 
with  their  ordinary  relations.  Those  most  plentifully  seen  are  quartz, 
griinerite,  muscovite,  and  biotite.  Occasionally  garnet,  and  chlorite  as  an 
alteration  product,  are  abundant.  Bordering  the  included  material,  the 
mao-netite  invariably  shows  crystal  outlines.  As  a  result,  each  area  of 
included  minerals  has  a  sen-ated  form. 

The  coarse  specular  hematites  are  made  up  mainly  of  large,  closely 
fittino-  flakes  of  hematite,  the  majority  of  which  take  an  imperfect  polish, 
and  liave,  therefore,  a  gray,  sheeny,  spotted  appearance.  The  flakes  which 
are  parted  along  the  cleavage  reflect  the  light  like  a  miiTor.  The  large 
number  of  individuals  of  this  kind  is  appreciated  only  by  rotating  the 
sections.  This  brings  successively  different  flakes  of  hematite  into  favor- 
able positions  to  reflect  the  light  into  the  microscope  tube.  In  some  sections 
cut  transverse  to  the  cleavage  the  schistose  character  of  the  rock  is  apparent 
in  reflected  light,  innumerable  laminae  of  hematite  giving  fine,  narrow, 
parallel,  dark  and  light  bands,  which  are  comparable  in  appearance  to  the 
polysynthetic  twinning  bands  of  feldspar.  As  both  the  magnetite  and 
the  hematite  are  usually  opaque,  the  two  minerals  in  general  can  not  be  dis- 
criminated, although  in  some  cases  the  crystal  forms  of  magnetite  are  seen, 
and  a  small  part  of  the  hematite,  much  of  it  in  little  crystals,  shows 
the  characteristic  blood-red  color.  The  important  accessory  minerals  are 
quartz,  griinerite,  feldspar,  and  muscovite.  Some  of  the  small,  detached 
areas  of  quartz  and  feldspar  appear  to  be  fragmental.  The  muscovite 
occurs  mainly  in  small,  independent  flakes,  but  some  of  it  is  apparently 
secondary  to  the  feldspar. 

The  fine-grained  specular  hematites  difi'er  from  the  so-called  micaceous 
hematites  chiefly  in  that  much  more  of  the  hematite  is  translucent,  and 


PETEOGRAPHICAL  CHARACTER   OF   NEGAUNEE   EORMATIOX.      375 

hence  at  the  edges  and  various  places  through  the  centers  of  the  sHdes  is  a 
brilliant  red  color.  The  slate  ores  in  reflected  light  show  the  laminated 
character  of  the  rock,  while  the  massive  ores  give  the  peculiar  spotty 
reflections,  exactly  the  same  as  magnetite. 

The  mottled  red  and  black  specular  ores  in  reflected  light  present  a 
pecviliar  appearance,  the  true  specular  material  giving  the  usual  brilliant, 
spotty  reflections,  while  the  soft  hematite  has  a  brownish-red  color. 

The  soft  hematites  in  transmitted  light,  in  many  slides,  show  the  char- 
acteristic blood-red  color  of  hematite,  although  for  the  most  part  the  sections 
are  so  thick  as  to  give  a  brownish  appeai-ance  or  are  opaque.  In  the  softest 
ores  in  reflected  light  a  dark  brownish-red  color  is  everywhere  seen,  which 
is  much  less  brilliant  than  that  presented  by  the  same  mineral  in  trans- 
mitted light.  In  some  of  the  soft  hematites,  however,  within  the  mass  of 
red  material  are  many  small  areas  which  reflect  the  light  in  the  same 
manner  as  the  specular  ores.  The  limonitic  hematites  difi"er  from  the  pure 
hematites  only  in  that,  in  both  transmitted  and  reflected  light,  in  many 
places,  the  reddish  colors  are  not  so  bright. 

The  foregoing  description  shows  that  there  are  gradations  from  the 
coarsest  magnetite  to  the  softest  limonitic  hematite. 

INTERESTING    LOCALITIES. 

The  localities  where  the  Negaunee  formation  is  exposed  are  so  numerous 
that  only  more  important  areas  of  exposures  will  be  here  mentioned. 

Michigamme  and  spurr. — At  Michigammo  aud  Spun'  mines  (Atlas  Sheet  V), 
and  in  the  area  connecting  them,  are  very  good  exposures  of  the  Negaunee 
formation  of  a  somewhat  exceptional  character.  At  the  lowest  horizon, 
adjacent  to  or  underlying  a  great  greenstone  ridge,  are  typical  exposures 
of  magnetite-griinerite-rock  and  magnetite-griinerite-schist.  Locally  the 
schist  mantles  areas  of  intrusive  greenstone  (PI.  XI),  and  the  latter  in  other 
places  includes  many  fragments  of  the  schist  (PI.  XII).  The  griinerite- 
magnetite-schists  are  ovei'lain  by  coarse  typical  red  jaspilite,  which  at  sev- 
eral places  is  just  above  the  greenstone.  The  jaspilite  of  this  locality  difl^ers 
from  that  of  most  of  the  district  in  that  the  pure  jasper  bands  are  of  unusual 
width,  sometimes  reaching  a  thickness  of  6  to  8  inclies.     Between  these 


37(3  THE    MARQUETTE    IRON-BEAEINd    DISTRICT. 

jasper  layers  are  belts  of  specular  micaceous  hematite,  the  lamiute  of  which 
show  slickeusides,  indicating  that  readjustment  has  occurred  between  them. 
The  jaspilite  varies  upward  into  a  banded  rock  consisting  of  alternate 
layers  of  pure,  white,  finely  crystalline  quartz  and  dark  bands  composed 
of  hematite  and  magnetite  (PI.  XXIII).  Intermediate  layers  show  the 
transition  between  the  rocks  having  bands  of  white  and  of  red  quartz. 
The  grains  of  the  jasper  and  white  quartz  belts  are  larger  than  those  of  the 
ordinarv  varieties  of  jasper,  and  are  to  a  large  degree  crystal-faced,  as 
shown  by  the  innumerable  reflecting  facets  when  held  in  the  sun.  At  the 
top  of  the  formation  is  a  thin  belt  of  ore,  making  up  a  part  of  the  ore  body 
of  the  Micliigamme  mine.  The  remainder  belongs  with  the  Ishpeming  for- 
mation. The  bands  of  white  chert  and  red  jasper  have  frequently  a  lenticular 
character.  The  rocks  are  often  folded  and  fractured  in  a  minor  way.  The 
cracks  are  filled  with  secondary  magnetite,  and  more  rarely  griinerite.  In 
some  places  the  folding-  was  so  severe  as  to  make  genuine  breccias.  At  one 
place,  a  short  distance  east  of  the  Spurr  mine,  the  inter-Marquette  erosion 
cut  away  all  of  the  jasper,  and  here  the  griinerite-magnetite-schist  is 
at  the  top  of  the  formation.  A  minor  fold  here  occurs,  so  that  in  a  single 
exposure  the  strike  may  be  seen  to  vary  from  an  east-west  direction  to  a 
northwest  and  finally  to  a  nortli  direction.  North  of  the  Spurr  mine 
minor  corrugations  are  seen,  which  give  local  northern  dips  in  the  general 
southward-dipping  formation. 

In  thin  section  much  of  the  Michigamme  and  Spurr  jaspilite  shows  a 
concretionary  arrangement  of  the  iron  oxide,  many  of  the  concretions  being 
made  up  of  a  large  number  of  concentric  rows  of  hematite  and  magnetite 
particles.  While  much  of  the  hematite  is  in  small  particles  or  areas  in 
these  concretions,  in  many  of  them  are  large  crystals,  which  look  like  later 
infiltrations.  The  quartz  is  much  more  coarsely  crystallized  than  the  quartz 
of  the  formation  in  the  main  ai'ea  about  Ishpeming  and  Negaunee  and  to 
the  south  and  east  of  these  towns,  the  average  grains  being  from  0.10  to 
0.15  mm.  in  diameter.  Each  of  these  quartz  grains  contains  a  large  number 
of  the  smaller  crystals  and  flecks  of  hematite.  Also  included  in  these 
quartzes  are  numerous  long,  minute,  curved  needles  of  rutile.  The  grii- 
nerite of  the  jaspers  has  usually  a  distinct  pleochroism,  giving  yellow  and 


INTERESTING    LOCALITIES   OF   NEGAUNEE   FORMATION.       377 

greenish  colors.  As  usual,  the  griuierite  has  a  tendency  to  be  associated 
with  the  magnetite  and  hematite.  In  one  case  an  opaque  crystal  of  hem- 
atite or  magnetite  was  found  to  he  surrounded  by  blades  of  griinerite,  each 
blade  being  parallel  to  one  of  the  sides  of  the  crj-stal.  Where  the  con- 
cretionary jasper  is  mashed,  the  concretions  have  an  oval  form,  the  longer 
axes  being  in  a  commoh  direction.  In  the  banded  ferruginous  rock  con- 
taining white  quartz  layers  (PI.  XXIII)  the  quartz  grains  contain  very  little 
oxide  of  iron.  Why  this  material  is  absent  here  and  present  in  the  jasper 
is  not  apparent.  The  grains  of  quartz  in  both  the  red  and  Avhite  siliceous 
layers  in  many  slides  have  crystal  outlines,  appearing  in  thin  section  as 
closely  fitting  polygonal  areas. 

By  an  increase  of  magnetite  and  griinerite  and  a  decrease  of  hematite 
the  jaspilites  pass  into  typical  griinerite-magnetite-schists.  Each  quartz 
grain  includes  hematite  and  magnetite  crystals,  and  often  blades  of 
griinerite.  A  concretionary  arrangement  of  iron  oxide  occurs  in  the 
transition  phases.  Pleochroic  hornblende  is  absent.  In  the  most  strongly 
griineritic  rock,  which  is  prevalent  near  the  base  of  the  tormation,  the 
quartz  almost  disappears,  and  there  is  a  background  composed  of  inter- 
locking blades  of  griinerite  which  include  a  large  amount  of  magnetite, 
and  thus  they  become  griinerite -magnetite -rocks.  The  iron  ores  are 
magnetites. 

Boston  and  Dexter  areas. — East  of  tlic  Micliigammc  mine  the  exposures  of  the 
Negaunee  formation  are  rare  for  nearly  15  miles.  However,  in  the  SW.  ^ 
sec.  32,  T.  48  N.,  R.  28  W.,  is  the  Boston  mine  (Atlas  Sheet  XVIII).  In 
sees.  3  and  4,  T.  47  N.,  R.  28  W.,  are  a  number  of  exposures  of  the  forma- 
tion (Atlas  Sheets  XIX  and  XXII),  and  north  of  the  center  of  sec.  3  is  the 
Dexter  mine.  On  the  line  between  sees.  3  and  4  is  the  contact  between 
the  Negaunee  formation  and  the  Groodrich  quartzite.  The  unconformity 
between  the  two  formations  is  here  not  marked.  The  two  are  slightly 
overturned,  so  that  the  quartzite  appeai-s  to  lie  under  the  Negaunee  jasper. 
A  short  distance  to  the  noi'theast  is  a  hill  composed  largely  of  the  Negaunee 
formation,  but  at  the  foot  of  its  northern  slope  is  found  the  Siamo  slate,  so 
that  at  this  place  we  have  the  Negaunee  formation  accurately  delimited 
above  and  below. 


378  THE  MAKQUETTE  IRON-BEAEING   DISTEICT. 

Excelsior  area — East  of  tliG  Dcxtei"  miue  tlid'e  are  again  no  exposures  of 
the  formation  for  2^  miles.  However,  in  sees.  4,  6,  and  6,  T.  47  N.,  R.  27  W. 
(Atlas  Sheet  XXV),  the  Negaunee  belt  is  exposed  at  very  numerous  locali- 
ties. The  formation  here  has  usually  a  somewhat  regular  east-west  strike 
and  a  southern  dip.  In  a  few  places,  especially  in  sec.  4,  minor  folds  and 
brecciation  were  observed.  At  the  old  Excelsior  mine,  just  west  of  the 
east  line  of  sec.  6,  the  contact  is  again  exposed  between  the  Negaunee  forma- 
tion and  the  Goodrich  quartzite,  and  here  the  evidence  of  unconformity 
is  strong,  the  quartzite  and  slate  appearing  to  mount  upon  and  mantle 
around  the  Negaunee  strata  on  the  east  side  of  the  pit.  In  these  exposui'es 
the  rock  is  mainly  ferruginous  chert.  In  thin  section  the  quartz  is  of  the 
finely  crystallized  kind  of  the  Ishpeming-Negaunee  area,  and  thus  contrasts 
with  that  of  the  Michigamme  and  Spurr  area. 

Lake  Bancroft  area. — Upou  tlic  soutli  slope  of  tlic  bluff  uorth  of  Lake  Ban- 
croft (Atlas  Sheet  XXVIII),  and  at  various  places  in  the  little  valley  separa- 
ting the  two  ridges  of  greenstone  north  of  this  lake,  are  found  exposures  of 
hematitic,  magnetitic,  griineritic  schists.  These  appear  to  have  been  caught 
in  the  intrusive  rocks.  It  is  interesting  to  note  that  all  of  the  rocks  here 
found  are  of  the  griinerite-magnetite-schist  variety,  while  the  ordinary 
2Dhases  of  the  formation,  both  to  the  west  and  to  the  east,  are  the  ferrugi- 
nous cherts  and  jaspers,  except  the  griineritic  and  sideritic  slates  adjacent 
to  a  greenstone  a  short  distance  east  of  Lake  Bancroft,  in  the  north  part  of 
the  city  of  Ishpeming.  In  thin  section  the  Lake  Bancroft  rocks  show  a 
peculiarly  finely  crystalline  or  partly  amorphous  siliceous  background. 
Also,  the  larger  part  of  the  iron  oxide  is  in  the  form  of  hematite,  this  being 
due  to  weathering.     The  griinerite  is  stained  deep-red  by  hematite. 

Teal  Lake  area. — The  uext  Importaut  cxposures  to  the  east  are  those  of  the 
Teal  Lake  iron  range,  just  south  of  Teal  Lake  (Atlas  Sheets  XXVII  and 
XXVIII).  The  interest  in  this  locality  lies  in  the  fact  that  the  ferruginous 
chert  of  the  Negaunee  formation  rests  directly  upon  the  Siamo  slate.  As 
has  been  said,  the  uppermost  horizon  of  the  latter  formation  is  here  a  slaty 
gray  wacke,  or  a  rock  approaching  a  ferruginous  quartzite.  At  many  places 
the  change  from  the  slate  to  the  iron  formation  is  sudden,  the  clean  ore  or 
the  ferruginous  chert  resting  upon  the  ferruginous  slate  or  graywacke  with 


INTERESTING  LOCALITIES   OF   NEGAUXEE   FORMATION.       379 

no  transition  horizon.  At  other  places  there  are  minor  iuterlaminations  of 
the  two.  The  ferruginous  chert  of  the  iron  formation  has  a  very  regular 
strike  and  dip,  being  remarkably  free  from  the  minor  folding  which  is  so 
prominent  in  the  iron  formation  to  the  southward. 

As  examined  in  thin  section,  the  ferruginous  slates  and  cherts  of  this 
locality  diifer  from  those  of  other  places  only  in  that  the  lower  horizons 
show  a  certain  amount  of  fragmental  quartz  mingled  with,  or  in  layers 
interbedded  with,  the  nonfragmental  material.  This  clastic  quartz  is  often 
enlarged.     Also  mica  is  occasionally  seen. 

Negaunee-ishpeming  area. — Soutli  of  the  Teal  Lake  range  are  numerous  expos- 
ures adjaceiit  to  the  mines  of  Negaunee-ishpeming  and  vicinity  (Atlas 
Sheets  XXV  and  XXVIII).  Here,  as  has  been  explained,  the  iron  forma- 
tion occupies  the  lower  lands,  usually  those  below  the  1,400-foot  contour 
(Pis.  XIII  and  XIV).  The  exposures  are  in  a  series  of  bay-like  areas,  which 
open  out  to  the  west,  but  are  surrounded  and  overtopped  to  the  north,  east, 
and  south  bv  amphitheaters  of  greenstone  (PI.  XIII).  In  these  bays  are 
found  some  of  the  great  mines  of  the  area,  such  as  the  Cleveland  Cliffs, 
Lake  Superior,  Lake  Angeliue,  and  Salisbury.  At  or  close  to  the  contact 
with  the  Goodrich  quartzite  the  rock  is  always  typical  banded  ore  and  jasper 
or  jaspilite  (Pis.  XVI,  XXIV-XXVII),  and  at  the  lower  horizons  it  is  the 
typical  ferruginous  chert  (Pis.  XX-XXXII).  Between  the  two  there  are 
often  gradations,  but  often  also  they  are  separated  by  a  dike  of  altered 
gi-eenstone.  Mining  has  shown  that  the  masses  of  greenstone  not  only 
border  but  underlie  the  bays  of  iron  formation,  being,  however,  deeper 
below  the  surface  in  passing  west,  thus  making  westward-plunging  basins 
of  greenstone  in  which  the  Negaunee  formation  material  rests  (PI.  XIII). 
At  the  bottoms  of  these  basins  are  the  great  ore  deposits  of  the  district. 

Thus  in  this  area  are  found  the  largest  ore  deposits  and  the  most 
numerous  varieties  of  the  ferruginous  chert  and  jasper.  The  strike  of  the 
formation  is  generally  east  and  west,  corresponding  to  the  close  north-south 
folds;  but  as  the  folding  is  highly  complex,  this  probably  being  in  part  due 
to  the  intrusive  greenstones,  strikes  in  all  directions  may  be  found.  The 
fen-uginoT^s  chert  and  jasper  are  most  intricately  crumpled,  and  are  broken 
and  fixulted   in   a  minor  wav.     The  brilliant  appearance  of  the  crumpled 


380  THE   MAEQUETTE   lEONBEARING  DISTEICT. 

and  sometimes  brecciated  jasper  may  be  particularly  well  seen  on  the 
so-called  jasper  bluff  southeast  of  Ishpeming  (Pis.  XXV  and  XXVI).  In 
the  exposures,  and  particularly  in  the  open  pits  and  waste-dump  material 
of  the  mine,  may  be  seen  all  stages  of  the  processes  of  replacement  of  the 
siliceous  bands  of  the  ferruginous  chert  and  jasper  by  iron  ore. 

At  Negaunee  a  section  from  the  Jackson  mine  to  the  southeast  (Atlas 
Sheets  XXVIII  and  XXXI)  gives  the  fullest  known  succession  from  the 
jasper  above  to  the  comparatively  little  altered  griinerite-siderite-slate  below. 
At  the  Jackson  open  pits  exposures  of  the  Negaunee  formation  is  beautiful 
typical  banded  jaspilite  (Pis.  XXIV  and  XXVII,  fig.  1).  To  the  south  the 
red  quartz  is  somewhat  suddenly  replaced  by  the  white  quartz,  and  in  place 
of  the  jasper  we  have  the  ferruginous  chert  (Pis.  XXI  and  XXII).  This 
jasper  and  ferruginous  chert,  while  having  a  general  northward  dip,  shows 
minor  crenulations,  faulting,  and  brecciation,  becoming  not  infrequently  a 
genuine  reibungsbreccia.  As  the  ridge  of  greenstone  is  neared  in  the 
southeast  part  of  sec.  1,  the  rocks  of  the  iron  formation  change  gradually 
from  the  typical  broken  ferruginous  chert  to  a  somewhat  regularly  lami- 
nated ferruginous  slate,  in  which  a  large  part  of  the  oxide  of  iron  is  limonite. 
The  change  from  this  ferruginous  slate  to  the  ore  is  very  beautifully  shown 
at  the  Grand  Rapids  mine.  To  the  south  of  the  greenstone  ridge  there  at 
once  appears  the  sideritic  griinerite- magnetite  slate.  While  the  section 
is  not  complete,  no  one  can  study  this  locality  without  becoming  con- 
vinced that  the  evenly  banded  sideritic  slate  (PI.  XVII,  fig.  1)  to  the  south 
is  the  rock  from  which  the  regularly  laminated  ferruginous  slates  and 
griineritic  slates  have  developed,  and  that  from  these  the  ferruginous  chert, 
jasper,  and  ore  bodies  have  been  formed  b)^  combined  dynamic  action, 
metasomatic  change,  and  infiltration.  In  thin  section  the  rocks  of  the 
Ishpeming  and  Negaunee  area  include  all  phases  of  the  ferruginous  cherts 
and  jaspers  found  in  the  eastern  part  of  the  district.  To  describe  them 
would  be  but  to  repeat  the  general  description  of  these  rocks. 

Area  southeast  of  Ishpeming. — In  the  broad  arca  south  of  Negaunee  and  east 
of  Ishpeming  (Atlas  Sheets  XXVIII,  XXIX,  XXXI,  and  XXXII),  very 
largely  composed  of  greenstone,  there  are  everywhere  found,  in  the 
valleys   between   the   greenstones,  exposures  of  sideritic   slates,  sideritic 


INTERESTING    LOCALITIES   OF   NEGAUNEE    FOKMATION,       381 

griinerite  -  magnetite  -  slates,  griinerite  -  magnetite  -  schists,  ferruginous  slate, 
and  occasionally  ferruginous  chert.  The  widesjiread  distribution  of  the 
griineritic  and  magnetitic  phases  of  the  formation,  in  connection  with  these 
greenstones,  at  once  suggests  that  the  intrusive  rocks  are  the  cause  of  the 
development  of  these  varieties  of  the  Negaunee  formation  from  the  sideritic 
slates,  rather  than  the  ferruginous  cherts  and  jaspers.  How  this  alteration 
occurred  has  already  been  explained  in  the  general  description  of  the 
griinerite-magnetite-schists  (pp.  359-361,  368-369).  The  presence  of  much 
residual  siderite  in  this  area  is  doubtless  partly  explained,  at  least,  by  the 
protective  influence  of  the  greenstones,  and  jjossibly  also  by  the  relatively 
impervious  character  of  the  secondary  grilnerite-magnetite-schist  as  com- 
pared with  the  broken  ferruginous  cherts  and  jaspers. 

In  thin  section  all  the  varieties  of  rocks  described  under  the  general 
description  (pp.  358-375)  as  clierty  siderites,  magnetitic,  griineritic,  and 
sideritic  slates,  magnetitic  and  griineritic  schists,  and  ferruginous  slates,  are 
found,  with  all  of  their  transition  varieties.  To  give  a  description  here 
would  be  substantially  to  repeat  that  already  given,  and  a  few  only  of  the 
peculiar  features  will  be  mentioned. 

It  is  in  this  area  that  the  very  finely  crystalline  and  apparently  partly 
amorphous  forms  of  silica  are  found.  In  some  cases  the  siliceous  back- 
ground seems  to  be  almost  nonpolariziug-.  In  a  more  advanced  stage  of 
alteration,  minute  opaline  droplets  or  granules,  averaging  perhaps  0.01  mm. 
in  diameter,  and  surrounded  by  films  of  iron  oxide,  constitute  the  back- 
ground. These  dro})lets  or  granules  are  rather  characteristic  of  the  early 
stages  of  the  rearrangement  of  the  silica.  The  silica  is,  however,  ordinarily 
completely  indi^^dualized,  and  occurs  either  in  granules  similar  to  the  drop- 
lets or  in  ordinary  chert,  the  grains  averaging  in  some  sections  as  much  as 
0.03  mm.  in  diameter.  The  hematite  in  the  ferruginous  slates,  even  where 
the  siderite  has  wholly  disappeared,  has  a  decided  tendency  to  occur  in 
rhombohedra.  At  the  lower  horizons  of  the  formation,  where  fragmental 
material  begins  to  appear,  certain  peculiar  varieties  are  found.  In  some 
cases  there  are  seen  large  feldspathic  areas,  which  appear  to  be  partly  altered 
into  or  replaced  by  the  magnetite,  griinerite,  and  quartz.  Small,  distinctly 
fragmental    grains  of  quartz   are   plentiful.     Not  infrequently   the   quartz 


382  THE   MAliQUETTE   IEONBEx!lEING   DISTRICT. 

grains  have  their  greatest  diameters  in  the  same  direction  and  have  a 
common  extinction.  In  some  slides  these  parallel-arranged  individuals  cut 
almost  at  right  angles  across  the  belts  of  magnetite  and  actinolite.  These 
facts  suggest  that  the  quartz  is  a  secondary  material,  which  has  arranged 
itself  as  demanded  by  the  differential  pressure.  In  other  slides  the  quartz 
has  a  peculiar  irregular  extinction,  Avhich  reminds  one  of  half-individualized 
material.  It  appears  unlike  truly  cherty  or  chalcedonic  quartz,  and  yet  is 
unlike  granulated,  coarse-grained  quartz.  Where  these  peculiar  varieties  of 
quartz  occur  the  iron  oxide  is  very  largely  magnetite,  mostly  in  the  form 
of  small  crystals.  In  many  slides  the  amphibole  is  decidedly  pleochroic, 
and  in  some  of  them  it  gives  beautiful  blue  and  violet  colors.  The  parti- 
cles are  so  small  that  they  could  not  be  isolated,  but  it  is  thought  that  this 
amphibole  developed  at  the  lower  horizons  because  in  the  mingled  uonclastic 
and  clastic  material  a  wide  variety  of  chemical  elements  were  available.  In 
passing  from  tliis  area  toward  the  Ishpeming  and  Negaunee  area  the  quartz 
shows  more  and  more  of  rearrangement  and  becomes  more  coarsely  crys- 
talline, grading  into  the  irregularly  laminated  varieties  which  have  been 
denominated  ferruginous  chert. 

Cascade  range. — Passlug  uow  to  thc  cast  cud  of  thc  southcm  belt,  at  the 
Cascade  range,  in  sees.  28,  29,  30,  31,  32,  and  33,  T.  47  N.,  R.  26  W.  (Atlas 
Sheet  XXXII),  we  find  the  most  extensive  exposures  of  ferruginous  chert 
and  jaspilite  in  the  district.  Also  there  are  here  complete  sections  from 
the  Ajibik  quartzite  below  to  the  Goodrich  quartzite  above.  Where  the  for- 
mation has  considerable  width,  as  in  sec.  28,  the  lower  horizons  of  the 
formation  are  the  typical  ferruginous  chert,  but  as  the  Goodrich  quartzite  is 
approached  the  rock,  as  usual,  becomes  typical  jasper.  In  the  W.  ^  sees.  29 
and  32,  and  in  sees.  30  and  31,  where  there  is  only  a  comparatively  narrow 
belt  of  the  Negaunee  formation  between  the  Ajibik  quartzite  and  the  Good- 
rich quartzite,  the  whole  of  the  formation  is  typical  banded  jasper.  It  is 
this  locality  which  strongly  suggests  that  the  position  of  the  iron-formation 
rocks  with  reference  to  the  overlying  Goodrich  quartzite,  rather  than  the 
particular  horizon  of  the  formation,  determines  whether  the  rock  is  mainly 
ferruginous  chert  or  jaspilite;  for  in  sees.  28  and  33  the  same  horizon  is  fer- 
ruginous chert  which  a  mile  or  two  to  the  west  is  typical  banded  jasper. 


INTEKESTING   LOCALITIES   OF    NEGAUNEE   FORMATION.       383 

In  the  great  exposures  in  sees.  29,  30,  31,  and  32,  the  folding,  brecciation, 
and  minor  faulting  of  the  formation  are  particularly  well  shown.  At  many 
places  between  the  Piatt  mine  and  Cascade  Brook  are  seen  the  transition 
phases  between  the  Negaunee  jasper  and  the  Ajibik  quartzite. 

In  thin  section  the  ferruginous  chert  and  jasper  of  the  Cascade  area  are 
in  no  respect  different  from  those  of  the  Ishpeming-Negaunee  area  except 
that  in  the  lower  horizons  fragmental  quartz  appears  in  the  slides  as  dissemi- 
nated grains  and  in  minute  layers. 

Foster-Lowthian  area. — Passiug  to  the  west,  tlicrc  are  again  great  exposures 
of  the  ferruginous  chert  in  sees.  21,  22,  23,  26,  and  27,  T.  47  N.,  R.  27  W. 
(Atlas  Sheets  XXVI  and  XXIX).  In  the  northeast  part  of  see.  20  is  a 
bluff  consisting  of  massive  greenstone,  greenstone-schist,  and  greenstone- 
conglomerate.  On  the  south  side  of  this  knob  the  greenstone-schist  and 
griinerite-magnetite-slates  appear  to  be  interbanded,  the  layers  varying 
from  a  few  inches  to  several  feet  across.  There  are  also  several  exposures 
of  griinerite-magnetite-schist  on  top  of  the  bluff.  Whether  the  greenstone 
is  an  intrusive  which  has  caught  fragments  of  the  Negaunee  formation,  or 
whether  it  was  a  contemporaneous  volcanic,  was  not  positively  determined, 
but  the  latter  is  perhaps  the  more  probable.  The  occun-euces  here  again 
strongly  suggest  that  the  igneous  rock  is  the  cause  of  the  development  of 
the  griineritic  and  magnetitic  kinds  of  the  Negaunee  formation.  Just  to  the 
north  of  the  greenstone,  at  the  open  pits  of  the  Lowthian  mine,  are  typical 
exposures  of  ferruginous  chert. 

Saginaw-Goodrich  area. — In  the  ueigliborliood  of  the  Saginaw  and  Goodrich 
mines,  in  sec.  19,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVI),  are  again  nearly 
continuous  exposures  from  the  Ajibik  quartzite  below  to  the  Goodrich 
quartzite  above.  This  locality,  however,  differs  from  the  Cascade  range 
in  that  the  southern  exposures  of  the  Negaunee  formation  are  the  typical 
griineritic  and  magnetitic  slates.  These  are,  however,  cut  through  by 
greenstones,  which  again  suggests  that  the  griineritic  and  magnetitic  char- 
acter is  due  to  intrusive  rocks.  At  the  bottom  of  the  formation  are  found 
ferruginous  quartzites,  which  stand  as  a  transition  horizon  between  the 
Ajibik  quartzite  and  the  Negaunee  formation.  For  the  most  j)art  the 
griinerite-magnetite-slates  have  a  somewhat  uniform  strike  and  dip,  but  in 


:^84:  THE   MARQUETTE   lEON-BEARING  DISTRICT. 

places  they  are  folded  into  a  series  of  minor  isoclinal  folds,  the  axes  of  which 
pitch  to  the  north  with  about  the  general  dip  of  the  formation.  In  horizontal 
plan  the  beds  of  one  of  the  folds  are  shown  by  fig.  23.  On  the  north  slope  of 
the  ridge  of  the  griinerite-magnetite-schists  this  material  grades  rapidly  into 
the  ferruginous  chert.  A  number  of  subordinate  folds  are  observable  at  the 
open  pits  of  the  Saginaw  mine.  These  have  superimposed  isoclinal  folds  of 
the  third  order.  As  a  result  of  the  many  minor  foldings  and  crinklings, 
the  rocks  are  much  broken.  The  strata  of  these  minor  folds  were  truncated 
bv  the  inter- Mai-quette  erosion,  and  consequently  the  Goodrich  quartzite 
cuts  across  the  bedding  of  the  Negaunee  formation  at  various  angles.  For 
instance,  at  the  old  Goodrich  mine,  at  one  place  the  strike  of  the  jasper  is 
almost  parallel  to  that  of  the  overlying  Goodrich  quartzite,  but  a  little  dis- 
tance to  the  east  abuts  pei'pendicularly  against  it  (figs.  20  and  21,  p.  335). 


PlQ.  23. — Horizimtal  plan  of  one  of  the  roinor  pitching  isoclinal  folds  in  the 
griinerite-magnetite-schist. 

Escanaba  River  area.— Wcst     of    tllC     Fltch    miuC,  SCC.    24,  T.  47    N.,    R.   28  W., 

there  are  no  exposures  of  the  iron-bearing  formation  for  more  than  4  miles. 
However,  in  sec.  20,  just  north  of  the  Escanaba  River,  and  iu  sec.  21,  T.  47  N., 
R.  28  W.  (Atlas  Sheet  XX),  are  exposures  of  griinerite-magnetite-schist, 
which  grade  below  into  a  novaculitic  rock  or  into  a  biotite-slate.  These 
are  apparently  transition  varieties  between  the  Negaunee  formation  and  the 
Ajibik  quartzite.  These  biotite-slates  are  very  similar  to  the  transition  rocks 
between  the  Negaunee  formation  and  the  Siamo  slate  at  Michigamme. 

In  thin  section  the  griinerite-magnetite-schists  are  in  all  respects 
similar  to  the  far  more  extensive  exposures  of  Mount  Humboldt,  described 
immediately  below.  The  biotite-slates  are  identical  with  those  near  the  top 
of  the  Siamo  formation  at  Michigamme,  even  in  the  matter  of  the  develop- 
ment of  a  certain  amount  of  hornblende  and  garnet.  The  novaculite, 
which  occurs  at  one  place,  has  a  fine-grained  quartzose  background,  and 
between  the  particles  are  innumerable  minute  flakes  of  sericite.     Coarser 


INTERESTING   LOCALITIES   OF  NEGAUNEE   FORMATION.       385 

bands  between  the  finer-grained  ones  distinctly  show  the  clastic  character 
of  the  quartz  g'rains. 

Humboldt  area. — Wcst  of  scc.  20  therc  are  no  exposures  for  a  mile  or  more, 
but  in  sec.  18  appears  the  southeastern  end  of  the  Mount  Humboldt  ridge 
(Atlas  Sheets  XVI  and  XIX).  This  is  of  minor  importance  in  sec.  18,  but 
in  sees.  11  and  12,  T.  47  N.,  R.  29  W.,  south  of  Humboldt,  becomes  an 
important  bluff,  with  steep-faced  sides  and  an  uneven  top.  Everywhere 
upon  the  ridge  are  large  and  numerous  exposures  of  the  Negaunee  forma- 
tion. Except  upon  the  north  and  west  borders,  the  rocks  of  the  Negaunee 
foiTnation  are  all  dense,  fine-grained,  but  distinctly  banded  griinerite- 
magnetite-schists.  The  strike  generally  corresponds  with  the  trend  of  the 
formation.  In  the  ledges  in  sec.  18,  southeast  of  the  road,  the  schistosity  is 
nearly  east  and  west,  while  the  true  bedding,  as  shown  b}'  the  minor  folds, 
is  southeast  and  northwest.  The  axes  of  these  minor  folds  plunge  to  the 
southeast.  On  the  north  and  west  faces  of  the  bluff  the  strikes  vary  with  its 
form,  being  parallel  to  its  face — that  is,  in  passing  from  the  north  face  of  the 
bluff  toward  the  west  the  strikes  gradually  change  to  the  southwest,  then 
to  the  south,  and  in  the  southwestern  part  even  to  the  southeast.  The  dips 
are  very  generally  to  the  north  or  northeast,  but  in  the  southwestern  part 
of  the  area  there  are,  for  short  distances,  reverse  or  southern  dips.  Between 
the  ledges  having  a  north  and  those  having  a  south  dip  there  is  a  little 
valley,  which  is  therefore  on  the  crown  of  the  anticline.  This  strongly 
suggests  for  this  part  of  the  area  a  quaquaversal  or  dome  structure,  although 
it  is  thought  that  the  anticline  spoken  of  is  of  a  second  order,  being  a  sub- 
ordinate bend  in  the  general  northward-dipping  beds.  The  secondary  fold 
shows  superimposed  folds  of  the  third  order,  and  these  again  those  of  the 
fourth  order,  and  so  on,  until  microscopic  plications  are  reached.  Where 
the  change  of  strike  is  the  most  rapid — that  is,  at  the  northwest  and  south- 
west corners  of  the  bluff — the  plications  are  closest,  and  in  some  places  the 
rocks  are  brecciated.  Throughout  the  central  part  of  the  bluff  the  expo- 
sures of  greenstone  are  almost  as  abundant  and  numerous  as  those  of  the 
griinerite-magnetite-schist  (PI.  XXXIII).  The  very  considerable  width  of 
the  belt  of  griinerite-magnetite-schist  in  sees.  11  and  12,  as  compared  with  its 
breadth  to  the  southeast,  may  be  in  part  due  to  the  large  amount  of  intrusive 
MON  xxviii 25 


386 


THE    MAKQUETTE   IRON-BEARING   DISTEICT. 


gi-eenstone,  the  formation  having  been  spread  out,  as  it  were,  by  the  entering 
material.  As  seen  in  cross-section,  the  greenstone  occurs  in  dome-like  forms 
under  the  schists,  or  as  masses  cutting  across  or  between  the  laj^ers.  In 
some  cases  the  dip  of  the  schists  is  comparatively  little  affected  by  the  intru- 
sive greenstone  (fig.  17,  p.  330),  but  in  other  cases  the  schist  to  some  extent 
mantles  over  the  greenstone,  although  the  schistosity  is  cut  across  on  one 


Fio.  24 Section  ahowins  relations  (, 


side,  and  the  dips  remain  prevailingly  to  the  north  (fig.  24).  As  seen  in 
plan,  the  greenstone  often  appears  as  oval  areas  surrounded  by  the  schists, 
the  latter  curving  about  the  intrusive  areas,  as  if  bent  by  it  (fig.  25). 

The  griineritic  and  magnetitic  rocks  of  Mount  Humboldt  rest  upon  the 
Ajibik  quartzite  below,  and  are  overlain  upon  the  north  and  west  slopes  of 
the  bluff  by  a  thin  belt  of  jaspilite,  connecting  the  row  of  mining  pits  which 


extend  from  the  old  Humboldt  to  the  Barron  mine.  This  jaspilite  is  in 
most  respects  like  that  at  Michigamme,  different  places  showing  beautifully 
the  white  and  red  siliceous  bands  and  varieties  intermediate  between  the 
two.  However,  at  Mount  Humboldt  the  jasper  is  extremely  plicated,  often 
brecciated,  and  the  ferruginous  bands  are  most  brilliant,  coarse-grained. 


INTERESTING   LOCALITIES   OF  NEGAUNEE   FORMATION.       387 

micaceous  hematite.  As  is  usual,  the  crevices  formed  by  tlie  foldiug,  both 
parallel  and  transverse  to  the  lamination,  are  healed  by  crystalline  mag- 
netite. No  transition  varieties  between  the  jasper  and  the  griineritic  rocks 
were  here  seen.  The  annular  area  where  such  rocks  would  occur,  if  they 
exist,  shows  no  exposures. 

At  the  south  end  of  the  Barron  mine,  at  the  southwest  end  of  the  bluff, 
it  is  said  that  a  diamond- drill  passed  at  once  from  the  jasper  to  the  granite. 
If  this  be  true,  the  Ajibik  quartzite  is  here  very  thin,  and  it  is  not  impossible 
that  the  whole  of  the  Lower  Marquette  series  for  some  distance  west  of 
Mount  Humboldt  was  cut  out  by  the  inter-Marquette  erosion. 

Under  the  microscope  the  griineritic  rocks  on  Mount  Humboldt  for  tlie~ 
most  part  pi'ove  to  be  but  slightly  quartzitic,  being  composed  almost  wholly 
of  griinerite  and  iron  oxides.  Many  of  the  slides  consist  of  a  nearly  solid 
mass  of  griinerite,  in  which  is  contained  comparatively  little  iron  oxide. 
While  the  blades  to  some  extent  are  in  various  directions,  there  is  a  distinct 
tendency  for  the  longer  axes  to  have  a  parallel  arrangement.  These  grii- 
nerite rocks  grade  into  those  in  which  the  magnetite  and  hematite  are 
plentiful.  Of  the  iron  oxides,  magnetite  is  predominant,  and  the  hematite 
seems  to  be,  in  part  at  least,  an  oxidation  product  of  the  magnetite.  Much 
of  the  magnetite  is  in  crystals  or  clusters  of  crystals.  Where  the  iron 
oxides  are  abundant  they  are  usually  more  or  less  concentrated  into  bands. 
Sometimes  the  parallel  blades  of  gi'iinerite,  following  the  schistosity,  are 
diagonal  or  perpendicular  to  the  bands  of  iron  oxide.  Not  infrequently  the 
magnetite-griinerite-rocks  are  gametiferous.  The  garnets  include  a  large- 
amount  of  griinerite  in  the  griinerite-rocks,  and  of  magnetite  and  griinerite 
in  the  magnetite-griinerite-rocks.  The  griinerite  needles  may  be  seen  pen- 
etrating the  garnets  in  all  directions.  The  garnets  appear  to  have  been  the' 
latest  development  and  to  have  included  or  absorbed  the  previously  existing 
minerals.  These  g-arnetiferous  varieties  are  particularly  abundant  adjacent 
to  the  greenstone  masses  and  at  low  horizons.  In  some  cases  between  an 
intrusive  greenstone  and  a  griinerite-magnetite-rock  there  is  an  almost  solid 
layer  of  garnet.  In  a  number  of  cases  associated  with  the  griinerite  is  a 
pleochroic  green  hornblende.  This  green  hornblende  occurs  as  independ- 
ent blades  and  as  parts  of  blades.     In  the  latter  case  a  blade  of  amphibole 


■388  THE   MAKQUETTE  IIION-BEARING  UlSTKIGT. 

•consists  in  part  of  gmuerite  and  in  part  of  green  hornblende.  The  cleav- 
age runs  from  one  to  the  other,  showing  that  both  are  parts  of  the  same 
crystal  individual.  The  two  are  discriminated  by  the  color  and  pleochro- 
ism  of  the  green  hornblende,  and  by  a  slight  difference  in  the  extinction. 
The  grlinerite-rocks  and  the  griinerite-magnetite-rocks  are  associated  with 
a  small  quantity  of  grilnerite-magnetite-schist.  In  the  passage  to  the  latter 
Tock  the  quartz  first  appears  as  small  oval  areas,  and  finally  as  distinct 
•bands.  The  quartz  grains  are  penetrated  through  and  through  by  the 
-griinerite  blades.  They  include  numerous  crystals  of  magnetite,  and, 
except  garnet,  therefore  appear  to  be  the  last  mineral  to  develop.  At  the 
iDottom  of  the  formation  in  the  southeastern  part  of  the  area  mica  and 
quartz  appear,  and  the  grlinerite-magnetite-schists  grade  into  ferruginous 
mica-slates  belonging  to  the  upper  part  of  the  Ajibik  quartzite,  or  more 
probably  the  equivalent  in  age  of  the  Siamo  slate.  These  transition 
varieties  are  frequently  gametiferous. 

In  thin  section  the  jaspilites  of  Mount  Humboldt  are  similar  to  those 
of  Michigamme.  A  small  amount  of  pleochroic  amphibole  is  present,  as 
at  that  locality,  and  in  one  case  this  has  partly  altered  to  chlorite.  The 
slides  show  remarkably  well  the  diff"erence  between  the  original  sheared 
hematite  and  the  secondary  magnetite.  The  former,  in  reflected  light, 
.may  be  seen  in  a  series  of  extremely  close  microscopic  folds,  the  laminsB 
•  of  which  are  often  broken  at  the  more  acute  bends.  The  crj-stals  of  mag- 
netite take  a  nearly  perfect  polish  and  give  brilliant  reflections.  These 
;are  found  to  be  largely  concentrated  at  the  places  of  fracturing  and  at 
.the  turns  of  the  folds.  As  is  well  known,  these  are  places  where  spaces 
■are  naturally  formed  by  the  folding  process.  So  marked  is  the  diff'erence 
between  the  reflecting  power  of  the  original  sheared  hematite  and  the 
magnetite  that  the  two  may  be  discriminated  with  the  naked  eye  in  section 
or  on  the  polished  surface.  The  slides  of  the  mashed  breccias,  looked  at 
with  the  naked  eye,  very  closely  resemble  the  mashed  conglomerates  of 
the  overlying  Ishpeming  formation.  The  broken  fragments  of  jasper  are 
flattened  in  a  common  direction,  the  different  ai'eas  overlap,  and  the  rock 
.has  a  very  strongly  conglomeratic  appeai'ance.  However,  when  examined 
under  the  microscope,  the  fragmental  quartz,  almost  invariably  present  in 
jthe  true  Ishpeming  conglomerates,  is  entirel}'  absent. 


INTERESTING  LOCALITIES   OF  NEGAUNEE   FOEMATION.       389 

Champion  area. — Passiiig  west  from  Mouiit  Humbolclt,  Ave  find  uo  expo- 
sures of  the  Negaunee  formation  for  about  3  miles.  Adjacent  to  and  south- 
east of  Champion  (Atlas  Sheets  XII  and  XIII)  there  again  jiitpear  numerous 
exposures  of  griinerite-magnetite-sehist,  constituting  a  high  ridge  running 
northwest  and  southeast.  The  rocks  are  very  dense  and  refractory,  retain- 
ing their  glacial  forms  almost  perfectly.  They  consist  of  alternate  bands 
which  vary  in  tlie  amount  of  contained  quartz.  The  strongly  griineritic 
bands  are  light-green;  those  with  little  griinerite  are  dull-white.  While 
the  rocks  have  a  strike  corresponding  in  a  general  way  with  that  of  tlie 
formation,  they  are  influenced  by  the  great  masses  of  intrusive  green- 
stone. This  is  well  shown  by  the  exposures  of  gi'iinerite-schist  in  the 
SE.  ^  sec.  31,  where  the  strike  curves  about  the  intrusive  mass  of  green- 
stone. West  of  this  intrusive  is  another,  of  less  magnitude,  and  again 
the  griinerite-schists  have  a  strike  parallel  to  it.  North  of  the  griineritic 
rocks,  constituting  the  foot-wall  of  some  of  the  mining  pits,  is  magnetitic 
chert  similar  to  that  of  the  Michigamme  mine.  The  (puu'tz  bands  are 
of  the  white  variety;  the  ferruginous  l^ands  are  largely  crystallized  mag- 
netite. In  thin  section  the  griineritic  rocks  of  Champion  are  very  similar  to 
those  of  Mount  Humboldt.  They  are,  perhaps,  somewhat  coarser  grained, 
and  quartz  is  rather  more  abundant.  The  slides  of  the  magnetitic  jas^jilite 
are  in  all  respects  similar  to  those  of  the  foot-walls  of  the  Michigamme 
and  Spurr  mines.  The  quartz  is  rarely  coarsely  crystalline.  Each  grairt 
includes  crystals  of  magnetite.  Also  the  magnetite  is  in  nearly  solid  bands 
between  the  siliceous  layers.  The  crystals  of  magnetite  at  the  borders  of 
the  bands  project  into  the  quartz.  The  amphibole  is  all  of  the  pleochroic 
variety,  giving  blue,  green,  and  yellow  colors. 

Republic  area. — Tlie  remaining  important  exposures  of  the  district  are 
those  adjacent  to  the  town  of  Republic  (Atlas  Sheet  XI).  These  are 
subsequently  described  in  a  separate  chapter  devoted  to  the  Republic  trough^ 
but  it  will  here  be  remarked  that  the  lower  part  of  the  Negaunee  formation 
is  griinerite-magnetite-schist,  while  the  upper  part  is  typical  coarse  jasper. 
The  lower  part  of  the  formation  is  much  intruded  by  greenstones,  which 
seem  to  follow  the  bedding  of  the  rock  in  a  general  way  for  short  distances^ 
the  same  as  at  Humboldt,  although  the  beds  are  also  crossed.  It  is  to  be 
noticed,  however,  that,  as  In  j)revIous  cases  where  tlie  greenstones  appear. 


390  THE   MAKQUETTE   lEON-EEAllINO  DISTKICT. 

the  Negaunee  rock  is  the  grunerite-maguetite-sehist.  Tliere  are  transition 
varieties  between  the  griiuerite  rocks  and  the  jaspihte.  In  certain  of  these 
the  layers  of  griinerite-schist  have  outer  zones  or  borders  of  jasper,  sug- 
gesting that  the  latter  developed  from  the  same  original  rock  under  conditions 
more  favorable  for  oxidation.  There  are  at  Republic  excellent  transition 
phases  and  interlaminated  beds  between  the  griinerite-magnetite-schist  and 
the  Ajibik  quartzite.  These  have  already  been  described  in  connection  with 
that  quartzite.  (See  pp.  292-294.)  Between  the  griinerite-magnetite-schist 
and  the  red  jasper,  which  occupies  the  upper  horizon  of  the  formation,  there 
is  a  rock  the  siliceous  bands  of  which  are  white  and  similar  to  those  which 
occur  at  the  topmost  horizon  at  Michigamme  and  Spurr.  In  a  phase  inter- 
mediate between  this  rock  and  the  jaspilite  the  white  bands  or  oval  areas 
of  quartz  have  a  jasper  border  (fig.  22,  p.  362).  The  relations  are  here, 
therefore,  just  the  reverse  of  those  at  Michigamme,  in  the  latter  place  the 
red  jasper  being  below  the  rock  with  white  bands,  while  at  Republic  the 
jasper  is  above. 

In  thin  section  the  jaspilites  and  griinerite-magnetite-schists  are  in 
nearly  every  respect  similar  to  those  of  Michigamme  and  Spurr,  but  a 
concretionary  arrangement  of  the  quartz  is  less  common.  While  the 
griinerite  generally  has  the  usual  radiating  arrangement,  in  one  case  it 
has  a  parallel  one,  resembling  that  of  muscovite  in  a  perfectly  lami- 
nated mica-schist.  At  low  horizons  beautiful  intergrowths  of  griinerite 
and  green  hornblende  are  found.  In  one  case  the  amphibole  of  alternate 
bands  consists  predominantly  of  griinerite  and  of  hornblende,  but  in  some 
bands  intergrowths  of  the  two  occur.  As  usual,  garnet  is  plentiful, 
•especially  adjacent  to  the  intrusive  greenstones  and  at  low  horizons. 
At  the  transition  horizons  to  the  Ajibik  quartzite  muscovite  and  chlorite 
also  appear. 

Magnetic  mine  area. — In  tlie  soutliwcst  tougue,  at  tlic  Maguctic  mine,  and  at 
various  places  to  the  south  (Atlas  Sheet  VII),  the  Negaunee  formation 
consists  of  a  coarsely  banded  griinerite-magnetite-schist.  The  amphibole, 
unlike  that  of  the  major  part  of  the  formation,  has  a  decided  green  color. 
The  quartz  of  the  siliceous  bands  is  also  more  coarsely  crystallized  than 
-anywhere  else  in  the  formation.     The  magnetite  occurs  to  a  great  extent  in 


INTERESTING  LOCALITIES   OF  NEGAUNEE   FORMATION.       391 

crystals  lai'ge  enough  to  be  distinct  to  the  naked  eye.  Many  bands  are 
composed  almost  wholly  of  griinerite  and  magnetite,  and  these  are  inter- 
laminated  with  those  in  which  the  quartz  is  equally  abundant  with  the  other 
minerals,  or  becomes  predominant,  serving  as  a  matrix  for  them. 

In  thin  section  these  griinerite  -  magnetite  -  scliists  are  seen  to  repre- 
sent the  extreme  phase  of  metamorphism  of  the  rocks  of  the  Negaunee 
formation.  The  quartz  is  more  coarsely  crystallized  than  in  any  of  the 
previously  described  rocks,  the  grains  averaging  from  0.2  to  0.4  mm.  in 
diameter,  and  in  one  of  the  coarser  varieties  averaging  about  1  mm.  The 
amphibole  is  of  two  kinds,  the  ordinary  white,  slightly  pleochroic  griiner- 
ite, and  a  green  amphibole.  The  griinerite  occurs  in  the  ordinary  blades 
and  crystals,  but  in  some  cases  has  a  very  uniformly  parallel  arrangement 
of  its  fibers,  Avhich  is  rather  unusual,  the  ordinary  varieties  in  other  parts 
of  the  Marquette  disti-ict  having  a  radiating,  sheaf- like  arrangement. 
This  may  indicate  that  dynamic  metamorphism  was  more  severe  in  the 
area  of  the  Magnetic  mine  than  elsewhere.  The  colored  amphibole  has  a 
tendency  to  occur  in  idiomorphic  crystals.  It- gives  beautiful  jjleochroism: 
C  is  light  pea-green,  b  is  dark  greenish-yellow,  a  is  light  transparent  j^ellow. 
The  absorption  formula  is  b  >  C  >  3-  In  one  case  the  angle  C'X  was 
found  to  be  16°.  In  the  same  shdes  different  bands  are  composed  pre- 
dominantly of  each  of  the  varieties  of  amphibole,  but  also  in  these  and  in 
other  slides  there  are  beautiful  and  complicated  intergrowths  of  the  two,  as 
at  Michigamme.  In  some  cases  the  green  variety  constitutes  the  outer 
bands  of  the  crystals ;  in  other  cases  the  reverse  is  true.  In  some  instances 
nuclei  of  the  colored  amphibole  are  entirely  surrounded  by  the  fibrous 
griinerite,  as  though  it  were  an  added  growth,  and  in  other  instances  the 
opposite  occurs.  The  magnetite  in  its  occurrence  is  the  same  as  in  the 
previously  described  griinerite-magnetite-schists.  In  the  gradation  phases  to 
the  Ajibik  quartzite,  biotite,  chlorite,  and  garnet  are  abundantly  associated 
with  the  green  amphibole. 

THE   IRON-ORE   DEPOSITS. 
THE    ORB    HORIZONS. 

The  ore  deposits  may  be  divided,  according  to  position,  into  three 
classes,  (1)  those  at  the  bottom  of  the  iron-bearing  formation,  (2)  those 


392  THE  MAKQUETTE   IKON-BEARING  DISTKICT. 

withiu  the  iron-beaiing  formation,  and  (3)  those  at  the  top  of  the  iron- 
beai-mg  formation.  (PI.  XXVIII,  fig.  1.)  By  the  last  is  meant  the  horizon 
immediately  below  the  next  overlying  formation,  the  Goodrich  quartzite. 
The  ore  deposits  of  the  second  class  frequently  reach  the  surface,  but  are 
not  at  the  uppermost  horizon  of  the  formation.  The  first  two  classes 
of  ores  are  generally  soft,  and  the  adjacent  rock  is  ferruginous  chert  or 
"soft-ore  jasper"  (Pis.  XX-XXII),  while  those  at  the  top  of  the  iron- 
bearing  formations  are  hard  specular  ores  or  magnetite,  and  the  adjacent 
rock  is  jaspilite,  also  called  "specular  jasper"  and  "hard-ore  jasper" 
(Pis.  XXIII-XXVII).  This  last  class  of  deposit  frequently  runs  up,  past 
the  unconformity,  into  the  Upper  Marquette  Goodrich  quartzite,  and  some- 
times some  of  these  ore  bodies  are  almost  wholly  in  this  position.  Strati- 
graphically  the  consideration  of  these  deposits  ought  to  be  deferred  until  the 
Goodrich  quartzite  is  treated,  but  they  are  so  closely  connected  genetically 
and  in  position  with  the  Lower  Marquette  ore  deposits  that  they  are  here 
treated. 

While  the  larger  number  of  ore  bodies  can  be  referred  to  one  or  another 
of  these  three  classes,  it  not  infrequently  happens  that  the  same  ore  deposit 
belongs  partly  in  one  and  partly  in  another.  To  illustrate:  The  inter- 
Marquette  erosion  may  have  cut  so  nearly  through  the  iron  formation 
that  an  ore  deposit  may  extend  from  the  bottom  of  the  formation  to  the 
top.  However,  in  these  cases  the  ore  bodies  are  usually  hard,  and,  upon 
the  whole,  are  more  closely  allied  to  the  third  class  than  to  the  first.  In 
many  places,  also,  the  upper  part  of  an  ore  deposit  may  be  at  the  topmost 
horizon  of  the  iron-bearing  formation,  and  be  a  specular  ore,  while  the 
lower  part  is  wholly  within  the  iron-bearing  formation  and  is  soft  ore.  In 
some  places  there  is  a  gradation  between  the  two  phases  of  such  a  deposit, 
but  in  more  instances  the  two  bodies  are  separated  by  a  dike,  now  changed 
to  soapstone  or  paint-rock. 

(1)  The  ore  deposits  at  the  bottom  horizon  (PL  XXIX,  figs.  3  and  4) 
c§,n  occur  only  where  the  lowest  horizon  of  the  formation  is  present;  that 
is,  they  are  confined  to  that  part  of  the  formation  resting  upon  the  Siamo 
slate  or  the  Ajibik  quai-tzite.  Hence  they  are  found  along  the  outer 
borders  of  the  formation,  and  do  not  occur  in  the  broad  Ishpeming-Negaunee 


PLATE    XXVIII. 


Plate  XXVIII.— THE  ORE  DEPOSITS. 

Fig.  1.  Generalized  section  showing  relations  of  all  classes  of  ore  deposits  to  associated  formations. 
On  the  right  is  soft  ore  resting  in  a  V-shaped  trough  between  the  Siamo  slate  and  a  dike  of 
soapstone.  In  the  lower  central  part  of  the  fiijure  the  more  common  relations  of  soft  ore 
to  vertical  and  inclined  dikes  cutting  the  jasper  are  shown.  The  ore  may  rest  upon  an 
inclined  dike,  between  two  inclined  dikes  and  upon  the  upper  of  the  two,  or  be  on  both 
sides  of  a  nearly  vertical  dike.  In  the  upper  central  part  of  the  figure  are  seen  the  relations 
of  the  hard  ore  to  the  Negaunee  formation  and  the  Goodrich  quartzite.  At  the  left  is  soft 
ore  resting  in  a  trough  of  soapstone  which  grades  downward  into  greenstone. 

Fig.  2.  Sharply  plicated  jasper  (black  belts)  and  ore  (white  areas),  showing  shattering  of  the  jasper 
and  concentration  of  the  ore.  The  ore  is  proportionally  greater  where  the  folding  has  been 
sharpest.     Drawn  from  photograph  from  southeast  corner  of  Republic  horseshoe. 

Fig.  3.  Horizontal  section  of  chimney  of  ore  on  east  side  of  Republic  horseshoe.  The  left  side  of  the 
ore  is  bounded  by  cross-joints.  The  right  side  is  bounded  in  part  by  a  sharp  flexure  passing 
into  a  joint,  and  m  part  grades  into  the  lean  banded  jasper  and  ore.     Scale:  20' =  1". 

Figs.  4,  5,  and  6.  Three  cross-sections  of  ore  in  trough  of  soapstone  grading  downward  into  green- 
stone. In  fig.  4  the  ore  deposit  is  solid.  In  fig.  5  a  dike  ofi'shoots  and  nearly  separates  this 
ore  body  into  two  parts.  In  fig.  6  the  two  dikes  divide  the  same  ore  body  into  three  parts. 
Scale:  200' =  1". 

Fig.  7.  Cross-section  of  National  mine.  On  the  left  is  soapstone  grading  into  greenstone.  Above  this 
is  hard  ore,  and  overlying  the  hard  ore  are  interstratified  conglomerate,  quartzite,  and  schist. 
The  ore  is  here  plainly  due  to  a  replacement  of  the  silica  of  the  difterent  sedimentary  bands 
by  ore,  although  the  original  conglomerate  was  heavily  ferruginous.     Scale :  200'  =  1". 


r^^^5^^^^^7T^5:j^^Tr^^x^ 


THE   ORE   DEPOSITS. 


lEONORE   DEPOSITS   OF   NEGALHS^EE   EOKMATION.  395 

area.  The  best  examples  of  these  deposits  are  those  occurriug  at  the  Teal 
Lake  raug-e  and  east  of  Negaunee  (Atlas  Sheets  XXVII  and  XXXI). 
Here  are  situated  the  Cleveland  Hematite,  the  Cambria,  the  Buffalo,  the 
Blue,  and  other  mines.  These  ore  deposits  have  as  their  foot-wall  the 
Siamo  slate.  A  striking  fact  about  these  deposits  is  that  all  of  those 
mentioned,  and  all  of  those  known,  occur  at  places  where  the  Siamo 
slate  is  folded  so  as  to  form  a  trough.  By  reference  to  the  maps  (Atlas 
Sheets  IV  and  XXVII)  it  is  seen  that  all  the  Teal  Lake  mines  occupy  a 
place  where  the  iron  formation  curves  to  the  north  and  then  swings  back  to 
its  original  course,  the  ore  deposits  thus  resting  upon  a  southward-pitching 
trough  of  the  slate.  Still  more  striking  is  the  occurrence  east  of  Negaunee. 
Here  the  ore  bodies  occur  at  places  where  the  slate  is  folded  so  as  to. 
furnish  sharply  pitching  synclinal  troughs,  which  plunge  to  the  west. 
(PL  XXIX,  figs.  3  and  4.)  It  is  further  found,  by  an  examination  of  the 
workings,  that  the  iron-bearing  formation  is  often  cut  by  a  set  of  steep  or 
vertical  dikes,  and  that  the  conjunction  of  these  dikes  with  the  foot-wall  slate 
forms  sharp  V-shaped  troughs.  This  is  particularly  clear  in  the  case  of  the 
Cleveland  Hematite  mine,  where,  between  a  series  of  vertical  dikes  and  the 
Siamo  slate,  the  ore  bodies  are  found.  By  comparing  this  occun-ence  with 
the  ore  deposits  of  the  Penokee  range,  ^  it  will  be  seen  that  they  are  ahnost 
identical,  in  each  case  there  being  on  one  side  of  the  formation  an  imper- 
vious slate  and  quartzite,  and  upon  the  other  an  impervious  dike,  the  two 
uniting  to  form  a  pitching  trough. 

(2)  The  typical  area  for  the  soft-ore  bodies  within  the  iron  formation 
is  that  of  Ishpeming  and  Negaunee.  Here  belonging  are  such  deposits  as 
the  Cleveland  Lake,  the  Lake  Angeline,  the  Lake  Superior  Hematite,  the 
Salisbury,  and  many  others.  "When  these  deposits  are  examined  in  detail 
it  is  found  that  the  large  deposits  always  rest  upon  a  pitching  trough  com- 
posed wholly  of  a  single  mass  of  greenstone  (PI.  XXVIII,  figs.  4-6),  or  on 
a  pitching  trough  one  side  of  which  is  a  mass  of  greenstone  and  the  other 
side  of  which  is  a  dike  joining  the  greenstone  mass.  The  underlying  rock  is 
called  greenstone,  although  immediately  in  contact  witli  the  ore  it  is  known 
as  paint-rock  or  soapstone  by  the  miners. 

'Tlie  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  K.  Van 
Hlse:  Mon.  U.  S.  Geol.  Survey,  Vol.  XIX,  1892,  pp.  268-294. 


396  THE  MARQUETTE  lEON-BEARING  DISTRICT. 

However,  a  close  examination  of  numerous  localities  shows  that  the 
greenstone  changes  by  minute  gradations  into  the  schistose  soapstone,  and 
this  into  the  paint-rock,  and  that  therefore  these  phases  are  merely  parts  of 
the  greenstone  which  have  been  profoundly  altered  by  mashing  and  leaching 
processes  and  which  have  been  strongly  impregnated  by  iron  oxide.  Many 
of  the  thinner  dikes  are  wholly  changed  to  paint-rock  or  soapstone,  or  to 
the  two  combined.  The  larger  number  of  these  troughs  are  found  along  the 
western  third  of  the  Ishpeming-Negaunee  area.  By  examining  the  maps 
(Atlas  Sheets  IV,  XXV,  and  XXVIII),  the  masses  of  greenstone  may  be 
seen  partly  inclosing  several  westward-opening  bays,  which  are  occupied  by 
the  iron  formation.  (See  also  PI.  XIII.)  Conspicuous  among  these  are  the 
Ishpeming  basin,  the  northern  Lake  Angeline  basin,  the  southern  Lake 
Angeline  basin,  and  the  Salisbury  basin.  In  each  of  these  cases  the  green- 
stone forms  an  amphitheater  about  the  rocks  of  the  iron-bearing  formation. 
Areas  of  iron  formation  open  out  to  the  west  into  the  main  area,  and  thus 
the  troughs  have  a  westward  pitch.  In  the  case  of  Lake  Angeline,  an 
east-and-west  dike  cuts  across  the  basin  south  of  the  center,  and  this,  com- 
bined with  the  greenstone  bluffs  to  the  north  and  to  the  south,  forms  two 
westward-pitching  troughs.  The  noi-thei-nmost  of  these  has  the  greatest 
ore  deposits  of  the  Marquette  district,  containing  many  millions  of  tons 
of  ore. 

(3)  The  hard-ore  bodies,  mainly  specular  hematite,  but  in  some 
deposits  including  much  magnetite,  occur,  as  has  been  said,  at  the  top 
of  the  iron-bearing  formation,  immediately  below  the  Goodrich  quartzite 
and  within  the  lower  horizons  of  the  Goodrich  quartzite.  (Pis.  XVI, 
XXVIII,  figs.  1,  3,  and  7,  and  XXIX,  fig.  2.)  As  typical  examples  of 
mines  of  this  class  may  be  mentioned  the  Jackson  mine,  the  Lake  Superior 
Specular,  the  Volunteer,  the  Michigamme,  the  Riverside,  the  Champion, 
the  Republic,  and  the  Bamum.  Also,  as  interesting  deposits,  giving  the 
history  of  the  ore,  may  be  mentioned  the  Kloman  and  the  Goodrich.  In 
all  of  these  deposits  the  associated  rock  of  the  iron  formation  is  jaspilite  or 
griinerite-magnetite-schist,  usually  the  former.  These  ore  deposits,  bridg- 
ing two  different  geological  series,  can  not  be  separated  in  description,  for 
frequently  they  weld  together   the  Upper  Marquette  Goodrich  quartzite 


PLATE    XXIX 


Plate  XXIX.— THE  ORE  DEPOSITS. 

Fio.  1.  Cross-section  of  Section  16  mine,  Lake  Superior  Iron  Company.  On  tlie  riglit  is  a  V-shaped 
trough  made  by  the  junction  of  a  greenstone  mass  and  a  dike.  The  hard  ore  is  between 
these  and  below  the  Goodrich  quartzite.  On  the  left  the  hard  ore  again  rests  on  soapstone, 
which  is  upon  and  iuterstratified  with  jasper,  and  is  overlain  by  the  Goodrich  quartzite. 
Scale:  200' =  1". 

Fig.  2.  Cross-section  of  the  Barnum  mine,  showing  hard  ore  resting  either  upon  folded  soapstone  or 
upon  jasjier,  and  overlain  by  soapstone.  At  the  right  of  the  figure  is  seen  a  layer  of  ore 
between  two  soapstone  dikes.     Scale:  200' =  1". 

Fig.  3.  Longitudinal  section  of  the  mines  operated  by  the  Buifalo  Mining  Company,  showing  the 
soft  ore  resting  upon  an  impervious  foot- wall  of  Siamo  slate  and  grading  upward  into  jasper. 
Scale:  200' =  1". 

Fig.  4.  Cross-section  of  same,  showing  the  slate  folded  into  two  troughs,  which  are  shown  by  the 
longitudinal  section  (fig.  3)  to  have  a  western  pitch. 
398 


.   GEOLOGICAL  SURVEY 


mmf^s^^^ 


Fic    4-. 


THE  ORE   DEPOSITS. 


IRON-ORE    DEPOSITS    OF   NEGAUNEE   FORMATION.  399 

formation  and  the  Lower  Marquette  Negaunee  formation.  As  in  the  cases 
of  (1)  and  (2),  all  of  the  large  ore  deposits  belonging  to  this  third  class  have 
at  their  bases  soapstone  or  paint-rock.  (PL  XXIX,  %.  2,  and  PI.  XXXIV, 
fig.  1.)  In  those  cases  in  which  the  soapstone  is  within  the  Negaunee 
formation  it  is  a  modified  diabase,  or  a  greenstone  mass  in  conjunction 
witli  a  dike  or  dikes.  Where  the  ore  deposits  are  largely  or  mainly  in  the 
Goodrich  quartzite  the  basement  rock  may  again  be  a  greenstone,  but  also 
it  may  be  a  layer  of  sedimentary  slate  belonging  to  the  Goodrich  quartzite. 
These  different  classes  of  rocks  are,  however,  not  discriminated  by  the 
miners,  but  are  lumped  together  as  soapstone  or  paint-rock.  Also,  as  in 
the  cases  of  (1)  and  (2),  wherever  the  deposits  are  of  any  considerable 
size  the  basement  rock  is  folded  into  the  form  of  a  pitching  trough,  or  else, 
by  a  union  of  a  mass  of  greenstone  with  a  dike,  or  by  a  union  of  either 
one  of  these  with  a  sedimentary  slate,  an  impervious  pitching  trough  is 
formed.  Perhaps  the  most  conspicuous  example  of  this  is  at  the  Republic 
mine  (PI.  XXXIV),  but  it  is  scarcely  less  evident  in  the  other  large  deposits. 
However,  a  few  small  deposits — chimneys  and  shoots — of  ore  occur  at  the 
contact  of  the  Negaunee  and  Ishpeming  formations  (PL  XXVIII,  fig.  3), 
where  no  soapstone  has  been  found.  As  examples  of  ore  deposits  which  are 
largely  or  wholly  within  the  Upper  jMarquette  may  be  mentioned  the  Volun- 
teer, Michigannne,  Champion,  and  Riverside.  These  are  partly  recomposed 
ores,  and  differ  in  appearance  from  the  specular  hematite  or  magnetite  of  the 
Lower  Marquette  in  having  a  peculiar  gray  color  and  in  containing  small 
fragmental  particles  of  quartz  and  complex  fragmental  pieces  of  jasper,  and 
frequently  also  sericite  and  chlorite  are  discovered  with  the  microscope. 

In  any  of  these  classes  the  deposits  may  be  cut  into  a  number  of 
bodies  by  a  combination  of  greenstone  dikes  or  masses.  A  deposit  which  in 
one  part  of  the  mine  is  continuous,  in  another  part  of  the  mine,  by  a  grad- 
ually projecting  mass  of  greenstone  which  passes  into  a  dike,  may  be  cut 
into  two  deposits,  and  each  of  these  may  be  again  dissevered,  so  that  the 
deposit  may  be  cut  up  into  a  number  of  ore  bodies  separated  by  soapstone  or 
paint-rock.  (PL  XXVIII,  figs.  4-6.)  In  some  cases  the  ore  deposits  have  a 
somewhat  regular  form  from  level  to  level,  but  the  shape  of  the  deposit  at 
the  next  lower  level  can  never  be  certainly  predicted  from  that  of  the  level 
above.     Horses  of  "jasper"  may  api)ear  along  the  dikes  or  within  an  ore 


400  THE   MAEQUETTE   lEON-BEAEIiTG   DISTEICT. 

body  at  almost  any  place.  The  ore  bodies  grade  above  and  at  the  sides 
into  the  jasper  in  a  variable  manner.  As  a  result  of  the  combination  of 
these  uncertain  factors,  most  of  the  ore  bodies  have  extraordinarily  irreg- 
ular and  curious  forms  when  examined  in  detail,  although  in  general  shape 
they  conform  to  the  above  descriptions. 

While  these  different  classes  of  ore  bodies  have  the  distinctive  features 
indicated  above,  they  have  important  features  in  common.  They  are  con- 
fined to  the  iron-bearing  formation.  They  occur  upon  imj)ervious  basements 
ip,^  pitching  troughs.  The  impervious  basement  may  be  a  sedimentary  or 
igneous  rock,  or  a  combination  of  the  two.  Where  the  ore  deposits  are  of  con- 
siderable size  the  plication  and  brecciation  of  the  chert  and  jasper  are  usual 
phenomena.  (Pis.  XX-XXIII  and  XXV-XXVII.)'  Frequently  this  shat- 
tering was  concomitant  with  the  folding  into  troughs  or  with  the  intrusion  of 
the  igneous  rocks.  When  the  passage  of  the  ore  bodies  into  the  chert  or 
jasper  is  examined  in  detail,  it  is  found  that  a  siliceous  band,  if  followed 
toward  the  ore,  instead  of  remaining  solid,  becomes  porous  and  frequently 
contains  considerable  cavities.  These  places  in  the  transition  zone  are  lined 
with  ore.  In  passing  toward  the  ore  deposit  more  and  more  of  the  silica  is 
found  to  have  been  i-emoved,  and  the  ore  has  replaced  it  to  a  corresponding 
degree.  (PI.  XXIII,  fig.  1.)  An  examination  at  many  localities  shows  this 
transition  from  the  banded  ore  and  jasper  to  take  place  as  a  consequence  of 
the  removal  of  the  silica  and  the  substitution  of  iron  oxide.  In  such  instances 
the  fine-grained  part  of  the  ore  is  often  that  of  the  ox-iginal  rock,  while 
the  coarser  crystalline  material  is  a  secondary  infiltration.  (Pis.  XXIII 
and  XXVI.)  It  is  not  infrequently  the  case,  however,  that  the  ore  deposits 
abruptly  terminate  along  a  joint  crack  or  fracture.     (PL  XXVIII,  fig.  3.) 

ORIGIN   OF   THE   ORES. 

The  facts  given  in  the  foregoing  pages  in  reference  to  the  iron-bearing 
formation  and  its  origin,  combined  with  the  peculiar  occurrence  of  the  ores, 
indicate  with  certainty  the  main  features  of  the  origin  ol  the  ore  deposits. 

While  the  ore  deposits  of  the  Lower  Marquette  series  have  a  greater 
variety  of  form  and  relations  than  those  of  the  Penokee  district,^  it  is  evident 

'The  Peuokeo  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  K.  Van 
Hise:  Mon.  U.  S.  Geol.  Survey.  Vol.  XIX,  1892,  pp.  280-290. 


ORIGIN   OF    IRON   ORES  OF   NEGAUNEE   FORMATION.  401 

that  the  conditious  governing  their  formation  are  much  the  same.  In  both 
districts  the  material  immediately  underlying  the  ore  is  relatively  impervi- 
ous to  water.  In  the  cases  of  the  deposits  resting  upon  soaprock  this  lack 
of  porosity  is  nearly  complete.  Most  of  the  ore  bodies  are  in  troughs  in 
both  districts;  the  ore  bodies  in  both,  in  longitudinal  section,  have  a  pitch; 
in  both  the  many  phases  of  material  found  in  the  iron  bearing  formation  are 
nearly  the  same;  and  in  both  is  found  plentiful  residual  iron  carbonate. 
It  is  therefore  thought  that  the  explanation  of  the  origin  of  the  ores  in  the 
Penokee  district  is  applicable,  with  a  few  modifications,  to  those  of  th" 
Marquette  district,  although  the  larger  number  of  the  deposits  of  the  latter 
belong  to  an  older  series. 

The  forms,  attitudes,  and  relations  of  the  ore  deposits  render  it  evident 
that  they  are  not  eruptives.  (Pis.  XXVIII  and  XXIX.)  No  eruptive 
would  be  found  in  such  strange  shapes  and  relations.  It  is  equally  certain 
that  these  irregular  masses  of  ore  are  not  altogether  formed  by  direct 
sedimentation,  althougli  a  considerable  part  of  the  iron  oxide  in  an  ore  body 
may  be  an  oxidation  product  in  place  of  a  sedimentary  iron  carbonate. 

All  these  facts  bear  toward  the  conclusion  that  the  ore  was  secondarily 
enriched  by  the  action  of  downward-percolating  water,  since  the  ore  deposits 
occur  at  places  where  percolating  waters  are  sure  to  have  been  concentrated 
The  soaprock  accommodated  itself  to  folding  without  fracture,  and,  while 
probably  allowing  more  or  less  Avater  to  pass  through,  acted  as  a  practically 
impervious  stratum  along  which  water  was  deflected  when  it  came  in  con- 
tact with  it.  It  is  a  common  opinion  among  miners  that  a  few  inches  of 
soaprock  is  more  eifective  in  keeping  out  water  than  many  feet  of  the  iron- 
bearing  formation.  On  the  other  hand,  the  brittle,  siliceous  ore-bearing 
formation  was  fractured  by  the  folding  to  which  it  was  subjected,  so  that 
where  this  process  was  extreme  water  passes  through  it  like  a  sieve.  That 
the  tilted  bodies  of  greenstone  or  soaprock,  especially  when  in  pitching 
synclines  or  forming  pitching  troughs  by  the  union  of  dikes  and  masses  of 
greenstone,  must  have  converged  downward-flowing  waters  is  self-evident. 
It  is  also  clear  that  the  weak  contact  plane  between  the  Goodrich  quartzite 
and  the  Negaunee  formation  was  one  of  accommodation  and  shattering 

MON  XXVIII 26 


40^  THE   MARQUETTE    IROI^-BEAEING   DISTRICT. 

Therefore  the  i)laue  of  unconformity  between  the  Upper  Marquette  and 
Lower  Marquette  series  must  have  been  a  great  horizon  for  downward- 
flowing  waters. 

It  has  been  seen  that  tlie  whole  of  the  iron-bearing  foi-mation  was 
probably  originally  a  lean,  cherty  carbonate  of  iron,  with  perhaps  some 
calcium  and  magnesium,  and  that  from  this  rock  the  ferruginous  cherts  and 
jaspers  developed.  If  we  now  go  no  further  back  than  the  ferruginous 
cherts  and  jaspers,  in  order  to  produce  the  ore  two  things  must  have 
occuiTed:  first,  the  further  concentration  of  iron  oxide  in  the  places 
where  the  ore  bodies  are  found;  and  second,  the  removal  of  silica  from 
these  places. 

The  final  concentration  of  the  ores  occurring  at  the  contact  of  the 
Upper  Marquette  and  Lower  Marquette  series  must  have  taken  place  later 
than  Upper  Marquette  time.  This  is  indicated  by  the  fact  that  the  uncon- 
formable formations  are  welded  together  by  the  iron  ore  at  many  places. 
The  relations  of  the  ore  bodies  within  the  ore  formation  to  the  greenstone 
masses  and  dikes  give  evidence  that  the  concentration  of  this  ore  occurred 
subsequently  to  the  intrusion  of  these  rocks.  It  is  certain  that  some  of  these 
igneous  rocks  were  intruded  during  or  later  than  Upper  Marquette  time, 
since  they  cut  across  the  Goodrich  quartzite.  Others  of  them  appear  to 
have  yielded  fragments  to  the  Upper  Marquette  series,  and  thel'efore  ante- 
date these  rocks.  Finally,  if  the  ore  bodies  were  concentrated  before  the 
Upper  Marquette  folding  and  erosion  their  invariable  positions  above 
the  impervious  formations  would  be  inexplicable.  The  folding  would 
perhaps  have  left  them  as  often  below  as  above  these  formations.  Taking- 
all  the  facts  together,  it  is  highly  probable  that  the  final  concentration 
of  all  the  ores  occurred  during  and  later  than  the  folding  and  erosion  subse- 
quent to  Upper  Marquette  time. 

Surface  waters  bearing  oxygen,  passing  downward  through  the  Upper 
Marquette  series  or  the  iron-bearing  formation  of  the  Lower  Marquette 
series,  would  decompose  the  iron  carbonates  with  which  they  came  in  con- 
tact and  thus  become  carbonated.  These  carbonated  waters  would  then 
be  capable  of  taking  other  iron  carbonates  into  solution.  What  proportion 
of  the  oriofinal  iron  carbonate  still  remained  in  the  ore-bearing  formation 


ORIGIN  OF   IRON  ORES   OF   NEGAUNEE   FORMATION.  403 

at  the  beginning  of  the  concentration  of  the  ore  deposits  is  uncertain, 
but  since  it  is  still  found  in  places  sheltered  from  percolating  waters,  such 
as  the  deeper  horizons  of  the  iron  formation,  adjacent  to  and  probably 
protected  by  greenstone  masses,  it  is  probable  that  the  quantity  was  very 
considerable.  The  oxides  or  carbonates  of  iron  may  also  have  been  taken 
into  solution  through  the  agency  of  organic  acids.  The  downward- 
moving  waters  passed  along  and  through  the  beds  of  the  iron-bearing 
formation  until  they  came  in  contact  with  an  impervious  substance.  Here 
were  also  converged  oxygen-bearing  waters  more  directly  from  the 
surface.  The  union  of  these  two  currents  pi-ecipitated  the  iron  oxide. 
The  abundant  waters  traversing  these  ore-bearing  localities  slowly  dis- 
solved the  silica,  its  place  being  taken  by  the  ore.  That  this  interchange 
actually  did  occur  is  known  of  the  localities  in  which  a  detailed  exami- 
nation has  been  made,  as,  for  instance,  at  Republic.  It  is  probable  that 
in  the  ore  deposits  associated  with  the  soaprocks  the  removal  of  silica 
was  due  in  part  to  them.  Originally  diabases,  they  must  have  contained 
alkalis.  The  alkaline  waters  produced  by  their  alteration  thus  furnished 
a  menstruum  capable  of  taking  the  silica  into  solution.  This  desilicification 
of  the  iron-bearing  formation  by  alkaline  waters  was  many  years  ago 
suggested  by  Brooks^  for  a  part  of  the  Marquette  district.  Rorainger-  not 
only  made  the  same  suggestion  in  reference  to  the  Jackson  mine,  but 
further  held  that  the  siliceous  matter  removed  was  replaced  by  oxide  of 
iron  carried  by  water  solutions. 

The  percolating  waters  which  carried  material  along-  the  readiest  paths 
to  form  the  ore  bodies,  and  which  removed  the  silica,  also  helped  to  jas- 
perize  the  upper  part  of  the  Negaunee  formation,  although  this  may  have 
been  partly  done  before  Upper  Marquette  time.  Whatever  the  time  at 
which  the  work  was  done,  the  pi'ocess  seems  to  have  been  as  follows :  The 
quartz  grains  of  the  ferraginous  chert  were  separated  by  mashing.  The 
upper  part  of  the  ore  formation  was  more  extensively  traversed  by  solutions 
than  the  deeper-lying  portions.  It  naturally  followed  that  the  ferruginous 
material  was  in  part  deposited  about  and  through  the  minute  particles  of 

'  Geol.  of  Michigan,  Vol.  I,  p.  134.  ^bid.,  Vol.  IV,  p.  75. 


404  THE   MAKQUETTE    IliON  liEARING   DISTRICT. 

quartz,  reddening  them  and  changing  the  material  from  white  chert  to  red 
jasper.  In  some  places  this  jasperization  extended  deeper  than  in  others, 
-and,  as  already  said,  at  other  places  it  abruptly  stopped  at  an  impervious 
mass  of  soaprock. 

One  or  two  questions  remain  to  be  considered:  first,  why  the  ore  is 
so  frequently  hard  and  specular  along  the  contact  horizon  or  in  the  jasper 
and  is  usually  soft  within  the  ferruginous  chert;  second,  why  the  magnet- 
ites, when  present,  occur  at  the  contact  horizon. 

An  examination  of  the  jasper  associated  with  the  hard  ores  shows  tnat 
crystallized  hematite  and  magnetite  occm*  in  many  cavities  formed  by  the 
removal  of  silica.  In  such  geodal  cavities  these  materials  were  deposited 
in  a  granular  crystalline  condition.  In  the  continuation  of  the  process  the 
silica  was  wholly  removed  and  its  place  taken  by  the  crystalline  hematite 
and  magnetite.  The  adjacent  jasper  also  shows  numerous  cracks  and 
fissures  filled  with  hematite  or  magnetite.  The  manner  in  which  these 
veins  of  coarser  crystallized  material  frequently  cut  across  the  finer- 
grained  substances,  which  represent  the  iron  oxides  present  before  the 
final  concentration  of  the  ore,  shows  conclusively  that  they  are  secondary 
infiltrations  later  than  the  last  orogenic  movement.  The  formation  of  the 
coarsely  crystalline  granular  hematite  and  magnetite  thus  appears  to  have 
been  connected  with  the  abundance  of  iron-bearing  solutions  along  the 
contact  plane. 

In  many  places,  however,  the  liard  ores  are  of  the  brilliant  micaceous 
or  specular  variety.  This  is  sometimes  called  slate  ore.  In  the  hand 
specimen  of  jaspilite,  composed  of  minute  alternating  layers  of  hematite 
and  quartz,  where  the  folding  has  been  severe  micaceous  ore  is  found 
between  the  rigid  bands  of  quartz.  Along  the  ferruginous  laminae  is  seen 
all  the  evidence  of  slickensides.  The  micaceous  character  of  the  ore  is 
in  this  case  plainly  due  to  the  accommodation  and  consequent  sheai'ing 
which  took  place  between  the  layers. 

The  micaceous  ore  from  the  large  deposits,  as  first  sugg'ested  by 
Pumpelly,  gives  the  same  evidence  of  shearing.  Wlien  it  is  remembered 
that  in  the  folding  of  thick  formations  readjustments  must  occur,  it  is  natural 
to  suppose  that  they  took  place  more  largely  at  the  contact  between  the 


OKIGIN   OF   IRON   ORES    OF   NEGAUNEE   FORMATION.  405 

Upper  Marquette  and  Lower  Marquette  series  than  at  any  other  one  horizon, 
for  this  is  emphatically  the  plane  of  weakness.  Thus  would  be  e^xplained 
the  finely  laminated  micaceous  variety  of  ore.  The  specular  hematite  may 
have  been  soft  ore,  for  it  is  not  impossible  that  shearing  along  the  contact 
plane,  with  the  heat  developed,  was  sufficient  to  cause  this  transformation. 

A  close  examination  of  the  slate  ores  ^lows  that  they  are  composed  of 
two  parts,  one  of  which  was  mashed,  the  other  being  granular  or  crystal- 
outlined  hematite  and  magnetite.  The  latter  material  fills  the  cracks  left 
as  a  result  of  the  mashing,  perhaps  occupies  the  place  of  residual  silica, 
and  welds  the  micaceous  leaves  together.  Thus  this  granular  ore  was 
certainly  deposited  after  the  folding.  How  much  was  introduced  during 
the  folding  it  is  impossible  to  say,  for  this  part  can  not  be  separated  from 
that  present  before  the  folding. 

That  it  is  easy  to  reduce  hematite  to  magnetite  is  well  known,  and  it  is 
probable  that  the  production  of  the  granular  infiltrated  variety  of  this  ore 
is  due  to  the  reducing  character  of  some  of  the  solutions  which  passed  down 
along  the  great  contact  plane  of  percolation,  where  the  magnetites  are  exten- 
sively found.  Reducing  power  could  readily  be  imparted  by  organic  acids, 
and  that  some  kind  of  reducing  agent  was  present  is  indicated  by  the  veins 
of  pyrite  which  are  frequently  associated  with  the  magnetic  ores. 

The  magnetite  of  the  griinerite-magnetite-schist  has  been  seen  to  be 
partly  due  to  an  imperfect  oxidation  of  the  original  iron  carbonate.  It  is, 
however,  doubtful  whether  any  considerable  quantity  of  the  magnetite 
of  the  greater  number  of  worked  ore  bodies  is  directly  of  this  derivation, 
although  some  of  the  lesser  magnetite  deposits  appear  to  be  an  em-iched 
gi-iinerite-magnetite-schist.  In  these  cases  there  is  no  particular  difficulty 
in  accounting  for  the  larger  part  of  the  magnetite,  but  the  same  difficulty 
exists  in  explaining  the  imperfect  oxidation  of  the  infiltrated  material  as  in 
the  other  instances. 

PROSPECTING. 

In  considering  the  advisabihty  of  prospecting  in  any  particular  locality 
the  foregoing  conclusions  as  to  the  relations  of  the  iron  ores  may  be  of 
assistance.  These  may  be  briefly  summarized  as  follows:  The  iron  ores 
are  always  confined  to  the  iron-bearing  formation.      They  always  rest 


406  THE   MAKQUETTE   IRON-BEARING    DISTRICT. 

upon  a  relativel)'  impervious  basement.  This  may  be  a  shale,  a  slate,  a 
greenstone  mass,  a  dike,  or  two  or  more  of  these  combined.  Adjacent  to 
the  ores  all  of  these  formations  are  apt  to  be  modified  and  impregnated 
with  iron  oxide,  and  are  hence  called  soapstone  or  paint-rock.  The  large 
ore  bodies  are  found  only  when  the  impervious  basements  are  in  the  forms 
of  jjitching  troughs.  These  pitching  troughs  are  particularly  likely  to 
bear  unusually  large  ore  bodies  when  the  iron-bearing  formation  is  much 
shattered  by  folding. 

In  prospecting  for  the  first  class  of  ores,  those  that  rest  upon  the  Siamo 
slate,  a  trough  in  the  slate  should  be  sought.  A  plunging  synclinal  trough 
may  be  formed  by  a  swing  of  the  boundary  line  between  this  formation  and 
the  iron-bearing  formation;  or  a  trough  may  be  formed  by  a  combination 
of  the  slate  with  a  cutting  dike  or  mass  of  greenstone ;  or  a  trough  in  the 
slate  may  be  supplemented  by  an  intersecting  greenstone. 

In  the  second  class  of  deposits — those  within  the  formation — the 
pitching  troughs  are  wholly  formed  by  the  iutrusives.  Here  valleys  of 
the  iron-bearing  formation,  when  nearly  surrounded  by  an  amphitheater  of 
greenstone,  furnish  a  particularly  favorable  area.  Where  the  iron-bearing 
formation  in  the  valley  is  the  ferruginous  chert,  rather  than  the  griinerite- 
magnetite-schist,  the  conditions  are  more  favorable.  Pitching  troughs 
bottomed  by  soapstone  may  exist  underground  which  can  not  be  discovered 
at  the  surface,  since^  where  an  intersecting  intrusive  is  of  small  size  and 
has  been  transformed  to  soapstone,  it  is  eroded  as  rapidly  as  the  iron  forma- 
tion, and  thus  its  existence  is  not  discovered  by  outcrop  or  any  topographic 
feature , 

The  third  class  of  deposits,  the  hard  ores,  must  always  be  prospected 
for  near  the  contact  of  the  Negaunee  iron  formation  and  the  Goodrich 
quartzite.  As  in  the  previous  cases,  the  ore  bodies  are  particularly  likely 
to  exist  if  the  two  are  folded  so  that  the  contact  forms  a  pitching  trough,  and 
if  this  be  bottomed  by  soapstone  the  conditions  are  still  more  favorable  for 
the  formation  of  large  deposits. 

The  general  map  (Atlas  Sheet  IV)  shows  several  extensions  of  the  iron- 
bearing  foi'matiou  which  have  not  been  prospected.  The  arm  rumiing  east 
of  Palmer  has  been  prospected  along  its  south  side,  but  as  yet  almost  no 
work  has  been  done  along  the  north  side.  The  exposures  here  are  not 
suflicient  to  indicate  the  minor  bends  of  the  iron-bearing  formation,  but 


PROSPECTING  FOR   IRON  ORE   IN  NEGAUNEE   FORMATION.      407 

the  break  across  the  quartzite  in  sec.  28,  T.  47  N.,  R.  26  W.,  suggests  that 
there  may  be  a  north  swing  of  the  formation  at  this  pLace ;  and  if  so,  this 
would  be  a  favorable  point  for  exploration.  Other  favorable  places  may 
exist  along  the  northern  side  of  this  syncllne,  but  their  exact  positions  can 
not  be  pointed  out.  The  second  synclinal  arm,  running  from  the  southeast 
comer  of  sec.  20,  T.  47  N.,  R.  26  W.,  in  a  northeasterly  direction,  has  not 
been  explored  at  all.  West  of  this  arm  is  the  great  anticlinal  dome  of 
Siamo  slate.  This  dome  is  folded  by  minor  rolls  in  an  east-west  direction, 
thus  furnishing  on  the  west  side  of  the  iron  formation  a  number  of  westward- 
pitching  synclinal  troughs,  in  which  are  large  deposits  of  ore.  Doubtless 
the  same  folded  condition  prevails  on  the  eastern  side,  producing  eastward- 
pitching  troughs,  although  here  outcrops  are  not  sufficient  to  accurately 
delineate  the  boundary  lines;  but  while  the  existence  of  a  swamp  in  sees. 
3,  4,  9,  and  10  makes  the  area  difficult  to  prospect,  the  sides  of  the  arm  are 
worthy  of  exploration.  In  the  south  part  of  sec.  3,  near  the  north-south 
quarter  line  of  the  section,  there  is  a  ridge  of  greenstone.  This  is  also  the 
end  of  the  syncline,  which  here  plunges  to  the  south.  The  junction  of  this 
greenstone  with  the  contact  line  between  the  Siamo  slate  and  the  Negaunee 
iron  formation  is  a  favorable  point.  Within  the  iron  formation  in  sees.  10 
and  15  a  great  mass  of  greenstone  forms  a  westward-facing  amphitheater, 
and  here  in  the  southwest  quarter  of  sec.  10  ^Vould  seem  to  be  a  favorable 
place  for  exploration. 

It  is  not  impossible  that  a  close  magnetic  survey  with  a  dial  compass 
and  dip  needle  across  the  approximate  boundary  lines  of  the  Siamo  slate  and 
the  Negaunee  formation,  for  these  eastern  arms,  would  enable  the  explorer 
to  more  accurately  delimit  the  iron -bearing  formation  and  to  determine 
the  probable  positions  of  pitching  troughs,  if  they  exist,  and  thus  point 
out  the  more  favorable  points.  This  attempt  ought  certainly  to  be  made 
before  money  is  spent  in  actual  underground  work.  Exposures  of  these 
eastern  arms  are  so  infrequent  that  it  is  not  certainly  known  that  the 
iron-bearing  formation  maintains  its  pure  nonfragmental  character.  If  it 
contains  interstratified  or  intermingled  clayey  material,  this  would  1)0 
unfavorable  to  the  development  of  merchantable  ore  deposits. 

In  the  foregoing  paragraphs  it  is  not  meant  to  imply  that  workable 
iron-ore  deposits  will  surely  be  found  in  these  eastern  arms,  but  merely  that 
the  conditions  are  sufficiently  favorable  to  warrant  a  search  for  them. 


CHAPTER    IV. 
THE  UPPER  MARQUETTE  SERIES. 

IKTRODUCTION. 

By  C.  E.  Van  Hise. 

Tlie  general  statement  has  been  made  that  the  Upper  Marquette  series 
appears  at  Negaunee  and  at  Palmer  in  two  detached  areas,  reappears  at 
Ishpeming,  and  from  this  place  toward  the  west  rapidly  widens  out  into 
a  broad  belt,  occupying  the  greater  part  of  the  area  of  Marquette  rocks. 
It  has  also  been  said  that  this  general  distribution  is  due  to  the  great 
north-south  transverse  anticline  east  of  Negaunee. 

Broadly  considered,  the  Upper  Marquette  series  was  predominantly  a 
great  shale  formation,  which  was  subsequently  modified  to  a  greater  or 
less  degree.  The  lowest  horizon  of  the  series  is,  however,  a  conglomerate 
and  quartzite,  which  marks  the  transgression  of  the  sea.  Replacing  this 
in  part  in  the  west  end  of  the  area  is  a  griinerite-magnetite- schist  horizon.' 
Following  above  this  is  the  great  slate  formation,  and  in  it  is  a  horizon 
which  originally  bore  a  considerable  quantity  of  iron  carbonate,  from 
which  various  ferruginous  rocks  have  developed,  and  also  small  ore 
bodies.  Finally,  during  Uj^jjer  Marquette  time,  in  parts  of  the  district 
there  was  contemporaneous  A^olcauic  action,  so  that  associated  with  the 
modified  shales  of  the  series  is  a  belt  of  volcanics  a  niimber  of  miles  long. 
As  in  the  case  of  the  Lower  Marquette,  later  intrusives  penetrated  the 
series  at  various  places. 

The  Upper  Marquette  series  is,  then,  structurally  divisible  into  a  lower 
belt  of  conglomerate,  quartzite,  griinerite-magnetite-schist,  and  associated 
rocks;  a  slate  formation;  and  a  belt  of  volcanics.  The  first  will  be  called 
the  Ishpeming  formation,  the  second  the  Micliigamme  formation,  and  the 
last  the  Clarksburg  formation. 


TUB   UPPER   MARQUETTE   SERIES.  409 


SECTION  I.— THE  ISIIPEMING  FOKSIATION'. 


The  Ishpemiug  formation  is  so  named  because  typical  exposures  of 
this  formation  suiTOund  the  city  of  Ishpeming  and  underlie  it.  For  the 
eastern  part  of  the  district,  and.  including  the  Ishpeming  area,  the  predomi- 
nant rocks  are  conglomeratic  quartzites  and  quartzites.  These  are  finely 
exposed  at  and  adjacent  to  the  Goodrich  mine  (Atlas  Sheet  XXVI),  and. 
this  rock  will  therefore  be  called  the  Goodrich  quai-tzite.  In  the  western 
part  of  the  district,  while  quartzites  are  present,  a  peculiar  schist,  which  is 
typically  exposed  at  the  lower  part  of  the  Bijiki  River  (Atlas  Sheet  VIII) 
aiid  will  therefore  be  called  the  Bijiki  schist,  occupies  a  large  part  of  the 
horizon  of  the  Goodrich  quartzite  and  is  equivalent  to  it  in  age. 

THE    GOODRICH    QUARTZITE. 
DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

The  easternmost  occurrence  of  the  Goodrich  quartzite  is  the  Palmer 
area  (Atlas  Sheet  IV).  From  the  village  of  Palmer  it  extends  east  and  west 
about  IJ  miles,  making  a  belt  3  miles  long.  From  near  its  center,  as  a 
consequence  of  subordinate  folding,  a  short  belt  projects  to  the  southeast. 
Small  isolated  patches  may  also  occur  capping  the  Ajibik  quartzite  of  the 
Ajibik  Hills.  The  second  subordinate  area  is  near  the  town  of  Negaunee, 
north  of  the  Jackson  mine.  On  account  of  the  close  folding  the  boundary 
line  of  this  area  is  very  irregular.  The  chief  area,  as  has  been  said,  begins 
at  Ishpeming.  From  this  area  a  rather  narrow  belt  extends,  in  a  course 
nearly  due  west,  to  west  of  Michigamme.  Another  arm,  of  irregular  Avidth, 
swings  to  the  south  and  southwest,  then  follows  a  general  westerly  course 
to  sec.  20,  T.  47  N.,  R.  28  W.,  where  it  swings  to  the  northwest  to  Hum- 
boldt and  Champion;  thence  it  extends  west,  southwest,  and  south  to  the 
end  of  the  Republic  tongue,  in  sec.  7,  T.  46  N.,  R.  29  W.,  passes  around 
the  end  of  this  tongue,  and  again  swings  to  the  northwest  to  sec.  20,  T.  47  N., 
R.  30  W.;  thence  it  swings  to  the  west  and  south,  beyond  the  limits  of 
the  district.     "West  of  this  belt  is  still  another. 


410  THE   MARQUETTE  lEON-BEAKING  DISTEICT. 

The  prominent  exposures  of  tlie  formation  are  usually  near  its  base. 
The  rocks  are  here  conglomerates.  These  grade  into  quartzites.  At  many 
places  in  passing  upward  the  quartzite  approaches  a  graywacke,  is  conse- 
quently softer,  and  therefore  not  so  frequently  seen.  Exposures  are  partic- 
ularly abundant  in  the  Palmer  and  Negaunee  areas,  about  the  Ishpeming 
basin,  and  as  far  west  on  the  southern  belt  as  the  Fitch  mine,  sec.  24, 
T.  47  N.,  R.  28  W.  For  the  last  3  miles  of  this  distance  it  constitutes 
a  rather  prominent  range.  West  of  this  place  exposures  are  infrequent 
until  Humboldt  is  reached.  Here  are  numerous  outcrops  north  of 
Mount  Humboldt.  At  Republic  are  large  and  fine  exposiires.  Many 
outcrops  are  found  in  the  northern  belt  south  of  the  Michigamme  and 
Spun-  mines. 

FOLDING. 

Broadly  considered,  the  Goodrich  quartzite  is  folded  into  a  great 
westward-plunging  synclinorium,  the  eastern  end  of  the  U  extending  from 
Ishpeming  southward.  This  eastern  border  of  the  formation  comprises  a 
series  of  reentrants  and  salients — reentrants  where  there  are  minor  syn- 
clines,  and  salients  where  there  are  minor  anticlines.  On  account  of  the 
flat  dip,  corresponding  to  the  westward  plunge  of  the  syncline,  the  forma- 
tion here  occupies  a  broad  belt.  On  the  south  side  of  the  formation  at  one 
place  the  Goodrich  quartzite  and  Negaunee  iron  formation  are  infolded  and 
overturned,  having  northward  dijDS  (Atlas  Sheet  XXVI).  At  this  point 
the  Goodrich  quartzite  has  a  tongue  running  east  into  the  iron  forma- 
tion, being  bounded  both  to  the  north  and  to  the  south  by  the  rocks  of  the 
Negaunee  formation,  which  dip  in  the  same  direction  as  the  quartzite. 
The  area  at  Negaunee  is  in  general  an  east-west  oval  synclinal  basin. 
Here  again  there  is  minor  folding,  so  that  the  formation  terminates  both  to 
the  east  and  west  in  a  number  of  fingers.  At  the  west  end  of  the  Jackson 
mine  the  Goodrich  quartzite  and  the  Negaunee  iron  formation  are  folded 
into  a  set  of  isoclinal  overfolds,  so  that  a  north-south  section  passes  three 
times  from  one  formation  to  the  other.  The  Palmer  belt  is  another 
east-west  synclinal  basin,  with  a  short  arm  extending  to  the  southeast  at 
one  place,  due  to  the  appearance  of  a  centi'al  anticline.  The  Republic 
tongue  and  that  to  the  west  are  two  closely  compressed  isoclinal  synclines. 


RELATIONS   OF  THE   GOODKICH   QUAIITZITE.  411 

At  IVIiclugamme  tlie  minor  folding  of  the  quartzite  is  beautifully  shown. 
(PI.  XXX.) 

RELATIONS   TO   AD.IACENT   FORMATIONS. 

The  details  of  the  relations  of  the  Goodrich  quartzite  to  the  underly- 
ing Negaunee  formation  are  so  fully  stated  in  connection  with  the  latter 
and  the  general  geology  that  they  need  not  be  repeated  here.  Grenerally 
stated,  they  are  those  of  unconformity,  the  advancing  sea  having  formed  a 
conglomerate  at  the  base  of  the  quartzite.  As  a  consequence  of  mining 
development  and  the  resistant  character  of  this  part  of  the  formation,  the 
conglomerate  may  be  seen  at  scores  of  localities  lying  upon  and  cutting 
across  the  bedding  of  the  underlying  formation  at  a-  greater  or  less  angle 
(figs  20  and  21). 

Where  erosion  cut  through  the  Negaunee  formation  the  basal  con- 
glomerate rests  upon  the  Ajibik  quartzite,  and  derives  the  majority  of  its 
fragments  from  it.  Where  the  latter  formation  is  also  cut  through,  as 
apparently  it  is  south  of  Palmer,  the  material  is  largely  derived  from  the 
Basement  Complex  This  fact,  that  the  Goodrich  quartzite  thus  comes  in 
contact  not  only  with  the  Negaunee  formation  but  with  inferior  formations, 
shows  that  the  unconformity  between  the  Upper  and  Lower  Marquette 
series  must  be  very  considerable 

For  much  of  the  district,  by  a  dying  out  of  the  coai'se  fragmental 
quartz  and  the  appearance  of  clayey  material  the  quartzite  gradually  passes 
into  the  Michigamme  formation.  This  gradation  is  usually  not  rapid,  and 
hence  the  location  of  the  boundary  line  between  the  two  is  somewhat  arbi- 
trary. At  the  western  end  of  the  district  the  quartzite  is  very  thin,  and  the 
formation  passes  quickly  upward  into  the  griinerite-magnetite-schists  of 
the  Bijiki  horizon.     (PI  XXXI.) 

PETROGRAPHICAL   CHARACTER. 

Macroscopicai. — A  couglomeratc  is  usually  at  the  base  of  the  Goodrich 
quartzite.  The  character  of  the  conglomerate  depends  upon  the  subjacent 
formation,  the  larger  portion  of  the  detritus  in  each  case  being  derived 
from  it.  As  has  been  stated,  this  inferior  rock  is  usually  the  Negaunee 
formation,  and  at  the  base  of  the  Goodrich  quartzite  is  ore,  recoraposed 
ore,  or  ore,  chert,  jasper,  and  quartz  conglomerate.  At  a  few  places  the 
subjacent  rocks  belong  to  the  Archean,  and  at  these  the  great  variety  of 


412  THE   MARQUETTE   lEONBEAROG   DISTRICT. 

materials  constituting  this  complex  ai-e  predominant  in  the  conglomerate. 
At  various  places — as,  for  instance,  in  the  great  conglomerate  at  and 
immediately  south  of  the  village  of  Palmer — there  are  abundant  quartzitic, 
gi-anitic,  and  schistose  bowlders,  derived  from  the  Archean,  and  also 
abundant  jaspilite  detritus  from  the  Negaunee  formation. 

The  basal  conglomerate,  of  varying  thickness,  grades  up  into  quartz- 
ites,  which  are  apt  to  contain  much  of  chert  and  jasper  in  minute 
fragments.  The  higher  horizons  of  the  quartzite  are  usually  feldspathic. 
Frequently  the  mashing  due  to  the  folding  was  so  severe  as  to  partly  or 
wholly  destroy  the  frag,  aents  of  ore  and  jasper,  making  the  rock  a  schist- 
conglomerate  or  schistose  quartzite.  This  change  is  complete  at  the  places 
where  the  close  infolding  which  has  been  spoken  of  occurs,  as  at  the  Jack- 
son mine,  at  Humboldt,  and  in  the  Republic  trough.  In  the  most  extreme 
stage  of  alteration  it  is  difficult  to  discriminate  the  mashed  recomposed  ore 
and  jasper  conglomerates  from  the  original  jaspilite.  In  passing  from  the 
least  altered  to  the  most  altered  phases  we  find,  first,  flattened  pebbles,  then 
those  which  are  elongated  into  layers,  and  finally  those  in  which  are  alter- 
nating layers  of  different  thickness,  which  simulate  original  lamination  in 
a  remarkable  degree. 

In  the  case  of  the  nonconglomeratic  recomposed  jaspers  the  rocks  are 
not  unlike  the  original  fonnation,  although  a  close  examination  usually 
shows  a  difference.  Ordinarily,  large  fragmental  grains  of  quartz  are  seen; 
flakes  of  mica  are  often  present;  and  the  banding  is  less  distinct  than  in  the 
original  jasper. 

Under  the  subject  "Negaunee  Iron  Formation"  the  development  of  ore 
bodies  within  the  Goodi-ich  quartzite  has  been  mentioned.  The  ore  usually 
occurs  at  places  where  the  detritus  Avas  rather  fine  grained,  and  thus  con- 
tained no  large  fragments  of  chert  and  jasper.  As  conspicuous  localities 
for  the  occurrence  of  these  recomposed  ore  bodies  may  be  mentioned 
the  Volunteer  mine,  the  Barron,  the  Humboldt,  the  Champion,  part  of  the 
Jackson,  and  part  of  the  Michigamme  and  Spurr.  As  a  consequence  of 
the  intense  mashing  which  the  formation  underwent,  numerous  cracks 
developed  and  minute  spaces  formed  between  the  laminae  and  between  the 
individual  particles.     Where  the  rock  was  enriched  so  as  to  become  an  ore, 


PETEOGRAPHICAL  CHAEACTER  OF  GOODRICH  QUARTZITE.      413 

as  has  been  before  explained,  secondary  magnetite  formed.  The  detrital 
micaceous  hematite  is  usually  easily  discriminated  from  the  crystal -outlined 
secondary  magnetite.  While  a  considerable  percentage  of  the  irou  oxide 
of  the  ore  was  present  as  detritus,  in  no  case  does  it  appear  that  the 
material  was  rich  enough  for  merchantable  ore  before  the  secondary  con- 
centration, and  often  the  secondary  magnetite  and  its  alteration  product, 
martite,  are  the  predominant  constituents  of  the  ore. 

Microscopical. — With  the  uiicroscoj^e,  the  basal  conglomerate  resting  on 
the  Ajibik  quartzite  is  found  to  have  a  background  consisting  of  quartz 
grains  set  in  a  more  or  less  abundant  sericitic,  cherty,  and  iron-impregnated 
matrix  This  matrix  may  be  so  abundant  as  to  separate  the  fragmental 
grains,  or  may  be  sparse.  In  this  background  are  found  complex  fragments 
of  quartzite  the  individual  grains  of  which  are  rounded,  and  fragments  of 
sericite-slate  and  sericite-schist,  all  identical  with  these  rocks  in  the  Ajibik 
quartzite. 

In  the  Republic  trough,  where  the  Goodrich  quartzite  in  part  rests 
directly  upon  the  Archean,  the  schist-conglomerate  found  at  the  bottom  of 
the  detrital  formation  has  as  a  matrix  a  micaceous  quartz-schist.  In  certain 
varieties  feldspar  is  abundant  in  the  background,  and  it  becomes  a  mica- 
gneiss.  In  this  background  are  oval  or  ribbon-like  areas  of  quartz  or  of 
feldspar,  which  represent  the  mashed  pebbles  of  the  conglomerate.  Occa- 
sionally these  pebbles  contain  both  quartz  and  feldspar,  and  represent 
complex  fragments  derived  from  the  granite.  The  quartz  is  always  and 
the  feldspar  is  usually  shattered,  and  along  the  crevices  of  the  feldspar 
mica  and  quartz  have  developed.  Frequently  the  residual  feldspar  and 
the  secondary  quartz  and  mica  form  an  interlocking  mass.  Were  it  not 
for  the  pebble-like  areas  these  rocks  would  be  regarded  as  completely 
crystalline  schists. 

Where  the  Goodrich  quartzite  rests  upon  the  Negaunee  formation 
there  are  three  main  phases  of  material:  (1)  Chert  and  jasper  conglomerate, 
(2)  recomposed  jasper,  and  (3)  ore. 

The  chert  and  jasper  conglomerate  may  have  a  sparse  or  an  abundant 
matrix.  In  the  first  case  the  matrix  consists  of  small,  simple,  fragmental 
grains  of  quartz,  complex  particles  of  ferruginous  chert  and  jasper,  and 
iron  oxide.     In  passing  to  the  less  strongly  conglomeratic  phases  the  matrix 


414  THE   MARQUETTE   IRON-BEARING  DISTRICT. 

is  a  continuous  ramifying  mass  which  contains  the  separate  pebbles  and 
bowlders.  This  matrix  may  be  composed  chiefly  of  any  one  of  the  sub- 
stances, iron  oxide,  chert,  jasper,  or  quartz,  or  of  any  combination  of  them. 
Not  infrequently  some  secondary  muscovite  has  also  developed.  Often 
the  quartz  grains  are  enlarged.  In  all  cases  the  simple  quartzes  show 
undulatory  extinction  and  fracturing.  In  the  resultant  and  other  crevices 
secondary  hematite  and  magnetite  were  deposited.  Where  the  mashing 
was  great  the  fragments  of  chert,  jasper,  and  quartz  were  flattened  into 
thin,  layer-like  areas,  and  in  this  caSe  a  slide  of  the  recomposed  rock 
differs  but  little  in  its  appearance  from  the  original  jaspilite.  Accompany- 
ing the  granulation  of  the  ore,  chert,  and  jasper,  the  hematite  was  sheared 
into  brilliant,  finely  laminated,  micaceous,  or  silky  fibrous  hematite.  Flakes 
of  muscovite  are  usually  seen.  The  secondary  magnetite  and  hematite  are 
easily  discriminated  from  the  sheared  micaceous  hematite  by  having  crystal 
outlines.  This  infiltrated  material  is  frequently  present  in  very  large 
proportion,  filling  all  the  interspaces  between  the  original  particles  and  the 
cracks  formed  within  the  fragments.  In  some  cases  the  secondary  hematite 
and  magnetite  have  such  relations  to  the  quartz  grains  as  to  show  that  the 
silica  was  actually  dissolved  and  replaced  by  the  iron  oxide.  To  what 
extent  this  occurred  where  the  rocks  are  much  mashed  it  is  difficult  to  say, 
but  in  the  little-altered  phases  we  find  crystals  of  hematite  and  magnetite 
which  not  oidy  pass  to  the  borders  of  the  cores  of  the  enlarged  grains  but 
into  them.  There  seems  to  be  some  relation  between  the  solution  of  quartz 
and  the  deposition  of  magnetite;  that  is,  when  the  conditions  are  favorable 
for  the  deposition  of  magnetite  they  are  also  favorable  for  the  solution  of 
quartz. 

The  most  mashed  phases  of  recomposed  jaspers  have  very  much  the 
same  appearance  as  those  of  the  original  jasper  formation,  but  when 
examined  with  a  low  power  the  overlapping  lenticular  leaflets  of  the 
mashed  chert  and  jasper  fragments  are  seen,  and  a  high  power  shows  in 
some  cases  a  micaceous  mineral  which  is  almost  invariably  absent  in  the 
original  formation.  In  the  less  mashed  phases  of  the  recomposed  jaspers 
their  genesis  is  more  plainly  indicated  by  the  presence  of  coarse-grained 
quartzose  material,  not  dei-ivable  from  the  immediately  subjacent  formation; 


PETROGRAPHICAL  CHARACTER  OF  GOODRICH  QUARTZITE.      415 

but  even  iu  such  cases  these  quartzes  were  often  granuh\te(l  into  jasper-like 
material. 

By  disappearance  of  the  siliceous  element  and  increase  of  the  second- 
ary hematite  and  magnetite  the  recomposed  rocks  pass  into  magnetic  or 
specular  ore.  Macroscopically  these  ores  often  show  a  peculiar  gray  color, 
and  in  thin  section  they  are  usually  easily  separated  from  the  ores  of  the 
Negaunee  formation  by  the  presence  of  brilliantly  polarizing  flakes  of 
muscovite  and  of  occasional  particles  of  fragmental  quartz. 

The  conglomerates,  recomposed  jaspers,  or  recomposed  ores,  by  a  les- 
sening of  the  amount  of  chert,  jasper,  and  iron  oxide,  grade  upward  into  the 
quartzites.  In  the  purer  phases  these  quartzites  consist  mainly  of  well- 
rounded,  simple  fragments  of  quartz,  many  of  which  are  enlarged,  but 
with  these  are  usually  complex  particles  of  chert  and  jasper.  The  quartz 
grains  generally  show  strong  pressure  eflfects,  such  as  undulatory  extinction 
or  fracturing  in  a  complex  manner.  This  fracturing  is  in  certain  cases  in  a 
rectangular  system  corresponding  to  the  shearing  planes.  In  other  phases 
there  is  an  abundant  matrix  composed  of  finely  crystalline  quartz,  with 
sericite,  biotite,  and  chlorite,  in  Avhicli  the  large  fragmental  grains  of  quartz 
are  set.  By  an  introduction  of  feldspar  the  quartzites  pass  into  feldspathic 
quartzites,  and  from  these  to  the  graywackes  of  the  Michigamme  formation. 
In  the  less  pure  quartzites,  sericite,  biotite,  and  chlorite  frequently  developed 
abundantly  from  the  clayey  background. 

In  the  more  mashed  phases  of  the  quartzite,  and  particularly  in  the 
Republic  trough,  the  rocks  are  micaceous  quai-tz-schists.  Feldsj^ar  is  plenti- 
fully associated  with  the  quartz  in  a  number  of  cases.  Usually  the  grains 
of  quartz  still  show  a  roundish  appearance,  but  they  interlock  intricately, 
and  show  no  evidence  of  original  cores.  In  the  finer-grained,  most  mashed 
kinds  the  background  is  a  finely  granular,  interlocking  mass  of  quartz. 
Between  and  wrapping  around  the  larger  grains  abundant  muscovite  is 
found.  In  some  of  the  rocks  muscovite  is  a  chief  constituent,  and  these 
may  be  called  muscovite  -  schists.  Muscovite -biotite -schists  and  biotite- 
schists  are  also  associated  with  the  quartz-schists.  These  are,  in  nearly 
all  aspects,  like  the  more  metamorphosed  rocks  of  the  Michigamme  forma- 
tion.    In  a  few  places  feldspar  is  a  chief  constituent  of  the  backgi'ound. 


416  THE   MAKQUETTE   lEON-BEAKING   DISTINCT. 

and  the  rocks  become  mica-gneisses.  Various  accessory  minerals,  such  as 
chlorite,  epidote,  and  zoisite,  are  found  in  the  quartz-schists,  mica-schists, 
and  mica-gneisses. 

At  various  localities  there  are  exceptional  varieties  of  rocks  which 
belong  to  the  Goodrich  quartzite,  but  these  will  not  be  considered  here. 
They  are  described  later  in  connection  with  the  localities  at  which  they 

OCCVU'. 

THICKNESS. 

The  thickness  of  the  Groodrlch  quartzite  is  variable,  and  no  average 
estimate  can  be  given,  as  it  is  not  sharply  delimited  above.  The  best- 
known  locality  to  determine  its  thickness  is  north  of  the  Saginaw  mine, 
where  It  has  a  surface  width  of  about  1,800  feet  and  an  average  dip  of 
probably  not  less  than  60°.  This  would  give  a  thickness  of  about  1,.550 
feet.  Since  the  transgression  horizon  here  rapidly  cuts  across  the  iron  for- 
mation, which  a  short  distance  to  the  west  is  reduced  to  a  narrow  belt,  it  is 
probable  that  this  thickness  Is  much  beyond  the  average  of  the  formation, 
even  for  the  Ishpeming  area,  and  that  it  is  several  times  as  great  as  in  the 
western  part  of  the  district. 

THE    BIJIKI    SCHIST. 

The  second  division  of  the  Ishpeming  formation  is  the  Bijiki  schist. 
The  rock  is  given  this  name  because  typical  exposures  of  It  occur  near  the 
mouth  of  Bijiki  River.  They  were  regarded  by  Brooks  as  anthophyllitic 
schists. 

DISTRIBUTION,    EXPOSURES,    AND    TOPOGRAPHY. 

This  schist  has  three  narrow  belts.  The  northernmost  one  extends, 
with  frequent  exposures,  from  the  west  end  of  the  district,  just  south  of  the 
Goodrich  quartzite,  to  sec.  28,  T.  48  N.,  R.  29  W.  (Atlas  Sheets  V,  VIII,  and 
XII).  The  belt  may  extend  somewhat  farther  to  the  east  than  this,  but 
there  are  no  exposures.  The  second  belt  is  a  short  distance  south  of  the 
first.  It  runs  along  the  northern  side  of  MIchigamme  Lake  to  a  point 
northeast  of  Champion.  The  third  belt  extends  from  the  southern  extrem- 
ity of  MIchigamme  Lake  to  Champion,  and  is  north  of  the  southern  belt  of 
Goodi-ich  quartzite.  The  Bijiki  schist  thus  appears  to  be  confined  to  the 
west  end  of  the  district.     In  the  time  scale  it  must  be  equivalent  to  a  part 


EXrOSUKES   OF   THE   BIJIKI   SCHIST.  417 

of  the  Goodrich  quartzite  to  the  east.  Where  the  Bijiki  formation  appears 
the  Goodrich  quartzite  becomes  an  exceedingly  narrow  belt,  too  small  to 
be  shown  on  the  atlas  sheets;  hence  the  two  are  mapped  together  as  the 
Ishpeming  formation. 

The  rock  is  of  a  resistant  character,  and  for  the  areas  outlined  there 
are  numerous  exposures.  South  of  Michigamme  and  Spurr  it  makes  con- 
spicuous east-and-west  ridges  just  north  of  the  railroad  For  most  of  the 
length  of  Lake  Michigamme  the  southern  border  of  the  central  resistant 
belt  forms  the  northern  lake  boundary.  However,  at  two  or  three  places 
the  schist  was  cut  through  and  the  lake  shore  follows  the  softer  formation 
to  the  north.  In  two  of  these  cases  the  Bijiki  schist  constitutes  headlands 
lapped  on  tlu'ee  sides  by  water  In  the  same  way  the  southeastern  shore 
of  Lake  Michigamme  is  limited  by  the  southern  belt  of  this  schist  At 
various  places  erosion  encroached  upon  the  belt,  but  the  rock  is  so  resistant 
that  the  lake  nowhere  cuts  entirely  across  the  formation. 

FOLDING. 

The  belts  adjacent  to  the  Goodrich  quartzite  OAve  their  position  to  their 
being  the  next  higher  horizon  in  the  general  synclinorium  of  the  district. 
The  central  belt  is  due  to  a  subordinate  anticline  which  rises  high  enough 
to  expose  the  Bijiki  schist,  but  erosion  has  not  reached  a  lower  horizon,  and 
thus  a  section  across  the  area,  including  the  tlu-ee  formations,  shows  two 
synclines  with  a  central  anticline. 

PETROGEAPHICAL   CHARACTER. 

Macroscopicai. — The  Bijiki  schist  is  a  banded  griinerite-raagnetite-schist. 
Associated  with  this  rock  are  also  phases  which  approach  the  Michigamme 
slate  above  and  the  Goodrich  quartzite  below.  These  are  usually  gradation 
phases,  and  occur  upon  the  outer  parts  of  the  belt.  (PI.  XXXI )  The 
griinerite- magnetite -schists  consist  of  bands  composed  mainly  of  the 
three  minerals,  quartz,  griinerite,  and  magnetite,  and  while  in  any  single 
band  one  of  these  minerals  may  be  predominant,  the  other  two  are  usually 
present.  The  rocks  are  gray  or  green,  and  in  their  nearly  pure  phases 
they  differ  from  the  griinerite -magnetite -schist  of  the  Negaunee  forma- 
tion chiefly  in  their  exceeding  toughness.  It  is  with  great  difficulty  that 
MON  xxviii 27 


418 


THE  MAEQUETTE   IRON-BEAKING   DISTKICT. 


the  rock  is  broken  into  pieces  parallel  to  the  stratification,  so  firmly  are  the 
different  plates  bound  together  by  the  long  griinerite  needles;  but  it  is 
comparatively  easy  to  break  the  rock  across  the  bedding.  This  peculiar 
toughness  and  the  more  coarsely  crystalline  character  of  the  griinerite  are 
the  chief  points  which  distinguish  it  from  the  similar  rock  of  the  Negaunee 
formation. 

An  analysis  of  the  typical  Bijiki  schist  (Specimen  25446,  sec  19, 
T.  48  N.,  R.  30  W.)  north  of  Michigamme  was  made  by  George  Steiger, 
in  the  chemical  laboratory  of  the  United  States  Geological  Survey,  with 
the  following  result: 

Analj/sis  of  tijpical  Bijiki  schist. 


Per  cent 

S102                                                         

65.42 
1.64 

27.08 
.31 
3.12 

F&oO)                                                                  

FeO                                                                 

CaO 

97.57 

This  analysis  indicates  that  the  essential  constituents  of  the  rock  are 
griinerite  and  quartz.  A  comparison  of  the  analysis  of  this  rock  with  the 
analyses  of  the  griinerite-magnetite-schists  of  the  Negaunee  formation 
(p.  338)  shows  how  very  similar  they  are.  The  first  analysis  given  of  the 
Negaunee  griinerite-schist  is  very  similar  indeed  to  that  of  the  Bijiki  schist, 
and  the  others  difi^r  from  the  latter  mainly  in  containing  more  magnetite. 
Other  specimens  of  the  Bijiki  schist  might  have  been  selected  which  are 
also  rich  in  magnetite. 

Microscopical. — The  Ivluds  of  tlic  sclilst  frcc  from  clastic  material  consist  of 
intricately  interlocking  griinerite,  magnetite,  and  quartz,  with  more  or  less 
hematite.  The  different  materials  may  be  uniformly  intermingled,  but 
more  commonly  each  is  alternately  predominant,  and  this  gives  the  rock  a 
banded  appearance.  Occasionally  the  amphibole  has  a  green  color,  and 
with  this  a  decided  pleochroism,  perhaps  indicating  that  it  is  common 
hornblende.     Not  infrequently  the  same  amphibole  individual  is  composed 


PETEOGRAPHICAL  CHARACTER  OF  THE  BIJIKI   SCHIST.     419 

in  part  of  the  hornblende  and  in  part  of  the  griiuerite.  In  different  indi- 
viduals and  slides  the  two  show  the  greatest  variety  of  iutergrowths.  In 
one  or  two  instances,  near  the  top  of  the  member,  siderite  is  an  important 
constituent,  constituting-  a  matrix  in  which  the  other  constituents  are  set. 
In  other  cases,  at  lower  horizons,  a  little  residual  siderite  is  seen,  which  is 
surrounded  and  penetrated  by  griiuerite  or  hornblende  and  magnetite, 
strongly  suggesting  that  these  minerals,  with  the  addition  of  silica,  devel- 
oped from  the  siderite.  Near  the  base  of  the  Bijiki  schist  rounded  and 
enlarged  grains  of  fragmental  quartz  appear  within  the  completely  crys- 
talline interlocking  griinei'ite,  magnetite,  and  quartz.  Still  nearer  the 
Goodrich  quartzite  we  have  a  fragmental  quartzose  background,  iu  the 
matrix  of  which  griiuerite  and  magnetite  have  developed.  In  both  the  pure 
and  the  impure  phases  a  great  deal  of  garnet  appears.  It  is  possible 
that  a  part  of  the  griinerite  and  magnetite  is  detritus  derived  from  the. 
Negaunee  formation,  but  the  extraordinary  likeness  of  the  Bijiki  schist  to 
the  griinerite-magnetite-schist  produced  by  metasomatic  processes  from  iron 
carbonate,  the  presence  of  siderite  in  the  formation  itself,  the  relations  of 
this  siderite  to  the  griinerite  and  magnetite,  the  absence  of  any  frag-mental 
appearance,  all  suggest  that  the  rock  developed  out  of  an  original  sideritic 
slate,  similar  to  that  of  the  Negaunee  formation.  It  is  not  improbable  that 
the  development  of  the  griinerite-magnetite-schist,  both  in  the  Upper  ]\Iar- 
quette  and  Lower  Marquette  series,  was  a  simultaneous  process,  occurring 
during  and  after  the  Upper  Marquette  folding. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

It  has  been  said  that  the  Goodrich  quartzite  grades  rapidly  upward 
into  the  Bijiki  schist.  The  Bijiki  schist  in  turn  grades  into  the  Michigamme 
formation,  the  intermediate  zone  again  being  half  fragmental.  Belonging 
with  or  immediately  above  the  griinerite-magnetite-schists,  in  the  western 
part  of  the  district,  are  the  ore  deposits  of  the  Upper  Marquette  series. 
These  ore  lx)dies  appear,  however,  to  be  rather  within  the  slates  than  to 
belong  Avith  the  griinerite-magnetite-schist;  but  if  exposures  were  suffi- 
ciently numerous  it  might  be  possible  to  map  in  the  Upper  Marquette  series 
a  continuous  iron-beariu"'  formation  which  would  include  these  ore  bodies 


420  THE   MAKQUETTE  IllON-BEAEING  DISTEICT. 

^nd  the  griinerite-magnetite-schists,  thus  making  the  hxtter  rocks  the  lowest 
Jiorizon  of  this  ore-beariug  formation. 

THICKNESS. 

The  formation  varies  from  a  considerable  thickness  to  disappearance, 
:and,'  as  in  the  previous  cases,  it  is  impossible  to  give  an  accurate  estimate 
•of  its  thickness  at  any  place.  At  some  places  it  apparently  has  a  surface 
width  of  600  feet,  with  a  dip  varying  somewhat,  but  perhaps  averaging  60°. 
This  Avould  indicate  a  maximum  thickness  of  about  520  feet. 

INTERESTING    LOCALITIES    OF    THE    ISHPEMING    FORMATION. 

Michigamme  and  Spurr. — Beginning  at  thc  uortliwest  part  of  the  district,  there 
are  large  and  typical  exposures  of  the  Goodrich  quartzite  and  Bijiki  schist 
from  the  Spurr  mine  to  east  of  the  Michigamme  mine  (Atlas  Sheet  V).  At 
these  mines  a  considerable  part  of  the  magnetic  ore  apparently  belongs  to  the 


Pig.  2() Section  showing  relations  of  jaspilite,  ore,  conglomerate,  and  quartzite  .^t  Michigamme  mine. 

basal  horizon  of  the  Ishpeming  foiTnation.  The  ore  deposits  weld  together 
the  unconformable  Ishpeming  and  Negaunee  formations.  The  hanging 
wall  of  the  ore  deposits  is  a  conglomerate,  the  pebbles  of  which  are  mainly 
from  the  underlying  Negaunee  jasper,  and  the  matrix  of  which  is  mainly 
magnetite.  At  the  Spun-  mine  the  pebbles  are  of  the  white,  cherty  rock; 
at  the  Michigamme  mine  they  are  of  the  red  jasper,  the  larger  ones 
being  6  to  8  inches  in  diameter.  This  suggests  that  the  underlying  Negau- 
nee rock  had  assumed  its  present  form  before  it  was  broken  up,  and  yielded 
detritus  to  the  Ishpeming  formation.  It  is,  however,  possible  that  at 
each  place  subsequent  changes  have  altered  the  ujjper  part  of  the  sili- 
ceous rock  of  the  Negaunee  formation  and  the  overlying  conglomerate  in 
a  similar  manner.  At  the  Michigamme  mine  the  conglomerate  is  in  places 
:at  least  20  feet  thick,  and  in  other  places  is  absent,  the  quartzite  directly 


INTERESTING   LOCALITIES   OF   THE   ISIIPEMIXG   FOKMATIOK       421 

overlying  the  ore.  The  relations  of  the  Negaunee  jasper,  the  ore,  and  the 
conglomerate  are  shown  by  fig.  26.  At  the  Spurr  mine  the  conglomerate  is 
at  least  40  feet  thick.  The  conglomerate  at  both  mines  passes  up  rather 
abruptly  into  a  greenish  or  grayish,  massive  quartzite.  This  quartzite  in 
turn  varies  by  interstratification  into  the  griinerite-magnetite-schist  which 
has  been  called  the  Bijiki  schist.  (PI.  XXXI.)  An  intermediate  variety 
is  a  fine-grained  biotitic  and  griineritic  graywacke.  Not  more  than  50 
feet  south  of  the  interlaminated  beds  are  typical  exposures  of  the  Bijiki 
schist.  This  quartzite  at  the  Michigamme  mine  is  folded  into  a  series  of 
minor  rolls,  which  are  cut  by  dikes.  (PI.  XXX.)  At  the  Spurr  nnd  Michi- 
gamme mines,  on  the  south  or  hanging-wall  side  of  the  jjits,  are  dikes  of 
chlorite-schist,  which  are  taken  to  be  modified  eruptives,  as  they  cut  the 
other  rocks  like  dikes.  It  is  in  and  adjacent  to  the  chlorite-schists  that 
the  large  garnets  and  chlorite  pseudomorphs  after  garnets  described  by 
Pumpelly  are  obtained.  The  entire  thickness  of  the  Goodrich  rock  is 
perhaps  not  more  than  100  feet.  The  Bijiki  schist  occurs  in  very  numerous 
exposures  on  the  hills  between  the  mines  and  the  railroad  track,  and 
particularly  on  a  prominent  ridge  just  north  of  the  railroad  track  between 
the  two  mines.  The  boundary  between  the  Ishpeming  formation  and  the 
Negaunee  formation  follows  a  low  and  sometimes  swampy  area.  In  all 
respects  the  Bijiki  schist  of  this  locality  corresponds  to  the  general  descrip- 
tion (pp.  417-418).  The  rock  is  minutely  crenulated,  and,  while  having  a 
general  southward  dip,  has  many  minor,  often  isoclinal  folds. 

As  seen  in  thin  section  the  majority  of  the  quartzites  are  composed 
of  rather  well-rounded  grains  of  quartz  having  a  matrix  of  chlorite  or  of 
chlorite  with  biotite  and  magnetite.  With  the  fragmental  grains  of  quartz 
at  the  Spurr  mine  are  also  fragmental  grains  of  feldspar  and  small  granitic 
pebbles.  Most  of  the  grains  of  quartz  are  so  large  that  they  could  not 
have  been  derived  from  the  Negaunee  formation,  and  they  very  probably 
come  from  the  granite  of  the  Basement  Complex.  That  this  is  their 
source-  is  indicated  by  the  feldspar  and  the  granitic  fragments.  Little  or  no 
fragmental  material  from  the  Negaunee  formation  was  detected.  It  thus 
appears  that  shortly  after  the  Goodrich  quartzite  began  to  form,  the  thin  bed 
of  conglomerate  buried  the  Negaunee  formation  in  the  immediate  vicinity,. 


422  THE   MARQUETTE    IRON-BEAEING    DISTRICT. 

and  the  fragmental  material  for  tlie  quartzite  was  transported  by  the  waves 
from  the  Basement  Complex,  which  at  some  not  distant  point  was  above 
the  water.  The  grains  of  quartz  all  show  pressure  effects  by  undulatory 
extinction  and  fracturing,  and  they  do  not  commonly  show  distinct  enlarge- 
ment, although  minute  irregularities,  which  indicate  that  they  have  prob- 
ably grown,  are  seen.  In  some  cases  the  enlargements  are  distinct.  The 
feldspars,  where  present,  show  partial  decomposition  into  interlocking  chlo- 
rite, muscovite,  biotite,  and  quartz.  The  chlorite  in  the  interstices  of  the 
grains  is  usually  in  aggregates  of  minute  leaflets,  but  rarely  it  occm's  in 
blades  of  considerable  size.  Hornblende  is  found  in  some  slides  between 
the  grains  of  the  quartzite.     Garnet  is  occasionally  present. 

In  the  variety  of  rock  intermediate  between  the  quartzite  and  the 
Bijiki  schist  the  coarse-grained  fragmental  quartz  is  clearly  discriminated 
from  the  fine-grained  quartz,  which  developed  in  another  way.  These 
rocks  may  be  described  as  hornblendic  and  magnetitic  schists  which  con- 
tain numerous  clastic  grains  of  quartz.  In  some  of  them  there  remains  a 
considerable  amount  of  siderite,  and  out  of  this  .siderite,  joined  with  silici- 
fication,  the  magnetite,  hornblende,  and  quartz  have  developed,  exactly  as 
from  the  sideritic  slates  of  the  Negaunee  formation.  In  the  half-frag- 
mental  phases,  wherever  the  siderite  is  found,  griinerite  and  magnetite 
appear;  where  fragmental  quartz  is  abundant  they  are  not  prominent.  It 
therefore  appears  that  the  original  transition  rock  was  here  a  siderite 
which  contained  a  certain  amount  of  fragmental  material.  By  compari- 
son (pp.  321-322,  333-334)  it  will  be  seen  that  the  rock  is  analogous  to  the 
transition  form  between  the  Ajibik  quartzite  and  the  Negaunee  formation. 
Another  analogy  between  this  transition  rock  and  that  of  the  Negaunee 
formation  is  the  fact  that  the  amphibole  which  develops  is  pleochroic  rather 
than  nonpleochroic.  The  cause  is  doubtless  the  same  in  both  cases — the 
presence  of  a  great  variety  of  chemical  elements  in  a  mingled  clastic  and 
nonclastic  sediment  from  which  material  could  be  drawn. 

By  disappearance  of  the  fragmental  quartz  and  replacement  of  the 
pleochroic  amphibole  by  the  nonpleochroic  griinerite  the  rock  passes  into 
the  typical  Bijiki  schist,  which  is  either  a  griinerite-magnetite-rock  or  a 
griinerite-magnetite-schist.     In  these  rocks  there  are  still  found  varieties 


INTERESTING  LOCALITIES   OF  THE   ISHPEMING  FOEMATION.       423 

which  are  largely  composed  of  siderite,  and  out  of  this  siderite  the  grii- 
nerite  and  magnetite  may  be  seen  developing.  The  descrii)tion  of  the 
griinerite-magnetite-rocks  and  griinerite-magnetite-schists  given  in  the  gen- 
eral description  (pp.  417-419)  applies  to  the  Spun*  and  Michigamme  area. 
They  are  in  all  respects  like  similar  rocks  from  the  Negaunee  formation, 
with  the  exception  that  the  griinerite  is  in  coarser  blades  and  crystals,  and 
that  frequently  there  are  numerous  minute  black  particles  throughout  the 
rock  Avhich  have  a  carbonaceous  appearance,  but  which  may  be  flecks  of 
iron  oxide.  The  quartz  of  the  griinerite-magnetite-schists  is  very  similar 
to  that  of  the  Negaunee  formation  of  the  Michigamme  area.  Upon  the 
whole,  however,  it  is  somewhat  more  tinely  crystalline,  the  grains  aver- 
aging, in  the  different  sections,  from  0.03  to  0.1  mm.  in  diameter.  In 
certain  slides  the  nonpleochroic  griinerite  and  the  pleochroic  amphibole  are 
intergrown,  exactly  as  they  are  in  the  Negaunee  formation  in  this  area. 
Where  the  rocks  are  exposed  to  weathering  the  griinerite  is  more  or  less 
altered  into  biotite  and  chlorite,  and  where  these  minerals  occur  there  is 
also  seen  brilliant  blood-red  hematite,  which  has  doubtless  developed  by 
oxidation  from  the  magnetite.  Not  infrequently  the  griineritic  and  mag- 
netitic  rocks  are  garnetiferous.  The  garnets  include  the  various  other 
minerals,  and  it  is  apparently  the  last  mineral  to  develop. 

Lake  Michigamme  area. — Upou  the  anticliual  Hdgo  bordcriug  the  nortli  side 
of  Lake  Michigamme  and  constituting  the  headlands  of  this  body  of  water 
are  numerous  exposures  of  typical  Bijiki  schist  (Atlas  Sheets  V  and  VIII). 
In  fact,  it  is  from  the  exposures  adjacent  to  the  mouth  of  the  Bijiki  River 
that  this  formation  is  given  its  name.  The  rocks  are  here  griinerite- 
magnetite-rocks  and  griinerite-magnetite-schists,  in  all  respects  like  those 
constituting  the  ridge  north  of  the  railroad  track  between  Michigamme  and 
Spurr.  In  thin  section,  also,  they  are  identical.  The  ordinary  hornblende 
is  the  nonpleochroic  griinerite,  but  at  the  northwest  point  of  the  westward- 
projecting  headland  in  sec.  28  there  are  beautiful  intergrowths  of  the  non- 
pleoclu'oic  and  pleochroic  amphiboles,  the  latter  giving  blue,  green,  and 
yellow  colors.  It  is  possible  that  the  phenomena  are  due  to  combinations, 
of  varying  proportions,  of  the  actinolite  and  griinerite  molecules. 

East  of  Lake  Michigamme  (Atlas  Sheet  XII)  this  anticline  is  repre- 
sented by  a  prominent  ridge  of  the   Bijiki  formation,  running  from  the 


424  THE   MAKQUETTE  lEON-BEARING  DISTRICT. 

NE.  \  sec.  31,  iu  a  course  somewhat  south  of  east,  uito  the  SW.  ^  sec.  33, 
a  distance  of  about  2  miles.  This  ridge  for  the  most  of  the  distance  has  an 
abrupt  southward-facing  slope,  which  overlooks  swampy  land  to  the  south. 
The  north  side  of  the  ridge  is  distinctly  but  not  so  sharply  marked.  For  the 
most  part  this  ridge  is  composed  of  typical  griinerite-magnetite-schists  and 
griinerite-magnetite-rocks  of  the  Bijiki  horizon.  However,  where  the  ridge 
breaks  down  upon  its  west  end,  in  the  SE.  ^  sec.  31,  adjacent  to  the  road,  at 
the  bottom  and  central  parts  of  the  exposure  are  conglomerates  containing 
fragments  of  chert,  coarse  quartzites,  and  fine-grained  novaculites ;  and  schis- 
tose and  dolomitic  graywackes  and  shales  intermediate  between  the  fore- 
going and  the  Bijiki  schists.  The  change  from  the  fragmental  rocks  to  the 
Bijiki  schist  takes  place  very  quickly.  The  elastics  are  in  most  respects 
like  those  at  the  bottom  of  the  Bijiki  schist  at  the  Spurr  and  Michigannne 
mines,  and  they  doubtless  mark  the  lowest  stratum  found  in  this  anticlinal 
ridge.  The  quartzites  and  novaculites  are  much  brecciated  by  the  shaq) 
folding.  To  the  south  of  the  fragmental  rocks  are  typical  griinerite- 
magnetite-schists.  To  the  north  the  Bijiki  rocks  are  very  quartzose,  and 
grlinerite  is  subordinate.  Both  north  and  south  they  show  extreme  plica- 
tion, and  occasionally  brecciation.  In  places  the  schistosity  does  not 
correspond  with  the  bedding. 

In  thin  section  the  rocks  of  this  ridge  are  in  almost  every  respect 
identical  with  those  of  Spurr  and  Michigamme.  The  siliceous  layers 
between  the  griinerite-magnetite  layers  are  very  often  composed  of  crystal- 
outlined  grains  similar  to  those  of  the  Negaunee  formation  at  the  Spurr 
and  Michigamme  mines.  In  coarseness  of  crystallization  the  quartz  of  the 
two  formations  is  identical.  On  account  of  their  crystal  outlines  the  grains 
do  not  interlock,  and  the  rocks  are  therefore  rather  friable.  In  some  sections 
films  of  hematite  occur  everywhere  between  the  grains,  thus  giving  a  con- 
tinuous net-like  area  of  translucent  red  hematite,  the  spaces  between  being 
occupied  by  the  pellucid  quartz.  This  is  best  seen  in  the  sections  from  the 
north  side  of  the  ridge,  where  the  grlinerite  is  rather  sparse  and  quartz  is 
the  predominant  constituent. 

Boston  and  Dexter  area. — In  passiug  to  the  east,  tlic  ucxt  exposurcs  fouud 
are  those  in  the  neighborhood  of  the  Boston  and  Dexter  mines  and  at 


INTERESTING   LOCALITIES   OF   THE   ISHPEMING   FORMATION.       425 

intermediate  points  (Atlas  Sheets  XVIII  and  XXII).  Here  the  rocks  are 
for  the  most  part  typical  quartzites,  although  at  one  place  interstratified 
graywacke  is  seen.  The  basal  portion  of  the  quartzite  is  a  conglomerate, 
bearing  detritus  from  the  Negaunee  formation. 

Lake  Corning  area. — The  uext  sct  of  exposures,  lu  secs.  4,  5,  and  6,  T.  47  N., 
R.  27  W.,  occur  at  intervals  west  of  the  Excelsior  mine  to  a  point  about 
2  miles  east  (Atlas  Sheets  XXV  and  XXVIII).  At  the  basal  horizon  there 
is  here  again  a  conglomerate,  the  debris  of  which  is  chiefly  from  the 
Negaunee  formation ;  the  ordinary  varieties  of  the  formation,  however,  are 
the  typical  quartzites,  but  with  these  are  slaty  phases. 

ishpeming  area. — The  uext  important  localities  are  those  at  Ishpeming  and 
in  the  various  basins  south  of  Ishpeming.  At  nearly  all  of  the  mines 
adjacent  to  this  city  the  contact  between  the  Negaunee  formation  and  the 
Goodrich  quartzite  is  seen.  East  of  Ishpeming  the  conglomerate  at  the 
contact  is  found  at  the  large  open  pit  of  the  Lake  Superior  mine  and  in 
open  pits  in  the  bays  both  to  the  north  and  south.  At  the  first  of  these  the 
contact  between  the  Ishpeming  formation  and  the  Negaunee  formation 
makes  the  so-called  Lake  Superior  "W."     (PI.  XVI.) 

South  of  Ishpeming,  the  conglomerate  is  again  seen  in  the  large  open 
pit  near  the  east  quarter  post  of  sec.  9,  at  open  pits  north  of  the  green- 
stone bluff  in  the  SE.  \  of  the  section,  and  at  numerous  localities  at  the 
large  open  pits  in  the  SW.  J  sec.  16.  In  the  localities  nearest  the  city  of 
Ishpeming  the  conglomerate  usually  rests  upon  the  Negaunee  formation, 
but  on  the  south  slope  of  the  greenstone  bluff,  adjacent  to  the  section  line 
between  secs.  3  and  10,  the  conglomerate  rests  upon  the  greenstone.  The 
same  relations  are  seen  in  the  southeast  part  of  sec.  16,  in  the  large  open 
pit  just  south  of  the  east-west  quarter  line. 

South  of  this  locality,  in  the  SW.  J  sec.  16,  are  the  handsomest  expos- 
ures of  this  conglomerate  in  the  Ishpeming  area.  The  rock  occupies  a 
well-marked  ridge  parallel  to  the  railroad.  Upon  its  smooth  glaciated 
surface  it  presents  a  beautiful  appearance,  due  to  the  pebbles  of  brilliant 
red  jasper. 

The  basal  phase  of  the  conglomerate  for  the  Ishpeming  area  is  usually 
very  feiTuginous  (PI.  XXVII,  fig.  2),  and  where  it  is  fine-grained  it  may 


426  THE   MAEQUETTE   lEON-BEARl^^G  DISTRICT. 

become  an  iron  ore.  These  iron  ores  usually  have  a  grayish  or  greenish 
color,  and  are  known  to  the  miners  as  the  hard  gray  ores.  As  looked  at  in 
hand  specimen,  the  detrital  hematite  and  the  heavily  ferruginous  fragments 
have  a  micaceous  appearance,  due  to  mashing.  The  larger  part  of  the 
ore  is,  however,  magnetite  in  crystals,  or  its  alteration  product,  martite. 
Associated  with  the  latter  is  a  great  deal  of  a  green  mineral.  As  these 
ores  become  impure  there  are  observed  in  them  fragments  of  jasper  and 
grains  of  quartz,  which  increase  in  quantity  until  the  rock  is  no  longer  an 
ore,  but  a  heavily  ferruginous  quartzite,  or  ore  and  jasper  conglomerate. 
There  are  all  gradations  between  a  conglomerate  which  consists  almost 
wholly  of  fragments  derived  from  the  Negaunee  formation  and  one  in 
which  the  detrital  material  is  mainly  grains  of  quartz,  in  which  case  the 
rock  becomes  a  ferruginous  quartzite. 

In  thin  section  the  general  description  of  the  jasper-conglomerate  given 
on  pages  413-415  applies  fully  to  the  conglomerate  of  the  area.  In  differ- 
ent localities  the  dynamic  effects  vary  greatly.  At  some  places  the  jasper 
pebbles  are  greatly  flattened  and  the  ore  pebbles  are  changed  to  hematitic 
slate.  In  other  places  the  dynamic  effects  are  slight,  the  rock  consisting 
of  rounded  or  angular  ore  and  jasper  pebbles,  cemented  by  finer  detritus  of 
the  same  kind  and  by  iron  oxide.  There  is  present  in  nearly  all  of  the 
conglomerates  a  small  amount  of  plainly  fragmental  quartz  not  derived 
from  the  Negaunee  formation;  and  also  a  small  amount  of  sericite  and 
chlorite  have  developed,  the  former  especially  in  the  mashed  varieties. 
The  gray  ore  is  found  to  consist  of  the  original  mashed  hematite,  and  of 
crystals  and  clusters  of  crystals  of  magnetite,  between  which  is  chlorite. 
In  the  impure  ores  there  is  a  certain  amount  of  fragmental  quartz,  which 
is  plainly  partly  replaced  by  magnetite.  Crystals  of  magnetite  project  into 
the  roundish  quartz  grains,  just  as  though  notches  had  been  filed  out  of  the 
clastic  grains  to  give  place  to  the  magnetite.  Also  numerous  crystals  of 
magnetite  are  wholly  included  within  the  quartz  grains.  It  thus  appears 
perfectly  clear  that  by  some  reaction  quartz  was  dissolved  and  the  mag- 
netite took  its  place.  What  relation  there  is  between  the  disappearance 
of  the  quartz  and  the  appearance  of  the  magnetite  is  not  clear,  but  one 
seems  to  be,  to  some  extent  at  least,  conditioned  on   the   other;   for  if 


INTEEESTING   LOCALITIES   OF  THE   ISIIPEMING  FORMATION.       427 

this  were  not  tlie  case  the  quartz  would  be  irregularly  dissolved  and  the 
magnetite  take  the  vacant  space.  It  is  in  the  rocks  which  are  half  way 
between  ore  and  ferrug-inous  quartzite  or  conglomerate  that  this  process  of 
replacement  is  best  seen. 

For  most  of  the  Ishpeming  area  onh^  the  belt  of  ore  and  ore  and  jasper 
conglomerate  is  exposed.  HoweA'er,  in  the  center  of  the  city  of  Ishpe- 
ming typical  quartzite  is  seen.  Also  in  sec.  16  the  exposures  are  found  at 
intervals  from  the  basal  conglomerate  to  the  top  of  the  formation.  Tlie 
conglomerate,  by  a  disappearance  of  the  ore  and  jasper  fragments  A-aries 
up  into  ferruginous  quartzite  and  ferruginous  slate,  and  this  into  ordinary 
quartzite,  which  is,  however,  interstratified  with  graywacke.  In  thin  section, 
taken  in  the  same  order  there  is  a  steady  lessening  of  the  chert  and  jasper 
fragments,  and  in  the  ferruginous  quartzite  there  is  a  very  consideraljle 
amount  of  complex  material  derived  from  the  Negaunee  formation.  Above 
the  lower  200  feet  the  material  was  derived  mainly  from  some  other  than 
the  Negaimee  formation. 

Negaunee  area. — Wcst  of  Ncgauuee  aud  uortli  of  thc  Jacksou  mine  is  a 
basin  of  the  Goodrich  quartzite  (Atlas  Sheet  XXVIII).  As  explained  in 
another  place,  the  Negaunee  formation  and  the  Goodrich  quartzite  at  the 
main  Jackson  pits  are  infolded.  As  a  consequence  of  this,  both  have  appar- 
ently the  same  dip,  nearl}-  vertical,  and  at  the  west  end  of  the  open  pit  the 
two  are  infolded  several  times,  so  that  a  horizontal  section  from  the  south 
toward  the  north  passes  from  the  Negaunee  formation  to  the  Goodrich  quartz- 
ite, thence  to  the  Negaunee  formation,  thence  to  the  Goodrich  quartzite, 
thence  to  the  Negaunee,  and  finally  to  the  Goodrich  quartzite;  that  is, 
there  are  two  tongues  of  the  Negaunee  formation  Ijetween  the  exposures  of 
the  Goodrich  quartzite.  The  basal  horizon  of  the  Goodrich  quartzite  here, 
as  at  Ishpeming,  was  composed  almost  wholly  of  the  fine  and  coarse  detri- 
tus of  the  Negaunee  formation.  As  a  consequence  of  the  close  infolding, 
the  detrital  rock  was  so  closely  mashed  that  it  is  difficult  to  discriminate  it 
from  the  original  Negaunee  formation.  Especially  where  the  detritus  was 
fine,  the  rock  simulates  to  a  remarkable  degree  the  typical  jaspilite,  and 
even  where  fragments  of  jasper  were  present  these  have  been  mashed  until 
they  resemble  jasper  laminae,  or,  where  not  so  much  altered,  they  have  an 


428  THE    MARQUETTE    lEON-BEAEING    DISTRICT. 

appearance  which  resembles  that  of  the  brecciated  jasper  of  the  Negaunee 
formation.  However,  a  close  examination  discloses  differences  between  the 
two  rocks.  The  jasper  laminse  in  the  conglomerate  have  not  the  continuity 
that  they  have  in  the  true  jasper.  Some  of  the  flattened  areas  have  a  rovmd- 
ish  appearance,  and  slightly  different  colored  jaspers  occur  close  together, 
whereas  in  the  brecciated  jaspers,  while  the  layers  may  be  broken  apart, 
there  is  usually  a  suggestion  of  former  continuity  and  a  likeness  of  character 
in  the  fragments.  The  cracks  in  the  broken  pebbles  and  in  the  matrix  are 
cemented  by  magnetite  in  crystals,  wliich  is  readily  discriminated  from  the 
detrital,  mashed,  lustrous,  micaceous  hematite. 

A  short  distance  to  the  north  of  the  main  pits  of  the  Jackson  mine 
the  relation  of  the  Negaunee  formation  and  Goodrich  quartzite  is  much 
clearer.  The  Negaunee  jasper  is  folded  into  a  number  of  westerly  pitching 
rolls,  the  strike  of  the  axis  of  the  central  fold  being  nearly  N.  75°  W.  The 
dip  of  the  jasper  or  the  dip  of  the  axes  is  45°  W,  The  overlying  quartzite 
has  a  north-south  strike,  and  dips  to  the  west  at  an  angle  of  20°.  At  this 
locality  the  unconformity  between  the  two  formations  is  apparent,  but  at 
the  main  Jackson  pit  the  severe  folding  obliterated  evidence  of  this.  The 
conglomerate  and  recomposed  ferruginous  schist  pass  upward  quickly  into 
the  plainly  fragmental  ferruginous  quartzite. 

As  examined  in  thin  section,  the  mashed  conglomerates  simulate  to  a 
remarkable  degree  the  mashed  and  brecciated  original  jasper.  Many  of 
the  pebbles  were  broken  by  the  pressure  into  a  number  of  angular  frag- 
ments, which  are  cemented  by  secondary  iron  oxide.  Other  pebbles,  where 
the  mashing  was  most  severe,  are  flattened  vmtil  they  approach  the  jasper 
bands  in  appearance.  In  the  recomposed  rock  in  which  all  of  the  material 
is  derived  from  the  Negaunee  formation,  it  would  be  impossible  to  state 
from  the  thin  sections  that  the  rock  is  clastic,  but  in  many  of  them 
there  appears  a  subordinate  quantity  of  small,  roundish  grains  of  distinctly 
fragmental  quartz,  larger  than  the  quartz  grains  of  the  Negaunee  forma- 
tion, and  evidently  derived  from  some  other  source,  probably  from  the 
Basement  Complex.  These  quartz  grains  show  undulatory  extinction  and 
fracturing.  By  an  increase  of  the  clastic  quartz  the  rocks  pass  into  the 
ferruginous  quartzites,  the  fragments  of  which  are  derived  almost  wholly 


INTERESTING  LOCALITIES  OF  THE   ISHPEMING   FORMATION.       429 

froiu  a  distant  source,  but  with  which  are  a  few  chert  and  jasper  fragments. 
The  whole  is  cemented  by  oxide  of  iron.  Tlie  fragmental  quartz  grains 
all  show  undulatory  extinction  and  fracturing,  and  many  are  arranged  with 
their  longer  axes  in  a  conunon  direction. 

The  presence  of  a  quartzite  above  but  a  moderate  thickness  of  the 
chert  and  jasper  conglomerate  shows  that  the  Negaunee  formation  in  this 
vicinity  was  quickly  buried  by  the  Groodrich  deposits,  and  that  the  major 
portion  of  the  material  for  the  Goodrich  quartzite  was  derived  from  the 
Basement  Complex  or  from  the  lower  part  of  the  Lower  Marquette  series. 

Cascade  area. — Soutli  of  Palmcr  there  are  a  number  of  localities  in  which 
the  Goodrich  quartzite  is  well  exposed  (Atlas  Sheet  XXXII).  The  whole 
area  is  a  basin,  like  that  at  Negaunee,  with  a  subordinate  fold,  which  pro- 
duces a  short  arm  or  liasin  running  southeast  from  the  main  area  and 
separated  from  the  latter  by  the  Negaunee  formation.  At  the  west  end 
of  the  Palmer  area  is  the  Volunteer  mine.  This  is  an  eastward-plunging 
syncline  at  the  bottom  of  the  Goodrich  quartzite.  The  ore  belongs  wholly 
to  the  Goodrich  quartzite,  a  part  of  it  being  the  gray  granular  ore, 
but  much  of  it  being  similar  to  the  ore  of  the  Goodrich  mine,  subse- 
quently described.  The  latter  is  a  micaceous  hematite,  in  which  are  seen 
numerous  little  eyes  of  fragmental  quartz.  The  ore  grades  up  into  ordi- 
nary ore,  jasper,  and  quartz  conglomerate,  and  this  into  a  finer -grained 
chert  and  quartz  conglomerate.  In  thin  section,  in  the  gradation  varieties 
between  the  ore  and  conglomerate  the  partial  replacement  of  the  frag- 
mental quartz  by  the  magnetite  may  be  seen.  Much  of  the  iron  oxide  was 
detrital,  and  this  has  been  changed  to  lustrous  hematite,  and  the  secondar}- 
impregnation,  as  usual,  is  represented  by  the  magnetite  in  crystals  and 
by  its  alteration  product,  martite. 

The  village  of  Palmer  is  itself  upon  the  area  of  the  Goodrich 
quartzite.  If  we  go  directly  south,  the  westward -plunging  anticline  of 
the  Negaunee  jasper  is  crossed,  and  we  reach  the  south  arm  or  basin 
of  the  Goodrich  quartzite.  Along  the  southern  side  of  the  basin  at  a 
number  of  points  may  be  seen  magnificent  exposures  of  the  great  basal 
conglomerate,  resting  directly  and  unconformably  upon  the  Negaunee 
jasper.     Large,  well-glaciated  areas  are  perfectly  bare,  in  which  the  great 


430  THE  MARQUETTE   IRON  BEARING  DISTRICT. 

variety  of  brilliantly  colored  pebbles  and  bowlders  presents  a  beautiful 
appearance.  At  no  fewer  than  four  places  the  actual  contacts  between 
the  Negannee  and  Goodrich  formations  are  seen.  In  places  the  dip  of  the 
conglomerate  is  low,  and  on  the  south  slope  of  the  hills  truncated  banded 
jasper  may  be  seen,  overlain,  with  slight  discordance,  by  the  conglomer- 
ate. While  the  conglomerate  is  studded  with  waterworn  bowlders  of  the 
Negannee  formation,  it  also  contains  very  numerous  pebbles  and  bowlders 
derived  from  the  Basement  Complex.  Immediately  adjacent  to  the  Negau- 
nee  formation  fragments  of  it  are  predominant,  but  a  little  higher  up  those 
from  the  Basement  Complex  are  equally  abundant.  The  conglomerate 
grades  up  in  the  central  and  northern  part  of  the  basin  into  a  quartzite,  but 
on  the  north  side,  adjacent  to  the  Negannee  formation,  the  fine-grained 
conglomerate  is  again  found.  A  short  distance  east  of  the  village'  of 
Palmer,  again,  the  basal  conglomerates  and  contacts  between  the  Goodrich 
quartzite  and  the  Negannee  formation  may  be  seen.  Here  the  phenom- 
ena are  the  same  as  at  the  contact  in  the  basin  just  described.  About  a 
mile  east  of  Palmer,  on  an  elevation  surrounded  by  a  swamp,  are  large 
exposures  of  quartzite  and  a  fine-grained  conglomerate,  which  constitute 
the  eastern  extremity  of  the  Palmer  basin. 

Goodrich-Saginaw  area. — To  thc  wcst,  uumerous  cxposures  of  the  Goodrich 
quartzite  constitute  a  marked  ridge,  running  from  about  one-fourth  of  a 
mile  north  of  the  center  of  sec.  21,  T.  47  N.,  R.  27  W.,  to  the  Fitch  mine, 
in  the  SE.  4  sec.  24,  T.  47  N.,  R.  28  W.  (Atlas  Sheets  XXIII  and  XXVI). 
South  of  this  ridge  are  the  Lowthian,  Saginaw,  and  Goodrich  mines.  At 
all  of  these  mines,  and  at  the  Fitch,  the  contacts  between  the  Negannee 
and  Goodrich  formations  may  be  seen.  At  all  of  them  the  unconformity 
between  the  two  is  perfectly  clear,  and  great  basal  conglomerates  are  pres- 
ent, the  debris  of  which  is  derived  chiefly  from  the  Negaunee  formation. 
For  the  most  part  the  discrepancy  in  the  folding  between  the  Negaunee 
and  Goodrich  formations  is  but  slight,  so  that  the  break  is  indicated  by 
minor  differences  in  strike  and  dip;  but  the  quartzite  is  deposited  in  the 
depressions  of  the  irregularly  eroded  Negaunee  formation.  At  the  Good- 
rich mine  the  Negaunee  jasper  is  plicated,  and  it  abuts  at  various  angles, 
up  to  perpendicularity,  against  the  beds  of  the  conglomerate  (figs.  20  and 


INTEKESTING  LOCALITIES   OF  THE   ISHPEMINU  FORMATION.       431 

21,  p.  335).  The  greater  portion  of  the  ore  taken  from  the  Goodrich  mine 
is  the  recomposed  material  of  the  Goodrich  quartzite.  In  all  respects  this  is 
like  that  at  the  Volunteer  mine,  except  that  the  detritus  mingled  with  the 
ore  is  coarser,  and  therefore  the  replaceiuent  of  the  siliceous  ingredients 
by  the  iron  oxide  is  very  incomplete.  The  ore  presents  the  appearance 
of  extreme  mashing,  being  composed  of  thin  plates  and  fibers  of  micaceous 
hematite,  the  thinnest  of  which  have  slickensided  faces,  showing  that  there 
has  been  shearing  between  them.  The  conglomerate  above  shows  the 
same  dynamic  effects,  the  lamina?  of  micaceous  hematite  wrapping  around 
the  more  resistant  quartz  and  jasper  pebbles.  As  usual,  in  places  between 
the  laminae  of  sheared  hematite  and  in  cracks  a  great  deal  of  magnetite  in 
crystals  has  been  formed.  The  exposures  of  the  basal  conglomerate  at 
the  Saginaw,  Goodrich,  and  Lowthian  mines  are  scarcely  less  Ix-autiful 
than  those  in  sec.  16  (see  p.  425)  and  at  Palmer.  The  conglomerate, 
composed  mainly  of  the  Negaunee  formation  material,  contains  pebbles 
of  white,  coarsely  crystalline  quartz,  and  is  in  places  at  least  300  feet 
tliick.  It  grades  gradually  upward  into  a  fine-grained  conglomerate,  in 
whicli  the  ore  and  jasper  are  still  prominent,  but  in  which  coarse-grained 
quartz  is  abundant.  This  passes  up  into  the  coarse-grained  quartzite,  in 
which  there  is  comparatively  little  of  the  chert  and  jasper,  and  this  into 
ordinary  quartzite,  which  in  the  higher  horizons  is  interstratified  with 
graywacke. 

At  the  east  end  of  the  ridge,  about  one-fourth  mile  north  of  the  center 
of  sec.  21,  the  conglomerate  is  infolded  with  the  Negaunee  formation  in  an 
isoclinal  manner,  the  Goodrich  rock  making  a  westward-plunging  tongue. 
A  section  from  south  to  north  passes  from  the  Negaunee  formation  to  the 
Goodrich  and  then  to  the  Negaunee,  the  dip  being  continuously  at  a  high 
angle  to  the  north.  At  this  locality  the  Goodrich  conglomerate  is  very 
much  mashed.  The  pebbles  of  jasper  are  flattened,  and  in  the  plane 
of  flattening  are  elongated  unequally  in  two  directions  at  right  angles  to 
each  other.  The  finer  detritus  shows  slickensiding  effects.  Some  of  the 
finer-grained  varieties  of  the  rock  are  gray,  siliceous  or  ferruginous  slates 
or  schists,  the  more  ferruginous  approaching  in  appearance  the  so-called 
slate  ore. 


432  THE  MAEQUETTE   IR02f-BEAEING  DISTRICT. 

In  Uiin  section  the  conglomerates  of  the  Saginaw  range  do  not  differ 
from  those  of  the  Ishpeming  area.  In  the  varieties  which  are  most  mashed, 
and  therefore  most  closely  resemble  the  original  jasper,  the  presence  of 
small,  distinctly  rounded,  and  often  enlarged  fragmental  grains  of  quartz 
and  secondary  sericite  shows  the  recomposed  character  of  the  rock.  It 
therefore  appears  that  while  nearly  all  of  the  detritus  came  from  the  sub- 
jacent Negaunee  formation,  the  sea  brought  in  fragmental  material  from  a 
more  distant  source.  In  the  fine-grained  quartz-conglomerates  and  the 
coarser  quartzites  much  chert  and  jasper  are  seen,  and  a  large  amount  of 
secondary  iron  oxide  is  in  the  matrix.  Certain  nonferruginous  qnartzites 
are  strongly  feldspatliic.  These  grains  and  those  of  the  quartz  are  of  large 
size,  some  of  them  almost  pebble-like.  It  therefore  appears  that  in  the 
shallow  sea  of  this  time,  after  the  Negaunee  formation  was  buried  to  a 
depth  of  a  few  hundred  feet,  most  of  the  detritus  came  from  the  gneis- 
soid  gi'anites  of  the  Archean.  The  slides  of  the  slate  ores  and  hematitic 
schists  of  the  Groodrich  mine  show  the  silky,  fibrous  appearance  of  the 
mashed  hematite  and  the  crystals  of  secondary  magnetite.  The  recom- 
posed character  of  the  ore  is  indicated  by  occasional  small  grains  of  plainly 
fragmental  quartz  and  by  flakes  of  secondary  sericite.  The  latter  mineral 
is  rarely,  if  ever,  present  in  the  ore  belonging  to  the  Negaunee  formation. 

Mount  Humboldt  area. — West  of  the  Fitcli  mine  there  are  no  known  exposures 
of  the  Ishpeming  formation  until  north  of  the  Mount  Humboldt  ridge,  in 
sec.  18,  T.  47  N.,  R.  28  W.  From  this  place  outcrops  of  the  formation 
are  found  at  frequent  intervals  to  some  distance  west  of  the  Barron  mine 
(Atlas  Sheets  XVI  and  XIX).  The  oval  Humboldt  ridge  is  therefore  sur- 
rounded, except  on  the  south,  by  concentric  layers  of  the  Goodrich  quartzite. 
The  lowest  horizon  of  the  quartzite  is  represented  by  a  quartz,  jasper, 
and  ore  conglomerate  similar  to  that  at  the  Goodrich  mine.  This  may  be 
seen  in  a  cut  on  and  adjacent  to  the  Republic  branch  of  the  Duluth,  South 
Shore  and  Atlantic  Railway,  and  at  the  various  open  pits.  Tliis  conglom- 
ei'ate,  in  common  with  that  at  the  Goodrich  and  at  Palmer,  differs  from  that 
of  the  Ishpeming  area  in  containing  more  material  derived  from  the 
Archean,  and  particularly  large  pebbles  of  white,  coarsely  crystalline  quartz. 
The  mashing  phenomena  spoken  of  as  occurring  at  the  Goodrich  are  even 


INTEEESTING   LOCALITIES   OF  ISHPEMING   FOEMATION.       433 

more  prominent  here.  At  the  Barron  mine  the  basal  recomposed  rock  so 
closely  resembles  the  original  jasper  that  it  is  difficult,  if  not  impossible, 
to  exactly  locate  the  place  at  which  the  Negaunee  formation  ends  and 
the  Goodrich  quartzite  begins.  The  two  have  been  mashed  into  apparent 
conformity,  and  at  the  contact  plane  is  the  ore  deposit  which  welds  them 
together;  but  upon  the  east  side  of  the  ore  deposits  it  is  plain  that  we 
have  the  Negaunee  jasper,  and  upon  the  west  side  the  Goodrich  qiiartzite. 
At  this  mine  and  at  the  west  end  of  the  Jackson  mine  are  the  two  places 
in  the  main  area  of  the  district  where  it  is  most  difficult  to  discriminate 
between  the  two  formations.  A  short  distance  from  the  Barron,  at  the  old 
Humboldt  mine,  north  of  Mount  Humboldt,  there  is  no  difficulty  in  making 
the  discrimination  between  the  formations,  as  the  mashing  was  less  severe. 
The  Humboldt  conglomerate  grades  up  into  a  coarse  graywacke  or  into 
a  feldspathic  quartzite.  At  one  or  two  of  the  open  pits  the  finer-grained 
detritus  has  been  mashed  into  a  perfect,  finely  laminated  mica-schist. 

In  thin  section  the  mashed  conglomerates  closely  resemble  the  mashed 
jasper  of  the  Negaunee  formation,  but  the  small,  distinctly  fragmental  grains 
of  quartz  of  larger  size  than  the  granules  of  jasper,  and  flakes  of  sericite, 
mark  the  difference.  The  finer-grained  conglomerate  has  become  a  fer- 
ruginous sericlte-schist.  The  large  clastic  grains  of  quartz  are  broken, 
elongated,  and  often  wholly  granulated.  Around  these  areas  the  mashed 
hematite  and  lustrous  sericite  wrap  in  the  usual  manner  in  such  rocks.  As 
these  conglomerates  pass  up  into  those  in  which  no  coarse  detritus  was 
present,  we  have  the  sericite-schists.  In  one  case  the  rock  consists  almost 
wholly  of  flakes  of  sericite  having  a  minutely  crenulated,  parallel  arrange- 
ment of  a  majority  of  the  blades.  There  are  present  a  few  larger  blades  of 
muscovite  which  are  arranged  transverse  to  the  others,  or  at  a  large  angle 
to  them.  These  are  doubtless  original  clastic  flakes.  The  quartzose  mica- 
schists  differ  from  the  pure  micaceous  rock  only  in  that  between  the  leaflets 
of  sericite  are  very  numerous  small  particles  of  quartz,  either  single  or 
clustered,  about  which  the  sericite  passes,  like  the  meshes  of  a  stretched  net. 
The  quartzite  close  to  Humboldt  has  largely  recrystallized,  and  approaches 
a  quartz-schist.  Many  of  the  original  fragmental  grains  have  been  granu- 
lated, and  the  new  quartz  which  has  developed  sometimes  approximates  in 
MON  xxviii L'8 


434  THE  MARQUETTE   IKOISr-BEARma  DISTRICT. 

coarseness  to  the  granulated  quartz,  and  between  the  grains  much  sericite 
and  chlorite  has  developed.  Where  the  fragmental  grains  were  large  their 
granulation  resulted  in  distinct  flattening.  These  granulated  areas  free 
from  the  mica  still  indicate  the  fragmental  character  of  the  rock.  At  the 
localities  southeast  of  Humboldt,  in  sec.  18,  where  there  are  a  number  of 
exjaosures  of  quartzite,  the  rock  is  sericitic,  less  crystalline  than  that  near 
Humboldt,  and  more  nearly  like  the  normal  Goodrich  quartzites. 

Champion  area. — 111  tlic  viciulty  of  Champion,  at  the  various  mining  pits, 
the  lowest  horizon  of  the  Ishpeming  formation  is  represented  by  the  granu- 
lar magnetic  ore  and  the  magnetitic  hematite-schist  of  the  mines  (Atlas 
Sheet  Xn).  The  impure  phases  of  the  ore  contain  jaspery  quartz  and 
green  chlorite.  These  grade  up  into  hematitic  and  magnetitic  quartzites 
and  quartz-schists.  These  latter  are  associated  with  or  overlain  by  biotitic 
graywackes,  biotite  -  slates,  or  biotite- schists,  many  of  which  are  garnet- 
iferous,  and  in  some  cases  are  griineritic.  Rarely  the  rock  approaches  in 
appearance  the  griinerite-magnetite-schist  as  developed  at  Bijiki  River,  as, 
for  instance,  southeast  of  Champion,  north  of  the  road,  a  short  distance 
east  of  the  west  quarter  post  of  sec.  4.  These  schists  in  places  are  acutely 
folded  in  a  minor  way.  On  the  road  just  east  of  the  Champion  mine  the 
schist  becomes  conglomeratic.  The  matrix  of  the  conglomerate  is  gamet- 
iferous  mica -schist.  It  holds  very  numerous  pebbles  and  bowlders  of 
many  varieties,  including  schistose  granite,  quartzite,  quartz -schists,  etc. 
Some  of  the  bowlder-like  areas  are  several  feet  in  diameter.  The  ques- 
tion arose  at  the  locality  as  to  whether  they  could  be  parts  of  folded  or 
broken  layers,  rather  than  true  bowlders,  but  this  seemed  hardly  possible. 
This  rock  is  one  of  the  most  crystalline  schist -conglomerates  of  the 
Marquette  district. 

In  places  the  mica-schist  is  almost  immediately  north  of  the  rocks  of 
the  Negaunee  formation.  The  presence  of  biotite  in  the  mica-schist,  the 
different  character  of  its  banding,  and  an  irregular, weathering,  due  to  cer- 
tain constituents  dissolving  out,  thus  giving  the  rock  a  ridgy  appearance, 
enable  one  to  discriminate  the  Ishpeming  rock  from  the  Negaunee.  In  the 
presence  of  the  mica-schists  and  schist-conglomerates,  and  in  the  absence 
of  any  considerable  quantity  of  pure  quartzite,  the  rocks  of  the  Ishpeming 


INTERESTING  LOCALITIES   OF  ISHPEMING   FORMATION.       435 

formatiou  at  Champion  are  different  from  those  at  any  other  locality.  It  i.s 
to  be  noted  that  just  nortli  of  the  Ishpeming  formation  in  this  vicinity  occur 
mingled  sedimentary  and  A^olcanic  rocks  of  the  Clarksburg  formation,  and 
further,  one  of  the  volcanic  centers  was  probably  just  east  of  Champion. 
It  is  doubtless  due  to  this  volcano  that  the  absence  of  pure  quartzites  is  to 
be  attributed.  The  peculiar  conglomerate  above  mentioned  may  perhaps 
be  really  volcanic  rather  than  sedimentary. 

In  thin  section  certain  of  the  impure  ores  have  a  background  consist- 
ing of  small  granules  of  quartz  between  which  are  leaflets  of  biotite, 
sericite,  and  chlorite,  and  in  which  are  very  numerous  crystals  and  clusters 
of  crystals  of  magnetite.  Where  the  quantity  of  the  micaceous  minerals 
is  small  and  the  granules  of  quartz  are  almost  wholly  derived  from  the 
Negaunee  jasper,  the  recomposed  rock  simulates  to  a  remarkable  degree 
the  original  formation.  Where  the  chlorite  becomes  very  abundant,  as 
it  sometimes  does,  the  background  is  predominantly  of  this  mineral,  but  it 
still  contains  clastic  grains  of  quartz  derived  from  the  Negaunee  jasper,  and 
the  whole  is  studded  with  crystals  of  magnetite.  There  are  also  present 
large  crystals  of  chloritoid.  As  usual,  the  micaceous  hematite  is  easily 
discriminated  from  the  brightly  reflecting,  secondary,  crystal-outlined  mag- 
netite. As  the  ore  body  at  the  Champion  mine  is  so  largely  magnetite,  it 
appears  that  the  secondary  replacement,  after  djmamic  action  had  ceased, 
was  the  most  important  process  in  the  production  of  the  ore  deposits. 

By  the  appearance  of  coarse-grained  fragmental  quartz  from  the 
Basement  Complex,  the  rocks  at  the  bottom  of  the  formation  grade  up 
into  biotitic,  sericitic,  and  chloritic  quartzites  or  quartz -schi.sts,  which 
contain  much  iron  oxide  as  a  secondary  impregnation.  In  the  more 
mashed  phases  the  quartz  grains  are  flattened  so  that  their  longer  axes 
are  in  a  common  direction.  Accompanying  the  flattening  is  granulation. 
Sericite  is  the  predominant  mica,  and  this  secondary  mineral  wraps  around 
the  particles  of  quartz  in  the  usual  mesh-like  fashion.  In  certain  of  the 
rocks  many  feldspars  are  seen,  and  these  are  partly  decomposed,  the  mica 
and  quartz  forming  from  them.  The  mica -slates  and  mica-schists  are 
in  all  respects  like  similar  rocks  of  the  Michigamme  formation,  described 
on  pages  449-450,  and  they  will  therefore  not  be  here  fully  considered. 


436  THE   MAEQUETTE   lEON-BEAEING  DISTRICT. 

The  different  jiliases  include  black  garnetiferous  mica-slates  and  coarse- 
grained chlorite-slates  and  biotite-slates.  The  finer-grained  varieties  are 
strongly  g-arnetiferous  and  andalusitic.  The  garnet  contains  few  inclu- 
sions, but  the  andalusite,  as  usual,  is  full  of  the  other  minerals  of  the  rock. 
The  griineritic  rocks  differ  from  the  biotite-slates  only  in  that  in  place  of 
part  of  the  mica  griinerlte  has  developed.     Garnet  is  also  abundant. 

Lake  Michigamme  area. — West  of  Champiou,  soutli  of  thc  ceutcr  of  scc.  36, 
and  at  several  places  near  the  water's  edge  on  the  south  arm  of  Lake  Michi- 
gamme, occur  griineritic  schists  (Atlas  Sheets  VI,  VIII,  and  IX).  In  sec.  36 
the  rocks  are  nearly  pure  griinerite-magnetite-schists,  very  closely  resem- 
bling those  of  the  Bijiki  formation  at  Michigamme.  At  the  remaining  places 
to  the  southeast  the  schists  are  biotitic  or  are  interlaminated  with  bands 
of  biotite-schist.  In  thin  section  the  nearly  pure  griineritic  schists  do  not 
differ  from  those  of  Michigamme.  The  quartzose  background  is  finely 
crystalline,  but  in  it  are  seen  occasional  lai'ger  fragmental  gi-ains  of  quartz. 
With  the  griinerite  is  a  certain  amount  of  pleochroic  amphibole,  and  the 
usual  intergrowths  of  the  two  occur.  In  certain  slides  the  chloritic 
decomposition  of  the  griinerite  is  seen.  The  rocks  are  usually  garnetif- 
erous. As  the  rocks  become  less  pure,  biotite  is  found  with  the  griinerite 
between  the  particles  of  quartz.  In  an  intermediate  variety  the  biotite  and 
griinerite  are  about  equally  abundant.  By  a  lessening  of  the  amount  of  grii- 
nerite and  an  increase  of  the  chlorite  and  biotite  the  rocks  pass  into  the 
ordinary  mica-slates  and  mica-schists  of  the  Michigamme  formation. 

Republic  area. — At  the  soutli  eud  of  tlie  Republic  trough  there  are  numer- 
ous large  exposures  of  the  Ishpeming  formation  (Atlas  Sheet  XI).  The 
predominant  variety  is  white  quartzite.  This  passes  downward  into  a  con- 
glomerate at  the  base  of  the  formation,  and  it  grades  upward  into  the  mica- 
schist  of  the  Michigamme  formation.  Beginning  with  the  basal  members 
of  the  formation  at  the  southwest  angle  of  the  Republic  horseshoe,  there  is 
the  usual  recomposed  specular  and  magnetic  ore,  often  "  micaceous,"  which 
bears  clastic  grains  of  quartz  and  complex  fragments  from  the  Negaunee 
formation.  At  the  same  horizon  or  above  this  are  magnificent  exposures 
of  conglomerate.  The  predominant  pebbles  and  bowlders  are  from  the 
Negaunee  formation,  but  with  these  are  found  bowlders  of  quartz  as  large 


INTERESTING   LOCALITIES   OF  ISHPEMING  FOIIMATION.       437 

as  2  feet  in  diameter.  Also,  there  are  j^reseiit  a  few  pebbles  which  appear 
to  be  dei-ived  from  the  giieissoid  granite  of  the  Basement  Complex.  At  the 
exposure  nearest  the  jasper  is  seen  a  distinct  cleavage  parallel  to  the  side 
of  the  Republic  tongue,  while  the  pebbles  and  bowlders  are  concentrated 
into  bands  which  cut  across  this  cleavage,  although  the  longer  diameters  of 
the  flattened  pebbles  are  in  the  plane  of  cleavage.  In  all  probability  these 
pebbles  were  deposited  with  their  longer  axes  parallel  to  the  bedding. 
They  have,  therefore,  been  rotated  to  their  present  position,  or  have  been 
flattened  by  pressure,  or  Ijoth  have  occurred.  A  close  examination  of  the 
jasper  below  shows  that  rotation  has  probably  taken  place  to  some  extent 
Here  the  folding  was  so  sliarp  as  to  bend  the  once  regular  belts  of  jasper 
into  zigzag  bands,  more  nearly  parallel  to  the  cleavage  than  to  their 
original  bedding.  We  therefore  conclude  that  the  pebbles  of  the  con- 
glomerate have  been  revolved  as  well  as  flattened.  The  quartzite  and 
conglomerate  are  also  found  at  various  places  from  the  Republic  mine  to  the 
village  of  Republic,  along  the  west  side  of  Republic  Mountain.  Wherever 
the  contact  is  found  between  the  Ishpeming  and  Negaunee  formations  .the 
latter  is  eroded,  the  former  is  a  conglomerate  bearing  numerous  fragments 
of  the  latter,  and  there  is  a  slight  discordance  between  the  two  formations. 
At  one  place  on  the  east  side  of  the  trough,  as  a  result  of  faulting,  the  contact 
is  a  double  one,  a  quartzite  tongue  appearing  within  the  jasper.  The  nature 
of  the  contact  between  the  two  formations  and  the  origin  of  this  tongue 
are  fully  discussed  by  H.  L.  Smyth  in  another  place.  (See  pp.  542-,547.) 
The  conglomerate  grades  up  into  a  gray  quartzite  or  quartz-schist. 
This  is  in  some  places  sufficiently  coarse  grained  to  show  distinctly  the 
fragmental  character,  but  at  certain  places  it  becomes  novaculitic  ni  appear- 
ance, being  apparently  composed  mainly  of  the  individual  grains  derived 
from  the  Negaunee  jasper.  The  quartzite  is  more  or  less  impregnated 
with  magnetite.  The  quantity  of  this  mineral  lessens,  upon  the  Avhole,  in 
the  higher  horizons.  Also,  in  the  quartzite  at  one  place  is  a  considerable 
quantity  of  epidote,  which  in  some  specimens  is  sufficiently  abundant  to 
give  a  distinct  greenish  tint  to  the  rock.  The  quartzite  in  its  upper  part 
passes  into  a  fine-grained,  micaceous  quartz-schist,  and  this  into  the  mica- 
schist  of  the  Michigamme  formation. 


438  THE   MAKQUETTE   IRON-BEARING  DISTRICT. 

In  thin  section  the  vecomposed  ore  has  a  hematite  and  magnetite  back- 
ground. In  this  are  included  many  flakes  of  muscovite,  broken  grains  of 
coarse  quartz,  and  simple  and  complex  particles  from  the  jasper.  The 
matrix  of  the  less  ferruginous  conglomerate  is  essentially  the  same  as  the 
quartzites,  except  that  a  considerable  quantity  of  feldspar,  including  both 
orthoclase  and  plagioclase,  is  contained  in  it.  At  some  places  this  feldspar 
has  partly  altered  into  quai-tz,  muscovite,  and  biotite,  many  of  the  roundish 
grains  being  now  an  interlocking  mass  of  these  minerals  and  the  resid- 
ual feldspar.  The  quartzites  show  the  effects  of  extreme  pressure;  all  of 
the  grains  show  undulatory  extinction  or  fracturing,  or  even  grainilation. 
While  many  of  the  grains  have  a  general  roundish  appearance,  they  rarely 
show  the  original  cores,  and  are  minutely  angular  upon  their  exteriors. 
These  larger  grains  intricately  interlock,  and  in  part  finely  crystalline 
quartz  has  developed  between  them.  In  the  rocks  in  which  the  dynamic 
effects  are  least  the  coarse-grained  original  quartz  is  discriminated  from 
the  finer-grained  secondary  quartz,  but  in  the  more  mashed  phases  of  the 
rock  the  two  are  similar  and  the  quartz  grains  interlock  in  an  intricate 
way,  giving  no  positive  evidence  of  their  original  fragmental  character. 
Between  the  grains  considerable  muscovite  has  developed,  making  the  rocks 
sericitic  quartzites,  and  where  most  mashed  sericitic  quartz -schists.  At 
one  place  on  the  east  side  of  the  trough,  in  a  coarsely  crystalline  quartzite, 
epidote  and  amphibole  are  so  abundant  as  to  become  chief  constituents. 
The  quartzite  is  coarsely  crystalline,  and  the  roundish  forms  of  the  quartz 
grains  still  show  the  fragmental  origin  of  the  rock.  The  epidote  occurs  in 
small  and  large,  distinctly  pleochroic,  irregular  grains,  some  of  which  show 
a  tendency  toward  crystal  outlines.  In  the  upper  part  of  the  Ishpeming 
formation  the  quartzite  is  fine-grained,  and  here  the  background  consists  of 
small  granules  of  quartz,  similar  in  appearance  to  those  making  up  the 
Negaunee  jasper,  and  this  is  doubtless  their  source.  The  rock,  however, 
differs  from  the  Negaunee  jasper  in  that,  everywhere  between  the  grains, 
flakes  of  biotite,  chlorite,  and  sericite  have  developed.  In  some  sections 
the  biotite  is  predominant,  in  others  the  sericite.  In  the  conglomerates, 
quartzites,  and  quai-tz-schists  alike,  numerous  crystals  of  magnetite  are  seen, 
which  are  included  indiscriminately  in  all  of  the  other  minerals  present. 


INTERESTING  LOCALITIES   OF  ISIIPEMING  FORMATION.       439 

Kioman  area. — To  the  uortliwest  of  Republic,  along  tlie  east  side  of  the 
Repvibhc  troug-h,  at  the  Khiman  luhie  the  contact  between  the  Negaunee 
formation  and  the  conghimerate  of  the  Ishpemiug  formation  is  beautifully 
exposed  (Atlas  Sheet  XI).  The  Negaunee  formation  has  Its  typical  diar- 
acters.  Resting  upon  this  is  recomposed  ore.  The  recomposed  material 
near  the  base  of  the  Ishpeming  formation  very  closely  simulates  the  Negau- 
nee jasper  in  appearance.  However,  minute  roundish  granules  of  quartz 
and  pebbles  of  jasper  show  the  character  of  the  rock  when  closely  examined. 
At  one  place,  where  the  ore  welds  the  two  formations  together,  it  is  impossible 
to  state  where  the  Ishpeming  formation  ends  and  the  Negaunee  formation 
begins.  The  ore  taken  from  the  mine  seems  to  occur  about  equally  in  the 
two  formations.  As  usual,  in  the  ore  two  kinds  of  iron  oxide  are  dist-rimi- 
nated,  the  original,  detrital,  mashed  hematite  and  the  secondary  crystalline 
magnetite.  In  cross  fracture  the  latter  is  more  evident ;  in  specimens  paral- 
lel to  the  lamination  the  micaceous  hematite  appears  to  be  predominant, 
presenting  its  usual  lustrous,  slickensided  surfaces.  The  heavily  ferruginous 
material  passes  up  into  a  quartzite  containing  numerous  fragments  of  ore 
and  jasper.  Layers  of  conglomerate  and  of  quartzite,  comparatively  free 
from  jasper  pebbles,  are  interstratifiied.  This  rock  passes  up  into  a  fine- 
grained gray  quartzite,  like  that  at  Republic.  In  thin  section  the  rocks  do 
not  differ  from  the  similar  rocks  at  Republic,  except  that  the  dynamic 
eflfests  are  less  severe.  In  the  quartzose  ore  the  coarse,  well-rounded,  little- 
distorted  grains  of  fragmental  quartz  derived  from  the  Archean  show  dis- 
tinct enlargements.  All,  however,  show  undulatory  extinction,  and  many 
are  fractured. 

Northern  Republic  and  Western  troughs. At  a  nUmber  of   plaCeS  betWCCU  tllC   Klo- 

man  mine  and  the  northwest  end  of  the  Republic  trough  there  are  exposures 
of  the  Ishpeming  formation.  The  majonty  of  these  are  at  the  mines  along 
the  trough,  but  at  various  other  places  important  outcrops  occur.  The  more 
important  localities  are  as  follows:  the  S.  ^  sec.  1,  T.  46  N.,  R.  30  W.,  the 
Metropolitan  mine,  the  Chippewa  mine,  the  Riverside  mine,  the  Standard 
mine,  the  Cannon  mine,  the  Erie  mine,  the  Magnetic  mine,  and  various 
places  to  the  east  and  west  in  sec.  20,  T.  47  N.,  R.  30  W.  In  the  Western 
tongue  there  are  exposures  of  the  Ishpeming  formation  in  sec.  30,  T.  47  N., 


440  THE  MARQUETTE   IRON-BEARING  DISTRICT. 

R  30  W.,  and  in  sec.  18,  T.  46  N.,  R.  30  W.  (Atlas  Sheets  VII  and  X.) 
To  describe  in  detail  each  of  these  localities  would  require  too  much  space. 
They  are  therefore  treated  together. 

At  the  various  mines  the  basal  horizon  is  a  coarse  magnetic  ore,  asso- 
ciated with  considerable  chlorite,  and  in  some  places  with  pyrite.  As  this 
ore  becomes  lean  it  contains  quartzose  bands,  which  show  large  clear  grains 
or  pebbles  of  quartz  that  appear  to  be  fragmental.  These  white  quartz 
bands  never  very  closely  simulate  the  red  or  white  jasper  of  the  Negaunee 
formation,  and  there  is  usually  no  sharp  differentiation  between  the  ferru- 
ginous and  nonfeiTUginous  bands,  as  is  the  case  with  the  jasper.  They 
are  therefore  discriminated  in  hand  specimen  from  the  true  jaspilite  of  the 
Negaunee  formation.  If  there  were  conglomerates  deposited  at  the  base  of 
the  formation,  at  most  places  these  have  been  wholly  destroyed  by  the 
dynamic  action.  In  some  cases  the  bands  of  quartz  have  a  lenticular 
appearance,  as  if  they  were  greatly  flattened  quartz  pebbles. 

At  two  places,  where  erosion  has  cut  away  the  Lower  Marquette  series, 
the  rocks  of  the  Ishpeming  formation  are  found  close  to  the  granite.  The 
first  of  these  localities  is  southeast  of  the  Erie  mine,  in  the  NE.  ^  sec.  28, 
and  the  latter  northwest  of  the  same  mine,  in  the  SE.  \  sec.  20.  At  the 
first  of  these  localities  the  rock  is  a  coarse-grained,  foliated,  micaceous 
quartz-schist,  which  at  one  place  contains  distinct  quartz  pebbles.  At  the 
second  locality  the  rock  is  of  a  similar  appearance,  and  it  contains  distinct 
pebbles  of  white  quartz  and  of  jasper.  This  rock  is  associated  with  a 
massive,  greenish-gray,  hornblendic  quartzite.  These  coarse  rocks  pass  up 
into  fine-grained,  micaceous  quartz-schists. 

The  heavily  ferriferous  rocks  of  the  mines  are  closely  associated  with 
or  grade  into  muscovitic  quartz-schists;  these  grade  into  completely  crys- 
talline, thoroughly  foliated  mica -schists,  like  those  of  the  Michigamme 
formation. 

Associated  with  the  mica-schists,  griineritic  and  magnetitic  schists  are 
found.  These  may  be  particularly  well  observed  west  of  the  Kloman 
mine,  adjacent  to  and  south  of  the  road.  This  griinerite-magnetite-schist 
contrasts  with  that  of  the  Negaunee  formation  in  having  a  rough  appear- 
ance upon  the  weathered  surface,  and  in  having  a  peculiar  toughness  which 


INTEUKSTIXCi    LOCALITIES   OF  ISHPEMING   FORMATION.       441 

prevents  it  from  being  cleaved  parallel  to  the  banding,  while  the  similar 
rock  of  the  Negaunee  formation  has  a  regular  lamination,  is  brittle,  and 
parts  readily  along  the  lamination;  also,  the  layers  of  the  former  are 
thicker,  and  many  of  them  have  more  of  the  appearance  of  a  crystalline 
schist  than  the  similar  rock  of  the  Negaunee  formation.  The  schist  is 
folded  in  a  most  complicated  fashion.  On  the  liorizontal  exposure  the 
layers  are  seen  to  turn  back  upon  themselves  in  repeated  sharp  v  folds. 

Upon  the  whole,  the  rocks  of  the  Ishpeming  formation  in  this  area  are 
the  most  extremely  metamorphosed  of  all  in  the  district.  While  occasional 
conglomeratic  schists  have  been  discovered  and  the  coarser  quartz-schists 
are  not  completely  crystalline,  the  finer-grained  rocks  have  been  changed 
to  completely  crystalline  schists.  The  evidence  of  imconformity  between 
the  Negaunee  and  Ishpeming  formations,  which  is  so  marked  at  Republic 
and  other  localities  in  the  district,  is  wholly  obliterated.  From  the  proxim- 
ity to  these  places  one  can  not  doubt  that  the  two  are  here  unconformable, 
and  that  there  was  an  irregular  erosion  contact  between  them;  but  if  this 
be  true,  the  mashing  was  so  severe  as  to  wholly  destroy  the  evidence  of  this. 
The  schistosity  of  the  Ishpeming  formation  and  the  bedding  of  the  Negaunee 
formation  are  in  apparent  conformity.  Where  the  Ishpeming  formation  is 
in  contact  with  the  Archean,  somewhat  similar  relations  obtain.  The  basal 
horizon  of  the  Ishpeming  formation  is  a  coarse  micaceous  quartz-schist.  It 
is  not  always  possible  to  tell  exactly  where  this  recomposed  rock  begins 
and  the  mashed  granite  ends.  At  two  localities  only,  one  between  the 
Erie  and  Magnetic  mines  and  the  other  southeast  of  the  former,  does  this 
basal  rock  distinctly  show  by  pebbles  its  fragmental  character. 

Under  the  microscope  a  few  of  the  slides  of  the  recomposed  materials 
at  the  mines  so  closely  resemble  the  original  Negaunee  jasper  that  the  two 
could  not  be  discriminated.  However,  there  is  usually  present  muscovite 
in  small  or  large  flakes,  and  biotite  and  chlorite  are  found,  in  some  places 
abundantly.  In  certain  cases  lenticular  areas  of  jasper  suggest  that  the  len- 
ticules  represent  mashed  pebbles.  While  the  quartz  of  the  siliceous  layers 
varies  considerably  in  coarseness,  there  is  usually  no  such  range  in  size  as 
at  Republic,  where  each  of  the  coarse  grains  derived  from  the  granite  can 
be  readily  discriminated  from  the  small  grains  derived  from  tlie  Negaunee 


442  THE   MAEQUETTE  lEOX-BEAKING  DISTKIGT. 

jasper.  If  coarse  grains  were  originally  present  tliey  have  been  gran- 
ulated by  the  mashing.  Toward  the  southeast  end  of  the  area,  near  the 
Kloinan  and  Republic  mines,  the  mashing  was  not  so  severe,  and  here  two 
classes  of  quartz  are  discriminated.  Although  no  grains  show  cores  and 
enlaro-ements,  in  some  cases  the  coarser  grains  are  cemented  by  a  fine 
mosaic,  as  in  ordinary  quartzites;  in  others  the  coarse  and  fine  grains  are 
in  alternate  layers;  while  in  a  third  case  they  are  indiscriminately  mingled. 

Where  the  Goodrich  quartzite  rests  upon  the  Archean,  the  basal  rock 
is  entirely  diff'erent  from  the  above,  being  in  two  places  a  conglomerate 
composed  chiefly  of  material  derived  from  the  Basement  Complex  The 
pebbles  of  the  conglomerate  were  simple  and  complex  fragments  of  quartz, 
large  grains  of  feldspar,  and  granitic  pebbles  composed  of  both.  These 
have  been  so  intensely  mashed  that  they  are  usually  in  elongated  oval  or 
ribbon-like  areas.  Much  of  the  coarsely  crystalline  quartz  is  granulated, 
so  that  the  quartzose  pebbles  consist  of  finely  granular  interlocking  quartz- 
The  feldspar  fragments  are  usually  much  shattered.  Along  the  cracks 
quartz  aud  muscovite  have  largely  developed.  In  one  case  the  stresses  have 
produced  an  unusually  strongly  marked  fissility  in  two  directions,  which  in 
section  resembles  the  double  cleavage  of  calcite.  Where  the  decomposition 
went  far  enough  the  areas  once  occupied  by  feldspar  consist  of  interlock- 
ing crystalline  masses  of  quartz,  muscovite,  and  residual  feldspar,  In  the 
pebbles  which  consisted  of  quartz  and  feldspar  together  the  effects  upon 
the  latter  mineral  were  the  same  as  in  the  pebbles  which  consisted  of  feld- 
spar alone.  The  matrix  of  the  conglomerate  is  a  muscovitic  and  feldspathic 
quartz-schist.  The  feldspar  comprises  both  orthoclase  and  plagioclase,  the 
latter  including  much  microcline.  In  some  of  the  sections  this  feldspar  is 
so  abundant  as  to  be  a  principal  constituent.  In  a  number  of  slides  the 
matri;t  is  completely  crystalline,  thus  becoming  mica-schist  or  mica-gneiss. 
If  it  were  not  for  the  conglomeratic  character  of  these  rocks  their  sedimen- 
tary origin  could  not  be  asserted  from  evidence  shown  by  the  thin  section. 
As  accessories  in  the  conglomeratic  schist  and  gneiss,  magnetite,  biotite, 
chlorite,  and  epidote  are  found. 

By  a  lessening  of  the  amount  of  magnetite  the  recomposed  ore-bearing 
rocks  at  the  mines  pass  up  into  muscovitic  quartz-schists.     Also  by  the 


INTERESTING   LOCALITIES   OF  ISHPEMING   FORMATION.       443 

disappearance  of  the  larger  fragments  the  conglomerates  resting  upon  the 
Archean  pass  up  into  like  rocks.  The  dynamic  forces  Avere  not  severe 
enough  to  entirely  destroy  the  large  original  rounded  fragmental  grains  of 
quartz.  These  are  represented  by  roundish  or  oval  areas,  which  may  show 
only  undulatory  extinction  or  fracturing,  or  may  be  oblong  granulated  areas- 
But  the  latter  differ  from  the  finer-grained  background  in  the  absence  of 
muscovite.  However,  the  quartz  grains  never  show  cores  with  enlarge- 
ments. They  completely  interlock.  Their  fragmental  character  is  therefore 
indicated  mainly  by  the  general  shapes  of  the  complex  areas.  In  the  finer- 
grained  varieties  of  the  quartz-schist  no  distinction  is  to  be  made  out  between 
detrital  and  nondetrital  grains.  The  rock  is  a  completely  crystalline,  mus- 
covitic  quartz-schist.  As  in  the  case  of  the  conglomerates,  a  greater  or  less 
amount  of  feldspar  is  present 

By  an  increase  in  the  amount  of  muscovite  the  quartz-schists  pass  into 
typical  crystalline  muscovite-schists.  In  many  of  the  rocks  biotite  also 
is  a  principal  constituent,  and  they  become  muscovite-biotite-schists.  By 
a  further  replacement  of  the  muscovite  by  biotite  the  rocks  pass  into 
biotite-schists.  In  some  cases  the  folia  of  muscovite  are  arranged  trans- 
versely to  those  of  biotite,  as  though  one  of  these  minerals  had  developed 
as  a  consequence  of  one  dynamic  movement  and  the  other  of  a  later  one. 
In  a  few  instances  the  amount  of  feldspar  in  the  background  is  so  great  as 
to  be  a  pnncipal  constituent,  and  the  rocks  are  muscovite-biotite-gneisses. 
The  feldspar  includes  orthoclase  and  plagioclase,  the  major  portion  of 
the  latter  being  microcline.  The  completely  crystalline  gneiss  occurs  in 
association  with  an  intrusive  greenstone. 

A  portion  of  the  feldspar  in  the  background  of  the  conglomerates, 
quartz-schists,  and  mica-schists  appears  to  be  detrital,  but  a  large  part  is 
apparently  a  new  development,  as  much  of  it  is  perfectl}^  clear,  showing  no 
decomposition,  and  including  magnetite  and  other  minerals.  In  the  quartz- 
schists  and  mica-schists,  magnetite,  epidote,  and  zoisite  are  often  abundant 
accessories,  and  in  the  mica-schists  garnet  also  is  plentiful.  The  mica-schists 
of  the  Ishpeming  formation  are  the  same  in  their  general  characters  as  those 
in  the  Michigamme  formation  (pp.  447,  449-450),  and  they  will  therefore 
not  be  here  described  in  detail. 


444  THE   MARQUETTE  lEOi^  BEARING  DISTRICT. 

At  the  Magnetic  mine,  and  east  and  west  of  it,  a  coarse-grained  gray 
quartzite  is  associated  with  the  ordinary  phases  of  quartzites.  In  this  quartz- 
ite,  hornblende  and  epidote  occur  as  chief  constituents  between  the  grains. 
These  rocks  are  in  all  respects  like  those  which  have  been  described  in  the 
quartzite  tongue  at  Republic.  In  some  of  the  micaceous  schists  of  the  same 
area  a  large  amount  of  gai-net,  chlorite,  and  chloritoid  is  present. 

By  the  replacement  of  mica  by  griinerite  and  green  hornblende  in  the 
mica-schists  these  rocks  grade  into  the  griineritic  rocks  which  have  been 
described  under  the  name  Bijiki  schist.  The  transition  varieties  and  the 
griineritic  rocks  which  are  free  from  mica  are  not  different  from  thpse  which 
occur  northwest  of  Champion  (pp.  434,  436).     As  usual,  garnet  is  abundant. 

SECTION  II.-THE  MICHIGAMME  FORMATION. 

By  C.  K.  Van  Hise. 

The  name  Michlgamme  is  given  to  the  upper  slate  and  mica-schist 
because  on  the  islands  of  Lake  Michigamme  and  on  the  mainland  adjacent 
to  the  shore  occur  extensive  exposures  of  this  formation. 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

The  Michigamme  formation  is  mainly  in  a  single  great  area  (Atlas 
Sheet  IV).  Beginning  west  of  Ishpeming,  it  sweeps  westward  as  a  broad 
belt  to  near  Humboldt;  here  it  becomes  somewhat  contracted,  and  east  of 
Lake  Michigamme  it  is  divided  into  two  belts,  a  narrow  northern  belt 
between  two  zones  of  Bijiki  schist,  and  a  broader  southern  belt  which 
includes  the  greater  part  of  Michigamme  Lake  and  the  country  to  the 
westward.  This  belt  widens  out  over  a  broad  area  and  occupies  a  great 
expanse  of  country  in  the  large  area  of  Algonkian  rocks  at  the  west  and 
south  part  of  the  district  covered  by  the  present  report.  From  this  broad 
area  two  arms  project,  forming  the  centers  of  the  Republic  and  Western 
tongues.  At  and  east  and  west  of  Humboldt  is  a  southern  lenticular  area 
about  6  miles  long. 

As  this  formation  was  originally  a  shale  or  grit,  where  it  has  not  been 
much  altered  the  exposures  are  not  prominent,  and  the  area  as  a  whole  is 
one  of  rather  feeble  relief,  occupying  lowlands  between  the  ridgy  country 


EXPOSURES  OF  THE  MICHIGAMME  FORMATION.  445 

of  the  formations  both  to  the  north  and  south.  This  is  particuhirly  the 
case  from  the  eastern  extremity  of  the  area  to  Lake  Michigamme.  At 
Lake  Michigamme  and  to  the  south  and  west  the  formation  was  mucli  more 
metamorphosed,  becoming  a  mica-slate,  a  mica-schist,  or  a  mica-gneiss,  and 
here,  on  account  of  the  increased  resistant  power,  the  exposures  are 
numerous  and  conspicuous,  especially  on  the  southern  side  of  the  west  arm 
of  the  lake  and  upon  the  islands  to  the  east.  In  the  Republic  tongue  the 
schist,  being  softer  than  the  lower  formations,  is  followed  by  the  Michigamme 
River. 

FOLDING. 

Broadly  considered,  for  most  of  the  area  the  Michigamme  formation  is 
in  a  great  syncline.  This  syncline  is,  however,  very  complex,  and  there 
are  many  subordinate  anticlines  and  synclines.  East  of  Lake  Michigamme 
one  of  these  anticHnes  is  of  sufficient  importance  to  bring  the  Bijiki  schist 
to  the  surface,  and  thus  to  divide  the  Michigamme  formation  into  two 
syncHnes,  the  southern  one  of  which  is  the  more  important.  Another 
probable  anticline  is  indicated  by  the  iron -ore  pits  in  sees.  29  and  35, 
T.  48  N.,  R.  29  W.,  as  it  is  thought  that  the  ferruginous  horizon  belongs  near 
the  base  of  the  formation  (Atlas  Sheets  XII  and  XV).  The  Republic  and 
Western  tongues  are  both  isoclinal  synclines.  If  the  exposures  are  exam- 
ined in  detail,  it  is  found  that  many  of  the  secondary  antichnes  and 
synclines  have  upon  them  tertiary  folds,  and  upon  these  are  folds  of  the 
fourth  order,  and  so  on  to  microscopic  plications;  so  that  in  many  places 
the  rocks  are  minutely  implicated.  This  is  particularly  well  seen  in  the 
schist  at  Lake  Michigamme. 

petroctRaphical  character. 

Macroscopicai. — Thc  Tocks  of  the  formatiou  comprise  two  main  varieties — 
little-altered  slates  and  graywackes,  and  mica-schists  and  mica-gneisses. 
Each  of  these  comprises  both  ferruginous  and  nonferragiuous  kinds.  The 
first  class  occurs  chiefly  in  the  area  east  of  Lake  Michigamme  and  the  sec- 
ond in  the  Lake  Michigamme  area,  although  representatives  of  the  first  are 
found  along  the  northern  side  of  the  Michigamme  formation  to  the  western 
limit  of  the  district  considered.     The  mica -schists  are  also  found  along 


446  THE  MAKQUETTE   IKOis'-BEARINa   DISTKICT. 

the  southern  part  of  the  belt,  several  miles  east  of  Clarksburg.  It  can  not 
be  said  that  these  divisions  are  in  any  way  stratigraphical,  unless  it  be  true 
of  the  ferruginous  phases,  which  appear  to  occupy  a  somewhat  persistent 
horizon;  but  these  latter  rocks  are  not  so  well  defined  that  they  can  be 
mapped  as  a  separate  formation. 

The  slates  and  qraywackes  differ  from  each  other  chiefly  in  coarseness 
of  grain,  the  two  often  being  interlaminated  in  the  same  exposure  or  ridge. 
There  are  all  gradations,  from  the  aphanitic,  black  shales  or  slates  to  a 
graywacke  so  coarse  as  to  approach  a  quartzite,  or,  in  one  case,  a  conglom- 
erate. The  rocks  vary  in  color  from  gray  to  black.  Where  they  are  fine- 
gi-ained  they  usually  have  a  well-developed  slaty  cleavage,  and  are  often 
carbonaceous,  ferruginous,  and  pyritiferous.  In  some  places  the  amount  of 
carbon  is  so  great  as  to  give  a  black  streak.  When  broken  apart  parallel 
to  the  cleavage  the  graphite  is  frequently  in  a  lustrous  form,  due  to  move- 
ments parallel  to  the  parting.  An  examination  of  two  specimens  (16671 
and  16678)  of  the  most  carbonaceous  rocks,  by  H.  M.  Stokes,  in  the  chem- 
ical laboratory  of  the  United  States  Geological  Survey,  showed  that  they 
contain  respectively  18.92  and  15.69  per  cent  of  carbon,  but  no  hydrocar- 
bons. A  portion  of  the  carbon  appears  to  be  in  the  form  of  anthracitic, 
coaly  substance,  but  much  of  it  has  been  transformed  to  graphite.  The 
pyrite  is  in  detached  crystals,  and  in  laminte  parallel  to  the  parting. 
The  least  altered  of  these  rocks  could  properly  be  called  shales  or  grits. 
In  places  where  they  are  more  altered  the  shales  pass  into  mica-slates,  and 
by  more  extreme  alteration  into  the  mica-schists. 

The  ferruginous  slates  and  graywackes  contain  much  iron.  In  the 
least-altered  phases  the  iron  compound  is  largely  siderite,  and  thus  the  rock 
.  is  a  sideritic  slate.  Rarely  the  siderite  becomes  the  predominating  constit- 
uent, and  in  this  case  the  rock  is  similar  to  the  sideritic  slates  of  the  Negaunee 
formation.  As  a  consequence  of  weathering  and  metasomatic  changes, 
ferruginous  slates,  ferruginous  cherts,  and  griinerite-magnetite-schists  have 
developed  from  these  sideritic  slates.  In  the  few  localities  where  the 
ferruginous  material  was  very  abundant  small  ore  bodies  also  have  formed. 
Such  are  known  at  three  places  north  of  Champion  and  at  one  south  of 
Spurr.     Pits  also  occur  in  the  south  halves  of  sees.  29  and  35,  T.  48  N., 


PETEOGRAPHICAL  CHARACTER  OF  MICHIGAMME  FORMATION.       447 

R  29  W.  (Atlas  Sheets  Xll  and  XV  )  These  ores  differ  from  the  soft  ores 
of  the  Negaunee  formation  in  that  the  iron  oxide  is  largely  limonite  and 
in  that  the  associated  rocks  contain  much  carbonaceous  and  graphitic 
material.  The  ferruginous  phases  are  particularly  prevalent  just  above 
the  Clarksburg  volcanics  and  the  Ishpeming  formation — i.  e.,  a  short 
distance  above  the  formations  which  are  immediately  subjacent  to  the 
Michigamme  formation.  All  of  the  pits  may  and  probably  do  belong  to 
the  same  horizon.  If  this  be  true,  the  central  belt  is  near  the  crest  of 
an  anticline  which  rises  high  enough  to  bring  this  low  ferruginous  horizon 
of  the  formation  to  the  surface.  The  foot-wall  of  the  ore  bodies  is,  so  far 
as  observed,  the  impervious  fragmental  Michigamme  slate.  The  ores  and 
pecuhar  associated  rocks  therefore  appear  to  be  in  bunches  or  in  lenses  in 
the  carbonaceous  slates,  strongly  suggesting  that  the  abundant  organic 
material  had  to  do  with  the  deposition  of  the  iron  compounds. 

The  ferruginous  and  nonferruginous  slates  and  graywackes,  by  an 
increase  in  metamorphism,  pass  into  the  mica-schists  and  mica-gneisses. 
At  Lake  Michigamme,  it  has  been  said,  mica-schist  is  abundantly  devel- 
oped in  its  typical  form,  This  mica-schist,  while  a  completely  crystalUne 
rock  having  well-developed  schistosity,  still  shows  in  places,  when  closely 
examined,  the  original  beddmg  and  an  alternation  of  coarse  and  fine 
material  such  as  occurs  in  the  slates  and  graywackes  to  the  east.  The 
schistosity  varies  from  parallel  to  perpendicular  to  the  bedding,  usually 
being  at  some  intermediate  angle  Where  the  schists  are  completely  crys- 
talline, garnet,  staurolite,  chloritoid,  and  andalusite   are  often  plentifully 

present. 

In  the  most  coarsely  crystalline  kind  the  rock  is  in  places  veined 
throughout  with  a  granitic-looking  mateiial,  and  feldspar  has  abundantly 
developed  within  the  rock,  forming  a  gneiss.  The  gneiss  is  pegmatized 
tlirough  and  through,  as  though  the  material,  either  as  a  magma  or  in  the 
form  of  a  water  solution,  had  penetrated  the  joints,  the  partings  parallel  to 
the  laminjE,  and  also  the  interspaces  between  the  constituent  particles,  and 
had  in  these  places  produced  quartz  and  feldspar.  A  close  examination 
shows  that  many  of  the  apparently  granitic  veins  are  but  the  coarser  beds 
strongly  pegmatized.     The  pegmatized  areas  grade  into  the  ordinary  mica- 


448  THE  MAEQCTETTE  IRON-BEARING   DISTRICT. 

schist.  It  is  concluded  that  the  pegmatization  was  not  the  result  of  an 
igneous  injection  from  an  extraneous  source,  particularly  as  there  are  no 
known  granite  intrusives  within  the  Upper  Marquette  series.  The  facts 
seem  rather  to  be  explained  by  water  action.  The  whole  rock  must  have 
been  penneated  by  hot  solutions,  from  which  the  new  minerals  separated 
in  the  interspaces  left  by  the  folding. 

If  this  explanation  be  coiTect,  the  rock  is  one  to  which  the  term  "meta- 
morphism"  is  applicable.  Why  the  rocks  of  this  part  of  the  formation  are 
so  thoroughly  metamorphosed  and  those  to  the  east  comparatively  little 
affected  has  not  been  certainly  determined.  The  beds  are  intensely  plicated. 
Such  plication  involves  a  large  amount  of  readjustment  of  the  layers  over 
one  another  and  within  the  layers  themselves — that  is,  the  mashing  was 
exceedingly  severe.  During  this  time  of  folding,  by  the  decomposition  of 
fragmental  feldspar  into  quartz  and  mica,  the  development  of  new  feldspars 
in  some  places,  and  the  granulation  of  the  coarser  crystalline  quartz,  the 
rock  changed  into  a  mica-schist  or  a  mica-gneiss. 

Microscopical. — The  sltttcs  aud  graywackes  consist  mainly  of  fragmental 
quartz  and  feldspar  set  in  a  clayey  and  micaceous  matrix.  Occasionally 
other  fragmental  constituents,  and  especially  mica,  are  found.  In  the  cases 
of  the  finer -grained  slates  the  clayey  matrix  is  predominant.  In  the 
coarser-grained  graywackes  the  plainly  fragmental  material  is  predominant. 
In  the  latter  we  often  have  closely  fitting  grains  of  quartz,  some  of  them 
well  rounded  and  enlarged,  with  a  few  of  feldspar,  set  in  a  sparse  matrix. 
This  rock  approaches  a  quartzite.  The  fragmental  constituents  generally 
show  pressure  effects,  the  larger  grains  being  broken  into  two  or  more 
fragments,  or  cut  by  fine  cracks,  sometimes  in  a  rectangular  manner, 
which  cause  undulatory  extinction.  The  fragmental  grains  of  feldspar 
have  largely  decomposed,  and  quartz,  biotite,  and  chlorite  have  developed 
from  them.  In  the  clayey  background  there  have  developed  many  minute 
flakes  of  biotite,  sericite,  leaflets  of  chlorite,  and  sometimes  needles  of 
actinolite.  These  usually  do  not  have  a  parallel  arrangement.  More  fre- 
quently than  not  there  is  also  present  a  greater  or  less  quantity  of  ferrite. 
Sometimes  crystals  of  tourmaline  also  occur. 


PETEOGRAPHICAL  CHARACTER  OF  MICIIIGAMME  FORMATION.  449 

In  proportion  as  the  feldspar  is  decomposed  and  the  quartz  is  granu- 
lated, the  rocks  approach  the  mica-schists,  an  intermediate  phase  being 
represented  by  the  mica-slates.  These  still  show  evidence  of  their  frag- 
mental  origin,  occasional  fragmental  grains  of  quartz  being  seen,  some  of 
which  are  enlarged.  Many  of  these  fragmental  grains  are  easily  separable 
from  the  newly  developed  quartz,  showing  as  they  do  undulatory  extinction 
or  fracturing.  The  quartz  grains  show  an  imperfect  arrangement,  with  their 
longer  diameters  in  a  common  direction.  The  folia  of  biotite  also  have  a 
parallel  an'angement.  In  a  further  stage  much  of  the  quartz  has  been 
granulated,  and  the  feldspar  is  largely  replaced  by  secondary  mica  and 
quartz. 

These  mica-slates  on  the  one  hand  grade  into  the  ordinary  slates  and 
gray  wackes  step  by  step,  and  on  the  other  hand,  by  greater  alteration,  they 
pass  into  mica-schists.  Where  the  process  of  metaraorphism  is  complete, 
the  fragmental  quartz  grains  are  wholly  granulated  by  the  mashing,  which 
has  kneaded  the  rock  throughout.  In  many  slides  each  folium  moved 
differentially  in  reference  to  those  above  and  below  it.  The  fragmental 
feldspar  is  wholly  changed  into  quartz,  mica,  and  chlorite.  The  folia  of 
new  mica  developed  with  their  longer  axes  in  a  common  direction.  In  pro- 
portion as  the  deformation  is  greater,  sericite  and  muscovite  become  prom- 
inent with  the  biotite.  Thus  in  place  of  the  fragmental  rock  a  completely 
crystalline  mica-schist  is  produced. 

The  details  of  the  processes  of  development  of  these  schists  will  not 
be  here  described,  but  they  are  similar  to  those  given  for  the  development 
of  the  mica-schists  in  the  Penokee  series.^  There  has,  however,  been  the 
difference  explained  above,  that  mashing  has  played  a  much  more  impoi'tant 
part  in  the  case  of  the  mica-schists  of  the  Marquette  district.  As  a  con- 
sequence, some  of  the  schists  are  strongly  foliated.  In  the  crystalline 
schists  a  large  amount  of  garnet,  staurolite,  andalusite,  and  chloritoid  has 
developed.  These  minerals  include  large  quantities  of  the  prior  quartz. 
They  show  no  evidence  of  strain,  and  they  are  believed  to  have  developed 

'  The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  R.  Van 
Hise:  Mon.  U.  S.  Geol.  Survey,  Vol.  XIX,  1892,  pp.  332-343. 
MON  XXVIII ^29 


450  THE   MAKQUETTE   IKON  BEARING  DISTRICT. 

in  the  quiescent  stage  after  dynamic  action  had  ceased,  but  while  the  heat 
still  produced  hot  solutions  which  bore  abundant  mineral  material. 

By  the  formation  of  the  secondary  feldspar,  probably  by  the  same  proc- 
ess both  within  and  between  the  grains,  quartz-mica-feldspar-rocks  or 
mica-gneisses  have  developed.  These  mica-gneisses  have  an  interlocking, 
granitic-appearing  background,  composed  of  quartz  and  feldspar  in  about 
equal  abundance.  That  the  rock  was  originally  fragmental  is  indicated 
only  by  occasional  roundish  grains  of  quartz  and  feldspar,  but  it  is  always 
difficult  to  determine  certainly  what  part  of  the  quartz  and  feldspar  is 
original  and  what  part  a  secondary  development.  Both  the  original  and 
secondary  feldspars  are  stained  with  limonite  and  are  decomposed  to  a 
greater  or  less  degree  into  chlorite,  biotite,  and  quartz.  Biotite  is  the 
predominant  micaceous  mineral,  but  muscovite  is  present,  and  chlorite  is 
abundant.  Magnetite  is  also  present  in  numerous  crystals,  and  a  small 
amount  of  hornblende  is  found.  The  veins  cutting  the  gneisses  are  com- 
posed mainly  of  iron-stained  feldspar,  with,  however,  much  chlorite  and 
quartz.  This  feldspar,  which  is  beyond  all  question  secondary,  is  identical 
in  its  appearance  with  that  contained  throughout  the  rock.  In  the  mica- 
gneiss  are  curious  black  concretionary-looking  areas,  which  in  thin  section 
are  seen  to  be  essentially  the  same  as  the  remainder  of  the  rock,  except 
that  they  contain  numerous  large  crystals  of  hornblende  and  much  zoisite. 
Each  of  the  hornblende  individuals  includes  many  of  the  other  mineral 
particles,  and  in  their  development  they  appear  to  be  analogous  to  the 
staurolite  and  garnet. 

The  occurrence  of  these  mica-gneisses  within  the  Michigamme  forma- 
tion is  of  great  interest  as  proving  the  development  of  this  kind  of  rock 
from  a  clastic.  In  almost  every  respect  the  coarsest  of  these  mica-gneisses 
are  similar  to  many  mica-gneisses  of  the  Basement  Complex.  The  only 
difference  between  the  two  is  that  in  the  Michigamme  formation  these  crys- 
talline forms  may  be  traced  by  gi'adations  to  phases  in  which  the  fragmental 
characters  are  clearly  apparent. 

The  purest  and  least-altered  phase  of  the  ferruginous  rocks  is  sideritic 
slate.  This  is  a  fine-grained  gray  rock,  composed  almost  wholly  of  siderite, 
which  upon  the  weathered  surface,  where  the  carbonate  passes  into  iron 


PETROGRAPHICAL  CHARACTER  OF  MICHIGAMME  FORMATION.  451 

oxide,  exhibits  a  reddish-brown  color.  From  these  ferriferous  carbonates 
there  have  developed  ferruginous  slate,  ferruginous  chert,  jasper,  griinerite- 
magnetite-schist,  and  iron  ore,  the  processes  and  results  being  identical 
with  similar  rocks  from  similar  materials  in  the  Negaunee  formation.  (See 
pp.  336-375.)  The  description  of  these  processes  will  therefore  not  be  here 
repeated. 

Certain  minor  differences  separate  these  rocks  from  those  of  the 
Negaunee  formation — the  griineritic  rocks  are  finer-grained,  the  iron 
oxide  is  largely  limonite,  and  in  all  phases  of  them  carbonaceous  material 
is  abundant. 

The  amount  of  pure  nonfragmental  material  is  subordinate,  but  because 
iron  ore  develops  from  it,  it  is  not  unimportant.  There  is  in  the  Michigannne 
formation  a  much  larger  quantity  of  material  intermediate  between  clastic 
and  nonclastic  sediments.  In  some  places  the  fragmental  and  nonfragmental 
material  is  largely  concentrated  in  alternate  bands.  In  other  places  the  two 
are  intermingled.  Where  least  altered,  these  intermediate  rocks  may  have  a 
background  consisting  of  siderite  and  cherty  quartz,  with  some  fen-ite,  which 
contains  numerous  well-rounded  fragmental  grains  of  quartz  and  feldspar. 
As  the  metamoi-phosing  processes  set  in,  the  siderite  goes  through  the  same 
set  of  transformations  as  where  it  is  alone,  and  the  same  is  true  of  the  frag- 
mental material,  so  that  there  results  a  great  variety  of  rocks.  Where  the 
processes  are  chiefly  metasomatic  the  siderite  changes  to  ferrite,  and  ferru- 
ginous graywackes,  ferruginous  slates,  cherty  graywackes,  and  cherty  slates 
are  produced.  In  a  common  variety. a  ferrite  background  contains  the 
clastic  constituents.  If  at  the  same  time  the  feldspar  alters  to  biotite  and 
chlorite,  the  slates  are  biotitic  and  chloritic.  Where  the  dynamic  effects  are 
stronger,  griiuerite  and  magnetite  develop  from  the  siderite,  the  secondary 
mica  has  a  parallel  arrangement  of  its  folia,  and  the  quartzes  are  arranged 
with  their  longer  axes  in  a  common  direction,  or  are  granulated,  so  that  there 
result  hematitic  and  magnetitic  mica-schists,  griineritic  mica-schists,  etc.  At 
different  places  there  are  all  gradations  from  the  least  to  the  most  metamor- 
phosed varieties,  and  from  those  which  originally  consisted  wholly  of 
nonfragmental  material  to  those  which  consisted  wholly  of  fragmental 
material.     In  the  first  case  the  pecuHar  rocks  of  the  iron  formation  were 


452  THE  MARQUETTE   IKON-BEAEING  DISTEICT. 

produced;  iu  the  second  case  the  mica -schists  and  mica -gneisses  were 
formed.     Between  one  extreme  and  the  other  there  is  every  gradation. 

RELATIONS   TO   THE   UNDERLYING   FORMATION. 

It  has  already  been  said  that  the  Michigamme  formation  grades  slowly 
down  into  the  Goodrich  quartzite  or  into  the  Bijiki  schist,  and  that  therefore 
the  line  of  separation  between  them  is  more  or  less  arbitrary.  The  relations 
to  the  Clarksburg  formation  are  considered  on  pages  461-463. 

THICKNESS. 

The  thickness  of  the  formation  must  be  considerable,  as  it  covers  a 
wide  area,  but  it  is  impossible,  on  account  of  the  subordinate  folding  to 
which  it  has  been  subjected  and  the  extensive  development  of  slaty  cleavage 
or  fissility  which  cuts  across  the  bedding,  to  make  even  an  approximately 
accurate  estimate.  It  is  possible  that  within  the  area  described  its  thickness 
is  not  more  than  1,000  to  2,000  feet,  but  it  may  be  much  more. 

INTERESTING   LOCALITIES. 
Spurr,  Michigamme,  and  Champion  area. Beginning    at    the    UOrthwCSt    part    of    thc 

area  (Atlas  Sheets  V,  VIII,  and  XII),  about  three-fourths  of  a  mile  south  of 
the  Spurr  mine  are  open  pits  of  soft  limonite  in  the  slate.  Exposures  occur 
in  the  valley  separating  the  two  belts  of  the  Bijiki  schist  along  the  north 
side  of  Lake  Michigamme  at  only  one  or  two  localities.  These  are  on  the 
north  side  of  East  Point.  Passing  to  the  eastward,  north  of  Champion, 
in  sees.  29,  30,  31,  and  32,  there  are  very  numerous  exposures  of  the  less 
altered  kinds  of  the  Michigamme  formation.  The  larger  and  better  exposed 
area  is  a  rough  elevated  plateau  north  of  the  Bijiki  schist.  The  rocks  of 
the  area  comprise  fine-grained,  black,  carbonaceous,  graphitic,  and  pyri- 
tiferous  slates;  coarser-grained  slates  of  the  same  varieties;  ordinary 
black  slates;  fine-grained,  dark-colored  graywackes;  coarse-grained  gray- 
wackes;  occasionally  rocks  which  approach  a  quartzite;  and,  at  one  place, 
a  conglomerate.  Between  the  different  varieties  of  rock  there  are  all  sorts 
of  gradations  and  interlaminations. 

The  conglomerate  contains  small  pebbles  of  chert  and  quartz,  and 
larger  pebbles  of  what  appears  to  be  dense  black  slate.  On  the  exposed 
surface  these  weather  out,  giving  the  rock  a  pitted  appearance.     The  finer- 


INTEKESTING   LOCALITIES  OF  MIOHIGAMME   FOIIMATION.      453 

grained  and  more  carbonaceous  rocks  stain  the  fingers,  and  are  so  soft  as 
to  readily  give  a  black  mark.  When  parted  along  the  cleavage,  many  of 
them  show  lustrous  graphite.  In  all  of  the  carbonaceous  varieties  of  rock 
pyrite  and  marcasite  are  very  plentiful  Even  the  fine-grained  graywackes 
have  a  dark  color,  due  to  the  contained  carbon.  For  the  most  part  the 
rocks  show  comparatively  feeble  dynamic  effects.  To  the  majority  of 
the  rocks  the  term  shales  and  grits  would  almost  be  applicable.  Slaty 
cleavage  is  present  only  in  the  fine-grained  varieties.  As  has  been  explained, 
occasionally  the  movements  have  been  sufficient  to  develop  lustrous  graphite 
between  the  laminse  of  the  slates.  In  places,  as  a  result  of  the  movements, 
the  slates  were  broken,  and  the  cracks  filled  with  vein  quartz,  the  veins 
varying  from  minute  seams  to  those  several  inches  across. 

In  thin  section  the  rocks  coiTCspond  in  their  characters,  in  most  respects, 
to  the  general  description  of  them  given  on  pages  448, 450-451 .  The  coarser- 
grained  graywackes  contain  comparatively  little  feldspar  and  a  small  amount 
of  interstitial  material.  They  are  largely  cemented  by  enlargement.  They 
therefore  approach  quartzites.  They  are,  however,  always  discriminated 
from  the  quartzites  of  the  Ishpeming  formation  by  the  presence  of  black 
carbonaceous  material  between  the  grains.  The  dynamic  effects  observable 
under  the  microscope  are  usually  slight,  the  grains  only  occasionally  being 
arranged  with  their  longer  axes  in  a  common  direction,  although  they 
commonly  show  undulatory  extinction  and  fracturing,  sometimes  in  a  rectan- 
gular manner.  In  the  few  slides  in  which  the  quartz  grains  have  a  parallel 
aiTangement  sericite  is  abundant,  this  appearing  to  be  developed  in  propor- 
tion to  the  other  dynamic  effects.  In  both  the  slates  and  graywackes  mica 
and  quartz  have  developed  by  the  decomposition  of  the  feldspar  in  the 
interstices.  Biotite  is  the  predominant  mica,  although  chlorite  is  rather 
plentiful.  This  process  is  of  far  greater  importance  in  the  slates  than  in  the 
quartzites.  The  cai-bonaceous  material  is  so  abundant  in  many  of  the  black 
slates  as  to  make  it  diflicult  to  determine  the  minerals  present,  and  especially 
the  amount  of  iron  oxides.  In  a  few  of  the  slates  the  movements  have 
been  sufficient  to  develop  folia  of  biotite  in  a  parallel  direction,  and  thus 
make  them  mica-slates.  In  the  transverse  sections  of  the  graphitic  slates 
the  contorted  laminae  of  gi-aphite  are  beautifully  shown. 


4.>4  THE   MARQUETTE  IRON-BEARING  DISTRICT. 

At  several  places  within  the  area  under  consideration  mining  has  been 
done  on  a  small  scale.  These  places  are  located  as  follows:  In  sec.  29  a 
number  of  pits  are  a  short  distance  north  of  the  center  of  the  section; 
others  are  about  one-fourth  mile  south  of  the  center;  and  still  others  some- 
thing- less  than  one-half  mile  to  the  east.  In  sec.  30  only  one  small  pit  is 
known,  and  this  is  about  one-fourth  mile  south  of  the  center  of  the  section. 
In  the  south  part  of  sec.  31  is  the  North  Champion  mine.  At  all  of  these 
places  the  country  rock  is  the  black  carbonaceous  and  pyritiferous  slate. 
The  ores  are  strongly  limonitic.  Associated  with  these  ores  are  sideritic 
slates,  ferruginous  cherts,  and  gi'iineritic  slates.  The  griineritic  slates  are 
usually  finer-grained  than  those  of  the  Negaunee  formation,  and  most  of 
them  appear  to  contain  a  considerable  amount  of  carbonaceous  material. 
The  majority  of  the  ferruginous  cherts  also  have  a  somewhat  different 
appearance  from  those  of  the  Negaunee  formation,  and  the  amount  of  this 
material  is  small.  Where  the  small  ore  bodies  occur  the  original  carbon- 
aceous formation  appears  to  have  been  sideritic.  By  its  oxidation  and  the 
concentration  of  iron  oxide  griineritic  rocks  and  ferruginous  clierts  devel- 
oped, by  processes  analogous  to  those  by  which  similar  rocks  formed  in 
the  Negaunee  formation. 

The  thin  sections  of  the  ferruginous  rocks  of  the  mines  are,  in  all 
essential  particulars,  the  same  as  the  similar  rocks  in  the  Negaunee  forma- 
tion, described  on  pages  366-375.  Like  them,  they  show  the  development 
of  the  different  varieties  of  the  rock  from  a  sideritic  slate.  However,  there 
are  minor  differences,  as  follows:  The  chert  is  usually  very  finely  crystal- 
line, and  sometimes  has  a  semiamorphous  appearance.  The  griinerite  is  at 
most  places  finely  crystalline.  The  iron  oxide  is  very  largely  limonite,  and 
all  of  the  minerals  are  everywhere  impregnated  with  black  carbonaceous 
material.     At  the  mines  the  amount  of  mingled  fragmental  material  is  small. 

Eastern  area.— (Atks  Shects  XV,  XVI,  XIX,  XXII,  aud  XXV.)— lu  sec.  35 
are  pits  which  contain  iron-bearing  rocks  similar  to  those  in  the  pits  north  of 
Champion.  Adjacent  to  these  pits  there  are  the  usual  black  carbonaceous 
and  pyritiferous  slates. 

To  the  east,  between  Mount  Humboldt  and  the  Clarksburg  forma- 
tion slates  are  found  at  vai'ious  localities.      However,  north  of  the  belt 


INTERESTING  LOCALITIES  OF  MICHIGAMME  FORMATION.     455 

of  Clarksburg  formation,  in  sec.  1,  T.  47  N.,  R.  29  W.,  and  in  sees.  5, 
6,  and  7,  T.  47  N.,  R.  28  W.,  there  is  a  complex  ridge  along  which  are 
very  numerous  exposures  of  the  slates,  novaculites,  and  graywackes  of  the 
Michigamme  formation.  The  different  varieties  of  tliese  rocks  are  particu- 
larly well  seen  adjacent  to  and  north  of  Clarksburg.  The  rocks  of  this 
area  differ  in  a  number  of  particulars  from  those  north  of  Champion.  The 
coarse-grained  graywackes  are  only  rarely  found,  but  in  their  place  there 
is  present  a  considerable  quantity  of  fine-grained,  banded  novaculite.  The 
slates  near  Clarksburg  are  not  so  carbonaceous  as  north  of  Champion,  and, 
finally,  they  are  somewhat  more  crystalline,  a  strongly  developed  slaty 
cleavage  being  prevalent  and  many  of  the  rocks  being  biotite-slates.  This 
slaty  cleavage  varies  from  parallelism  to  the  bedding  to  right  angles  to  it. 
The  cleavage  is  in  general  approximately  east  and  west,  and  its  dip  is 
usually  nearly  vertical. 

In  thin  section,  the  semicrj'stalline  appearance  observed  macroscop- 
ically  is  borne  out.  The  mica-slates,  which  are  rare  north  of  Champion, 
are  the  usual  rocks  north  of  Clarksburg.  With  the  biotite  there  is  abun- 
dant chlorite.  In  some  slides  one  is  predominant,  in  others  the  other.  In  a 
number  of  slides  the  original  bedding  is  recognized  by  alternating  layers  of 
different  degrees  of  coarseness,  and  across  the  bedding  the  cleavage  cuts,  as 
indicated  by  the  parallel  folia  of  mica.  The  novaculites  have  a  fine-grained 
background,  consisting  of  small,  closely-fitting  grains  of  quartz.  Between 
these  quartz  grains  more  or  less  of  sericite,  biotite,  and  chlorite  have  devel- 
oped, and  also  occasionally  grunerite.  The  pure  nonfragmental  rocks  of 
the  iron  formation  are  only  rarely  found  in  this  area,  but  many  of  the  rocks 
are  half  fragmental — that  is,  mingled  with  the  clastic  material  are  much 
siderite  and  its  alteration  products.  The  alteration  of  the  siderite  usually 
results  in  the  production  of  needles  of  griinerite  and  crystals  of  magnetite, 
although  the  other  oxides  of  iron  occur.  Commonly  in  the  strongly  griin- 
eritic  rocks  chlorite,  rather  than  biotite,  is  found. 

In  the  broad  belt  of  the  Michigamme  formation  running  from  Clarks- 
burg to  Ishpeming  the  area  is  low  and  swampy,  with  only  occasional  out- 
crops. The  most  important  cluster  of  these  extends  north  of  the  Clarks- 
burg formation,  through  sees.  9,  10,  11,  12,  T.  47  N.,  R.  28  W.,  and  sec.  18, 


456  THE   MAllQUETTE   lEON-BEAKlNG  DISTRICT. 

T.  47  N.,  R.  27  W.  These  exposures  mark  a  moderate  ridge,  which,  how- 
ever, is  not  continuous.  Both  macroscopically  and  microscopically  these 
rocks  are  in  all  respects  like  those  north  of  Clarksburg,  and  they  therefore 
need  no  further  description. 

Lake  Michigamme  area. — Retumiug  now  to  tlic  Lako  Micliigamme  area  (Atlas 
Sheeis  V,  VI,  and  VIII),  at  the  east  end  of  the  lake  mica-slates  are  found. 
On  the  islands  in  the  center  of  the  lake  and  on  the  mainland  adjacent 
there  are  very  numerous  exposures  of  mica-schist.  These  appear  to  be 
completely  crystalline  rocks.  For  the  most  part  they  have  a  moderately 
strong  foliation.  In  many  places  they  are  thickly  studded  with  crystals  of 
garnet  and  staurolite.  These  are  especially  prominent  upon  the  weathered 
surface,  as  the  background  has  dissolved  and  left  these  crystals  projecting. 
At  most  places  the  schistosity  is  the  only  structure  observable,  but  in  some 
places  alternating  bands  of  coarse  and  fine  material,  cutting  across  the 
schistosity,  indicate  the  probable  original  bedding.  It  will  be  remembei'ed 
that  adjacent  to  Champion  the  rocks  of  the  Michigamme  formation  are  but 
little-altered  shales  and  graywackes.  At  the  east  end  of  the  lake  the  rocks 
are  intermediate  in  their  crystallization,  being  mica-slates.  We  therefore 
have  a  progressive  increase  in  the  metamorphism  of  the  rocks  from  their 
little-altered  condition  to  completely  crystalline  schists,  the  change  occur- 
ring in  a  distance  along  the  strike  of  about  4  miles. 

"West  of  the  islands,  and  probably  a  continuation  of  the  same  horizon, 
running  through  sees.  32  and  33,  T.  48  N.,  R.  30  W.,  is  a  ridge  upon  which 
are  very  numerous  exposures,  in  all  respects  like  those  of  the  islands  and  the 
shore.  The  exposures  also  extend  to  the  water's  edge  in  sees.  28  and  29. 
For  the  most  part  the  ledges  are  of  the  same  general  character,  but  in  some 
places  coarser  layers  are  interstratified  with  finer  ones.  These  apparent 
beds  are  in  a  series  of  rolls  which  are  cut  across  by  the  schistosity.  In 
the  southwest  part  of  sec.  29  in  this  area  are  the  peculiar  pegmatized 
exposures  of  mica-schist,  which,  on  account  of  their  exceptional  characters, 
are  fully  described  in  the  general  characterization  on  pages  447-448,  450. 

Toward  the  western  end  of  the  lake,  in  sec.  30,  T.  48  N.,  R.  30  W., 
and  in  sees.  25  and  36,  T.  48  N.,  R.  31  W.,  there  are  occasional  exposures, 
and  here  the  rocks  are  somewhat  less  crystalline,  containing  little  or  no 


INTERESTING   LOCALITIES   OF   MICIIIGAMME   FORMATION.     457 

staurolite  and  garnet.  Just  west  of  the  west  end  of  the  lake  the  rock  is  so 
coarse  and  quartzose  as  to  become  a  micaceous  quartzite. 

Along-  the  southwest  arm  of  the  lake,  in  sees.  5,  7,  8,  17,  and  20, 
T.  47  N.,  R.  30  W.,  at  various  places  mica-schists  are  found.  The  rocks  of 
this  area  are  very  similar  to  those  on  the  islands  and  those  to  the  west  on  the 
mainland,  but  upon  the  whole  they  are  more  strongly  foliated  and  coarsely 
crystalline,  being  rather  coarse-grained,  typical,  foliated  mica-schists.  The 
crystals  of  garnet  and  staurolite  are  very  abundant,  and  are  of  larger  size 
than  in  the  area  to  the  north.  At  one  place  in  sec.  17  the  rock  has  a  strongly 
feldspathic  appearance,  and  is  apparently  a  garnetiferous  mica-gneiss. 

The  mica-schists  of  the  Lake  Michigamme  area  are  very  similar  to 
those  of  the  Penokee  district.^  The  most  important  difference  between  the 
two  areas  is  that  mashing  has  played  a  more  important  role  in  the  Mar- 
quette than  in  the  Penokee  district.  Thei'e  has  evidently  been  movement 
throughout  the  rock,  each  particle  having  been  rearranged  with  reference 
to  the  surrounding  particles,  and  where  the  rock  becomes  strongly  foliated 
the  slickensided  surfaces  between  the  folia  show  that  these  had  differential 
movements  with  respect  to  one  another.  Accompanying  this  more  strongly 
crystalline  character  are  abundant  garnet  and  staurolite,  and  a  less  quantity 
of  chloritoid. 

In  thin  section,  by  using  the  gradation  varieties,  the  processes  of  trans- 
formation of  the  fragmental  rocks  to  the  completely  crystalline  schists  is 
made  out  with  great  clearness.  The  processes  are  almost  identical  with 
those  which  have  been  described  as  taking  place  in  the  mica-schists  which 
occupy  a  similar  horizon  in  the  Upper  Huronian  series  of  the  Penokee 
district  and  in  the  Black  Hills.  A  full  description  of  these  j^i'ocesses  will 
therefore  not  be  here  given,  but  a  few  supplementary  notes  may  be  made. 

The  ordinary  mica-schists  of  the  area  have  a  quartzose  background,  in 
which  biotite  is  very  abundant.  Frequently  a  large  amount  of  chlorite  is 
'associated  with  the  biotite,  and  very  often  also  muscovite  is  present.     In 

'  The  Penokee  irou-bearing  series  of  Michigan  and  Wiseousiu,  by  R.  I).  Irving  and  C.  R.  Van 
Hise:  Mon.  U.  S.  Geol.  Survey,  Vol.  XIX,  1892,  pp.  332-343. 

The  pre-Cambrian  rocks  of  the  Black  Hills,  by  C.  R.  Van  Hiso :  Bull.  Geol.  Soc.  America,  Vol.  I, 
1890,  pp.  222-229. 


458  THE  MARQUETTE   IRON  BEAEING  DISTRICT. 

many  cases  much  feldspar  occurs  Avith  tlie  quartz,  so  that  technically  the 
rocks  are  mica-gneisses.  As  the  rock  becomes  more  foliated  the  mus- 
covite  is  relatively  more  important,  and  occasionally  it  is  predominant. 
Oxide  of  iron,  and  especially  magnetite,  is  found  as  minute  specks  and 
as  crystals  included  within  all  of  the  foregoing  minerals.  The  amount 
varies  greatly;  some  slides  are  comparatively  free  from  iron  oxide;  in  others 
all  of  the  minerals  have  a  blotched  appearance  throughout,  due  to  the 
inclusion  of  innumerable  minute  flecks  of  iron  oxide,  and  perhaps  also  of 
carbonaceous  or  graphitic  material.  Between  the  two  extremes  there  are 
all  gradations. 

In  the  coarse-grained  micaceous  graywackes  adjacent  to  the  border  of 
the  lake,  where  metamorphism  is  partial,  the  processes  of  development  of  the 
mica-schists  are  best  made  out.  Here  the  decomposition  of  the  coarse  frag- 
mental  feldspar  into  interlocking  secondary  chlorite,  biotite,  muscovite,  and 
quartz  may  be  beautifully  seen.  Also  the  original  roundish  fragmental 
grains  of  quartz  are  recognizable.  They  show  undulatory  extinction,  and 
oftentimes  fracturing  into  two  or  more  individuals,  whereas  the  secondary 
quartz  is  in  small  granules  which  do  not  show  pressure  effects.  In  a  coarse 
variety  of  the  ordinary  biotite-schist,  not  of  tlie  completely  crystalline  type, 
two  classes  of  quartz  are  still  recognizable.  There  are  coarser  grains,  which 
have  a  distinct  roundish  appearance,  averaging  from  0.15  to  0.20  mm.  in 
diameter.  Some  of  these  have  their  longer  axes  transverse  to  the  schis- 
tosity.  These  larger  particles  are  taken  to  be  clastic.  They  are  associated 
with  much  more  abundant,  finer-grained  quartz,  averaging  from  0.04  to 
0.05  mm.  in  diameter.  Much  of  this  quartz  is  plainly  a  secondary  devel- 
opment, but  also  a  part  of  it  may  be  original  fragmental  material.  In  both 
the  micaceous  graywackes  and  the  fine-grained  biotite-slates  the  mica 
has  in  general  a  parallel  arrangement,  is  in  small  flakes,  and  is  of  secondary 
origin. 

In  passing  to  the  completely  crystalline  biotite-schists  the  recognizable 
coarser  grains  of  quartz  gradually  disappear  by  granulation,  and  we  have 
a  background  of  quartz  grains  of  approximately  the  same  size,  generally 
arranged  with  their  longer  axes  in  a  common  direction.  The  mica  also 
becomes  more  coarsely  crystalline   and    has   a  greater  uniformity  in  its 


INTERESTING  LOCALITIES   OF   MICHIGAMME   FORMATION.     i59 

parallel  arrangement.  In  this  variety  of  the  rock  it  is  evident  that  the 
mineral  particles  have  been  flattened  or  moved  differentially,  or  both,  and 
thus  were  adjusted  to  one  another.  In  many  of  the  slides  a  great  deal  of 
feldspar  is  found  mingled  with  the  quartzose  background.  In  those  which 
show  an  intermediate  degree  of  alteration  a  portion  of  this  feldspar  is  clearly 
clastic.  In  the  rocks  which  are  nearly  completely  crystalline,  what  part  is 
clastic  and  what  part  a  secondary  development  is  very  difficult  to  determine. 
In  the  most  coarsely  crystalline  mica-gneisses  found,  orthoclase,  microcline, 
and  plagioclase  are  all  seen.  The  individuals  for  the  most  part  average 
about  the  same  size  as  those  of  the  quartz.  Certain  of  the  larger  feldspar 
areas  have  a  fragmental  appearance,  but  the  finer-graiued  background  has 
clearly  recrystallized. 

In  the  more  metamorphosed  varieties  of  the  schists  garnet  and  staurolite 
are  abundant.  The  garnets  often  have  very  well  developed  crystal  outlines. 
As  they  grow  they  seem  to  be  able  to  absorb  or  push  aside  nearly  all  of 
the  other  constituents,  as  they  are  comparatively  free  from  inclusions, 
although  oftentimes  a  considerable  amount  of  quartz  and  feldspar  is 
contained  The  staurolite  occurs  in  the  ordinary  twinned  forms.  Very 
often  their  outlines  are  ragged;  at  other  times  they  have  sharp  crystal 
boundaries.  As  the  staurolites  have  developed,  they  have  grown  around  the 
quartz  and  feldspar,  so  that  these  minerals  within  the  staurolite  crystals  are 
nearly  as  abundant  as  in  the  remainder  of  the  section.  However,  in  the 
growth  the  staurolite  has  absorbed  or  pushed  aside  the  muscovite,  biotite, 
and  chlorite,  as  these  are  rarely  included  in  it.  The  staurolite  shows 
no  evidence  of  strain.  Occasionally  large  blades  of  chloritoid  are  seen. 
These,  like  the  staurolite,  include  the  quartz  and  feldspar,  but  exclude  the 
mica.  These  blades  are  in  general  arranged  with  their  longer  diameters 
and  cleavage  transverse  or  at  a  large  angle  to  the  foliation  of  the  rock.  It 
is  believed  that  the  lack  of  dynamic  effects  in  the  staurolite  and  the  trans- 
verse arrangement  of  the  chloritoid  are  evidence  that  these  minerals  and 
the  garnet  developed  under  static  conditions,  after  movements  ceased.  If 
this  be  true,  it  is  probable  that  the  micas  and  chlorite  had  largely  developed 
at  an  earlier  time,  and  if  so  the  chloritoid,  garnet,  and  staurolite  must  have 
grown  by  absorbing  the  micas  and  chlorite. 


460  THE  MARQUETTE   IKOX-BEAEING   DISTRICT. 

SECTION  III.— THE  CLARKSBURG  FORMATION. 

By  W.  S.  Bayley. 

The  Clarksburg  formation  differs  from  the  other  formations  of  the 
Marquette  Algonkian  in  that  it  embraces  a  large  quantity  of  volcanic 
material  interbedded  with  sediments  as  regularly  as  the  beds  of  an  ordi- 
nary clastic  series.  The  series  embraces  surface  flows  of  basic  lava,  beds 
of  tuff,  conglomerates,  and  breccias,  interleaved  with  well-banded  layers  of 
graywacke,  slate,  or  quartzite.  These  are  cut  by  dikes  and  irregularly 
shaped  intrusive  masses  of  "greenstone"  similar  in  macroscopic  appearance 
to  the  "diorites"  cutting  the  iron  formation.  No  acid  igneous  rocks  have 
been  discovered  anywhere  in  the  formation,  either  as  lava  flows  or  as 
pebbles  inclosed  in  the  conglomerates. 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

The  rocks  of  the  formation  form  a  belt  extending  westward  from  the 
high  bluffs  north  of  Stoneville  Station,  on  the  Duluth,  South  Shore  and 
Atlantic  Railway,  in  sec.  18,  T.  47  N.,  R.  27  W.,  to  the  center  of  sec.  31, 
T.  48  N.,  R.  29  W.,  a  distance  of  about  12  miles  (Atlas  Sheet  IV).  Passing 
east  from  its  western  end,  near  Champion  the  belt  grows  wider  for  several 
miles;  then  it  narrows,  and  again  widens  as  it  swings  southeastward 
toward  Clarksburg,  to  the  southeast  of  which  village  the  formation 
reaches  its  greatest  width  of  about  1^  miles.  From  this  point  the  belt 
swings  to  the  east  again,  and  becomes  gradually  narrower  until  it  disap- 
pears in  sec.  18. 

It  is  noticeable  that  where  the  belt  has  its  maximum  width  the  under- 
lying formations  swing  southward,  and  that  as  the  belt  narrows  to  the  east 
and  west  of  Clarksburg  they  reassume  their  normal  courses. 

The  formation  is  a  local  one  in  the  sense  that  it  occurs  on  one  side  only 
of  the  syncline  in  the  Upper  Marquette  series.  The  central  vent  from  which 
most  of  the  lavas  and  tuffs  were  erupted  is  thought  to  have  been  in  the 
widest  portion  of  the  belt,  a  little  to  the  southeast  of  Clarksburg.  Here 
the  tuffs  and  sedimentary  rocks  are  rare  and  the  intrusive  boss-like  knobs 
of  "greenstone"  are  most  numerous.     But  east  and  west  of  this  place  other 


THE  CLARKSBURG  FORMATION.  461 

similar  knobs  are  found,  and  these  are  taken  to  indicate  that  there  were  a 
number  of  vents  from  which  the  lavas  were  extruded.  From  these  vents 
as  centers  the  lavas  and  tuffs  were  sent  out  over  the  surrounding  country, 
but  not  to  great  distances,  for  the  coarser  materials  did  not  span  the  width 
of  the  basin  in  which  the  Michigamme  slates  were  deposited.  The  suppo- 
sition of  the  existence  of  a  number  of  vents  situated  along  a  line  parallel  to 
the  axis  of  greatest  folding  of  the  Marquette  series,  but  to  the  south  of  it, 
together  with  the  north-south  compression  to  which  the  Clarksburg  beds 
were  subjected,  in  company  with  all  of  the  other  Algonkian  formations  of 
the  region,  will  explain  satisfactorily  the  occurrence  of  the  beds  as  a  belt  on 
one  side  only  of  the  Marquette  synclinorium. 

The  topography  of  the  area  underlain  by  the  Clarksburg  rocks  is 
not  essentially  different  from  that  of  other  portions  of  the  Marquette  range 
where  greenstones  are  prominent.  It  is  characterized  by  the  occurrence  of 
numerous  small  and  large  rounded  knobs  and  long  narrow  ridges,  often 
bare  at  their  summits,  and  separated  from  one  another  by  stretches  of  swamp 
land  or  by  sand  plains.  Where  the  bedded  rocks  are  in  excess  of  the  intru- 
sive ones  the  hills  often  possess  precipitous  southern  exposures,  but  this 
feature  of  the  topography  is  not  sufficiently  striking  to  be  characteristic. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

The  relations  of  the  Clarksburg  rocks  to  the  surrounding  formations  are 
often  difficult  to  interpret.  In  a  few  cases  where  contacts  are  plainly  visible 
the  interpretation  is  clear.  On  the  south  the  volcanic  series  is  bounded 
by  grUiierite-magnetite-schists  of  the  Negaunee  formation  and  by  Goodrich 
quartzites  and  Michigamme  slates.  Southeast  of  Champion  the  volcanics 
appear  to  rest  for  a  short  distance  upon  grlinerite-schists.  Although  actual 
contacts  of  the  two  formations  have  not  been  seen,  well-characterized  ledges 
of  the  schists  and  of  the  volcanics  are  met  with,  separated  by  covered  inter- 
vals of  but  a  few  feet  in  width.  The  schists  appear  to  strike  directly  into 
hills  composed  of  the  Clarksburg  rocks,  and,  what  is  more  significant,  great 
bowlders  of  the  schists,  some  sharped-edged  and  others  rounded,  are  found 
thickly  strewn  through  the  lower  beds  of  the  volcanic  series. 

With  respect  to  the  relations  with  the  Goodrich  quartzite  and  the 
Michigamme  slate  there  is  somewhat    greater  obscurity.     In  the  NW.  ^ 


462  THE   MARQUETTE  IRON-BEARING   DISTRICT. 

sec.  18,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXV),  for  instance,  the  volcanic 
conglomerates  are  in  contact  with  graywackes  or  with  arenaceous  slates. 
The  exact  locality  of  the  contact  in  question  is  the  east  end  of  the  top  of 
the  hills  north  of  Stone ville  station.  The  Clarksburg  rocks  at  this  place 
consist  principally  of  conglomerates  with  a  green  schistose  matrix  and 
of  tuifs.  The  pebbles  of  these  conglomerates  are  fine-grained  diabases, 
quartzites,  slates,  and  granites.  The  south  side  of  the  hill,  which  is 
composed  principally  of  the  greenstone-conglomerates,  is  faced  with  the 
graywackes,  which,  near  the  top  of  the  hill,  appear  to  be  beneath  the  con- 
glomerates unconformably.  At  the  east  end  of  the  hill  again  are  other 
arenaceous  slates,  and  these  apparently  strike  directly  into  the  hill.  Here 
the  volcanics  again  appear  unconformably  upon  the  graywackes. 

A  little  farther  west,  however,  near  the  center  of  sec.  13,  T.  47  N., 
R.  28  W.  (Atlas  Sheet  XXV),  at  the  base  of  the  large  hills  on  the  north  side 
of  the  railroad  track,  are  graywackes  like  those  in  sec.  18,  but  at  this  place 
they  seem  to  grade  up  into  tuffs,  which  are  interbedded  with  the  greenstone- 
conglomerates.  At  many  other  localities  the  same  relations  are  observed 
between  tuflfaceous  and  lava  beds  and  graywackes.  The  latter  are  inter- 
leaved with  the  former,  and  are  much  more  abundant  among  the  lower  beds 
than  among  the  higher  ones  of  the  formation. 

On  its  northern  side  the  belt  of  Clarksburg  rocks  is  everywhere  bor- 
dered by  the  Michigamme  slate,  the  relations  between  the  two  formations 
being  very  similar  to  those  between  the  volcanic  formation  and  the  forma- 
tions to  the  south.  At  Clarksburg  (Atlas  Sheet  XIX)  these  relations  are 
plainly  seen.  In  the  little  dome-like  hill  north  of  the  railroad  track  and 
east  of  the  station  sedimentary,  graywacke-like  beds  and  conglomeratic 
greenstones  are  regularly  interbedded.  On  the  hills  northwest  of  the  village 
are  slaty,  tuffaceous  rocks,  in  which  the  included  fragments  become  smaller 
and  smaller  as  we  pass  northward  into  the  Michigamme  slates,  until  at  a 
short  distance  northwai-d  typical  slates  are  met  with.  No  sharp  line  of 
demarcation  between  the  tuffs  and  the  slates  can  be  detected,  the  former 
apparently  grading  into  the  latter  by  a  gradual  diminution  in  the  amount 
of  tuffaceous  material  intermingled  with  the  sedimentary  substance. 


THE  CLARKSBURG   FORMATION.  4G3 

Wherever  the  bedded  volcaiiics  are  studied  the  same  relations  are 
found  to  exist  between  them  and  the  sedimentary  beds  beneath  and  above 
them.  The  volcanic  formation  appears  to  be,  in  g-eneral,  either  between  the 
Goodrich  and  the  Michigamme  formations  or  near  the  base  of  the  latter. 
At  its  eastern  and  its  extreme  western  ends  it  seems  to  be  a  little  above  the 
base  of  the  lowermost  members  of  the  Michigamme  slates,  while  toward 
the  center  of  the  belt  beds  belonging  with  the  Goodrich  quartzites  are 
interleaved  with  undoubted  volcanic  conglomerates.  The  probable  explana- 
tion of  these  seemingly  contradictory  phenomena  is  that  the  rocks  of  the 
eastern  and  western  ends  of  the  formation  are  a  little  younger  than  those 
in  the  central  portions  of  the  belt,  where  the  greatest  volcanic  activity  was 
exhibited,  or,  in  other  words,  the  volcanic  energy  first  found  vent  in  the 
central  portions  of  the  area  now  occupied  by  the  formation,  and  from  here 
traveled  both  eastward  and  westward. 

THICKNESS   AND   FOLDING. 

The  thickness  of  the  volcanic  formation  does  not  admit  of  accurate 
measurement,  although  it  must  amount,  in  places,  to  several  thousands  of 
feet.  The  individual  beds  in  the  center  of  the  belt  can  not  be  certainly 
separated  from  one  another.  In  other  places,  east  and  west,  the  bedding 
is  more  definite,  but  even  here  no  single  bed  can  be  traced  for  any  great 
distance.  In  the  western  portion  of  the  area  the  layers  are  much  contorted, 
large  and  small  folds  crowding  one  another  in  an  almost  endless  succession. 
The  strikes  of  these  small  folds  point  in  all  directions,  though  the  prevailing 
one  seems  to  be  toward  the  east  and  Avest.  Generally  the  beds  dip  at  high 
angles  towai-d  the  north  or  northeast.  The  series  is  much  more  highly 
contorted  than  the  Ishpeming  formation,  a  result  probably  due  to  the 
fact  that  a  set  of  mixed  volcanic  ash  beds,  lavas,  and  sediments  was  less 
resistant  to  pressure  than  the  quartzites. 

PETROGRAPHICAL   CHAEACTER. 

A  general  survey  of  the  entire  formation  presents  a  good  illustration  of 
a  series  of  deposits  formed  by  submarine  volcanoes.  The  most  important 
vents  of  the  volcanoes  were  near  Clarksburg,  though  minor  vents  existed  also 


464  THE   MARQUETTE   IKON-BEARING  DISTRICT. 

east  and  west  of  this  place.  Around  these,  lava  flows  and  a  few  tuff  beds 
accumulated.  As  one  passes  away  from  the  center  along  the  belt  compact 
sheets  of  lava  become  less  and  less  noticeable,  while  Avell-bedded  tuffs  and 
eruptive  conglomerates  and  breccias  become  more  abundant,  and  sedimen- 
tary layers  are  interleaved  with  them.  Toward  the  edges  of  the  belt  the 
latter  rocks  increase  in  importance  and  the  well-characterized  tuffs  diminish 
in  quantity.  The  volcanic  activity  continued  from  the  later  portion  of 
Ishpeming  time  into  the  earlier  portion  of  Michigamme  time,  beginning 
and  ending  gradually.  The  volcanoes  were  evidently  submarine,  or  at  any 
rate  their  products  were  deposited  in  water,  even  if  the  apices  of  some  of 
them  were  above  the  water's  surface  for  a  part  of  the  time.  The  submarine 
chai'acter  of  the  volcanoes  explains  not  only  the  interbedding  of  tuffs  and 
sediments  and  the  formation  of  triie  conglomerates  containing  pebbles  of 
the  iniderlying  rocks,  but  it  also  explains  the  existence  of  bedded  breccias 
composed  of  fragments  of  volcanic  origin  in  a  tuffaceous  base  and  the 
presence  of  conglomerates  formed  of  volcanic  fragments  in  a  sedimentary 
groundmass. 

The  sediments,  tuffs,  conglomerates,  breccias,  lavas,  and  coarse  green- 
stones have  all  suffered  a  great  amount  of  alteration,  but  in  many  cases 
their  original  nature  can  still  be  made  out.  The  recognition  of  the  true 
character  of  the  tuffs  depends  mainly  upon  their  field  relations  and  field 
habits,  but  a  pyroclastic  structure  can  be  detected  in  many  of  their  sections. 
Almost  all  the  rocks,  except  the  best-preserved  sediments,  are  now  partly 
or  wholly  crystalline.  This  condition  has  been  brought  about  mainly  by 
the  development  within  them  of  hornblende,  biotite,  and  quartz.  The 
change  from  the  fragmental  to  the  crystalline  texture  is  most  nearly 
complete  in  the  groundmass  of  some  of  the  conglomerates.  This  ground- 
mass  is  a  biotite-schist,  not  very  unlike  the  biotite-schists  of  the  Southern 
Complex. 

THE   MASSIVE   GREENSTONES. 

The  coarse  crystalline  greenstones  that  occur  so  frequently  as 
knobs  in  the  area  southeast  of  Clarksburg  and  farther  east  have  the  same 
composition  as  the  "diorites"  of  the  Negaunee  and  other  pre-Clarks- 
burg  formations,  but  a  somewhat  different   structure.     A  few  specimens 


THE   CLAKKSIiUEG   FORMAT  ION.  465 

show  evidences  of  the  diabasic  structure,  while  many  of  them  appear  to 
have  been  porphyritic. 

In  the  hand  specimens  these  rocks  resemble  in  many  respects  the 
greenstones  north  t»f  Lake  Michigamme  (see  pp.  5()0-.503).  They  are  prin- 
cipally massive  rocks  of  a  dark-green,  almost  black  color,  which  vary  in 
grain  from  very  fine  to  very  coarse.  On  freshly  fractured  surfaces  the 
finer-grained  jjhases  have  a  more  or  less  fibrous  appearance,  due  to 
the  presence  in  them  of  abundant  acicular  hornblende  and  plagioclase 
crystals.  The  fresh  surfaces  of  the  coarser  rocks  seem  to  be  made  up 
almost  exclusively  of  large  areas  of  black  or  dark-green  hornblende. 

Under  the  microscope  some  of  the  thin  slices  of  the  finer-grained  rocks 
show  numerous  square  or  quadratic  sections  of  an  altered  idiomorphic  plagio- 
clase in  a  groundmass  composed  of  (quartz,  chlorite,  hornblende,  calcite, 
biotite,  and  a  little  newly  formed  feldspar.  The  quartz  and  calcite  are  often 
present  in  largest  quantity,  the  former  as  little  interlocking  grains,  forming 
a  matrix  in  which  the  other  silicate  components  lie,  and  the.  calcite  filling 
little  interstices  between  these.  The  amphibole  is  a  yellowish-green  variety 
in  spicules,  and  the  biotite  a  reddish-brown  variety  in  small  plates.  These 
same  minerals  are  also  found  embedded  in  the  altered  plagioclase,  by  whose 
decomposition  they  were  probably  formed.  All  the  components  of  the 
groundmass  of  these  rocks  are  apparently  new  products  in  their  present 
positions.  They  were  probably  derived  by  secondary  processes  from  a 
very  fine  grained  or  possibly  a  glassy  groundmass  of  a  basaltic  porphyrite. 

In  other  sections  there  are  mottlings  of  a  brown  color  on  a  white  back- 
ground when  the  sections  are  viewed  against  white  paper.  The  brown 
areas  contain  a  great  deal  of  biotite,  while  the  colorless  areas  are  free  from 
this  mineral.  The  latter  consist  of  aggregates  of  quartz,  a  little  feldspar, 
and  a  few  hornblende  needles,  and  the  former  of  the  same  minerals  Avith  an 
abundance  of  biotite  flakes  and  large  masses  of  spongy  magnetite.  The 
light  areas  suiTOund  the  brown  ones  as  the  matrix  surrounds  the  phenocrysts 
in  a  porphyrite. 

All  the  original  structures,  except  the  porphyritic,  have  disappeared 
from  these  rocks,  so  that  it  is  impossible  to  learn  much  concerning  their 

MON   XXVIII 30 


466  THE   MAKQUETTE   lEON-BEAKIFG  DISTEICT. 

original  character.  It  is  believed,  however,  that  they  were  basic  porphyrites 
which  occurred  either  in  volcanic  necks  or  as  moderately  thick  lava  flows. 

The  coarse-grained  greenstones  are  the  fillings  of  volcanic  orifices, 
or  they  constitute  great  dikes  cutting  through  the  bedded  members  of 
the  Clarksburg  series.  In  these  the  green  amphibole  is  sometimes  in 
large,  well-characterized,  ophitic  areas,  between  which  are  the  alteration 
products  of  plagioclase.  The  interior  of  the  hornblende,  which  is  more  or 
less  chloritized,  is  of  a  yellowish-green  color,  and  around  this  is  a  periphery 
of  dark-green  amphibole,  more  particularly  where  the  mineral  is  in  contact 
with  undoubted  remnants  of  plagioclase.  The  color  of  both  nucleal  and 
peripheral  amphibole  is  bluish-green  in  a  direction  approximately  parallel 
to  the  cleavage,  but  the  peripheral  hornblende  is  darker  than  the  nucleus. 
Ai'ound  the  borders  the  plates  are  all  fringed  with  long  needles  of  the  gi-een- 
blue  amphibole,  and  similar  needles  penetrate  in  all  directions  the  materials 
of  the  light  interstitial  substance  between  the  amphibole  plates.  This  inter- 
stitial mass  is  an  aggregate  of  the  decomposition  products  of  plagioclase, 
among  the  more  prominent  of  which  are  biotite,  calcite,  epidote,  and  quartz. 
In  addition  to  the  long  spicules  of  green-blue  amphibole  that  cut  this 
aggregate,  others  with  the  characteristics  of  actinolite  also  penetrate  it. 

In  a  very  few  cases  a  typical  diabasic  structure  is  noticed  on  weathered 
surfaces  of  ledges.  In  the  thin  section  of  these  rocks,  however,  a  porphy- 
ritic  stnicture  is  also  observable.  Decomposed  feldspars  with  quadratic 
cross-sections  are  embedded  in  the  usual  plexus  of  hornblende,  biotite, 
altered  plagioclase,  quartz,  and  magnetite,  to  which  is  often  added  kaolin. 
The  plexus  may  be  in  areas  with  an  ophitic  outline,  but  the  lines  between 
the  porphyritic  crystals  and  the  matrix  in  which  they  lie  are  rendered  so 
obscure  by  the  many  secondary  substances  that  have  arisen  from  the  alter- 
ation of  the  feldspars  that  it  is  difficult,  and  in  many  cases  impossible,  to 
make  them  out.  The  larger  crystals  of  plagioclase,  of  whose  feldspathic 
nature  there  can  be  no  doubt,  are  so  filled  with  kaolin,  sericite,  quartz, 
biotite,  and  hornblende  that  it  would  seem  probable  that  many  of  the  same 
minerals  in  the  matrix  must  have  likewise  been  derived  from  feldspar. 

Certain  compact  hornblendic  rocks  differ  from  the  specimens  just 
described  simply  in  the  possession  of  a  great  quantity  of  amphibole.     The 


THE    CLARKSBUEG    FORMATION.  467 

mineral  is  of  the  same  nature  as  that  in  the  other  greenstones,  but  is  in 
mucli  larger  quantity.  It  occurs  as  comjiact  anhedra/  fringed  Avith  long 
acicular  crystals  that  form  a  network,  in  whose  meshes  are  areas  of  altered 
plagioclase  and  leucoxene. 

From  the  above  rapid  survey  of  the  coarse  greenstones  in  the  Clarks- 
burg series  it  will  be  seen  that  these  rocks  do  not  differ  essentially  from 
the  "greenstones"  subsequently  described  as  intrusive  in  the  pre-Clarksburg 
beds.  They  are  strikingly  similar  to  those  in  the  western  portion  of  the 
district  (see  pp.  499-506).  They  were  originally  of  the  same  composition  as 
these,  and  they  have  suffered  similar  alterations.  Their  structure,  where 
it  can  be  detected,  was  a  little  different  from  that  of  the  lower  intrusive 
gi-eenstones  in  that  there  was  a  tendency  to  the  production  of  porphyritic 
feldspars.  This  difference  may  be  due  to  the  fact  that  the  Clarksburg  rocks 
were  cooled  in  the  ducts  of  volcanoes  or  in  dike  fissures  that  were  near  the 
surface,  while  the  lower  greenstones  were  cooled  at  greater  depths.  In  this 
connection  it  is  interesting  to  note  that  rocks  like  these  greenstones  are  not 
foimd  intrusive  in  horizons  higher  than  the  Clarksburg. 

THE   LAVAS. 

The  lavas  that  are  interbedded  with  the  sediments  and  tuffs  of  the 
formation  are  not  very  abundant.  Some  of  the  finer-grained  greenstones 
abeady  described  may  be  portions  of  lava  flows,  but  that  this  is  certainly 
the  case  has  by  no  means  been  shown.  A  few  layers  identical  with  the 
former  in  structure  and  composition  are  unquestionably  sheets. 

The  clearest  evidence  that  genuine  lava  flows  were  laid  down  among 
the  more  abundant  tuff  beds  of  the  old  Clarksburg  volcano  is  afforded  by  the 
amygdaloids.  These  have  been  found  in  a  few  widely  separated  localities 
within  the  limits  of  the  Clarksburg  belt,  but  they  are  not  at  all  common. 
The  rocks  are  fine-grained,  light-gray,  and  often  schistose.  They  contain 
few  or  many  amygdules  filled  with  calcite  or  chlorite,  and  these  are  often 
flattened  as  though  by  flowage.  Under  the  microscope  they  present  no 
peculiar  features.     Small  laths  of  plagioclase,  with  forked  extremities,  are 

'This  term  has  recently  been  proposed  by  Pirssou  to  designate  those  crystalline  constituents  of 
rocks  that  do  not  possess  ciystal  outlines.     Bull.  Geol.  Soc.  Am.,  vol.  7,  1896,  p.  492. 


468  THE   MARQUETTE    IKONBEAKING   DISTRICT. 

scattered  through  a  groundinass  composed  of  numerous  microlites  of  feld- 
spar in  an  altered  basic  glass.  The  various  phases  of  the  amygdaloids  are 
so  similar  to  one  another,  and  so  like  diabasic  lavas  elsewhere,  that  they 
demand  no  special  description. 

THE    SEDIMENTS   AND   TUFFS. 

The  sediments. — PetrogTaphically  most  removed  from  the  lavas  are  the  sedi- 
ments, which  grade  imperceptibly  into  the  tuffs.  Pure  sediments  are  found 
only  along  the  borders  of  the  Clarksburg  belt,  beyond  the  horizon  at  which 
the  volcanic  series  is  regarded  as  beginning  and  ending.  They  consist  of 
quartzites,  gray  wackes,  and  slates,  many  of  which  are  much  mashed.  The 
sediments  interbedded  with  the  tuffs,  conglomerates,  etc.,  are  composed 
principally  of  the  waste  of  preexisting  rocks,  but  intermingled  with  this 
debris  is  a  greater  or  smaller  quantity  of  basic  material  which  is  supposed 
to  be  of  volcanic  origin.  Many  of  the  beds  are  now  thoroughly  crystalline, 
so  tliat  an  accurate  separation  of  their  sedimentary  and  volcanic  compo- 
nents is  not  possible.  Often  some  of  the  thicker  beds  consist  of  alternating 
layers  containing  respectively  large  and  small  quantities  of  basic  material, 
and  these  pass  into  others  in  which  the  volcanic  substance  can  scarcely  be 
detected.  In  some  of  the  former  beds  a  well-marked  tuffaceous  structure 
is  recognizable,  but  in  the  majority  of  cases  all  evidences  of  a  well- 
characterized  original  structure  have  been  obscured  by  recrystallization. 
As  a  rule  the  structure  of  the  more  purely  sedimentary  rocks  is  much 
better  preserved  than  is  that  of  the  tuffaceous  ones. 

The  nearly  pure  sedimentary  rocks  are  dark-gray  or  light-gray  and 
fine-grained,  with  an  even  or  a  contorted  bedding,  marked  by  parallel  bands 
of  different  shades  of  color.  The  coarser  bands  exhibit  very  plainly  their 
fragmental  character  on  the  weathered  surface.  Little  eyes  of  quartz  can 
be  seen  against  a  background  which  has  the  appearance  of  a  graywacke. 

The  least  altered  of  the  sedimentary  rocks  consist  principally  of 
rounded  qixartzes,  altered  feldspar  grains,  and  a  few  flakes  of  a  dirty, 
greenish-brown  biotite.  In  the  finer-grained  bands  biotite  is  probably 
more  abundant  than  it  is  in  the  coarser  ones,  but  with  this  exception  there 
is  little  difference  between  them.     The  lighter  and  darker  shades  noticed 


PLATE    XXXII. 


Plate  XXXII.— THIN  SECTIONS  P'ROM  CLARKSBURG  FORMATION  AND  REPUBLIC  GREEN- 
STONE. 

Fig.  1.  Thin  section  of  sedimentary  bed  from  Clarksburg  formation,  showing  secondary  hornblende 
crystals.  No.  17640,  from  200  steps  N.,  1,800  steps  W.,  of  SE.  corner  of  sec.  17,  T.  47  N.,  R.  28 
W.  From  one  of  the  beds  constituting  the  Clarksburg  formation.  The  large  anhedra  are  of 
yellowish-green  amphibole,  idiomorphio  in  cross-section,  but  very  irregular  in  outline  in 
longitudinal  section.  The  groundraass  consists  of  biotite  flakes  between  which  is  a  crypto- 
crystalline  aggregate  of  quartz.     The  black  mineral  is  magnetite.     Natural  light.     X  50. 

Fig.  2.  Thin  section  of  fragmental  rock  from  near  base  of  Clarksburg  formation.  No.  14785,  from 
SW.  i  sec.  32,  T.  48  N.,  R.  29  W.  In  ordinary  light  the  fragmental  structure  is  plainly  revealed. 
Between  crossed  nicols  the  quartz  grains  (one  of  which  is  indicated  by  the  dotted  line  in 
the  figure)  break  up  into  differently  orientated  portions,  so  that  tlie  section  appears  like  that 
of  a  very  quartzose  gneiss.     Polarized  light.     X  22. 

Fig.  3.  Thin  section  of  banded  tuff  from  Clarksburg  formation,  SE.  i  sec.  4,  T.  47  N.,  R.  29  AV.  The 
figure  shows  a  large  Carlsbad  twin  of  plagioclase  in  a  fine-grained  crystalline  groundmass 
composed  mainly  of  quartz,  biotite,  and  magnetite,  with  a  little  green  hornblende.  The 
"streaming"  of  the  biotite  around  the  upper  end  of  the  feldspar  crystal  may  be  seen  upon 
close  inspection.     In  polarized  light.    X  22. 

Fig.  4.  Thin  section  of  greenstone  from  Republic,  showing  secondary  hornblende  crystals.  No.  16485, 
from  SE.  corner  of  Republic  Mountain,  SE.  i  of  SE.  i  sec.  8,  T.  46  N.,  R.  29  W.  The  rock  is 
associated  with  the  members  of  the  irou  formation.  The  section  shows  the  typical  struc- 
ture of  the  secondary  amphibole  in  many  of  the  western  greenstones.  To  the  right  is  the 
cross-section  of  a  small  idiomorphic  grain.  The  greater  portion  of  the  hornblende  exhibits 
the  cellular  structure,  which  has  been  regarded  as  the  characteristic  structure  of  contact 
minerals.  The  clear  white  areas  represent  quartz  grains  and  the  cloudy  areas  altered 
plagioclase.  The  large  light  areas  at  the  top  and  toward  the  lower  right-hand  edge  of 
the  figure  represent  spaces  in  the  section.  Natural  light.  X  55. 
470 


Fig.    I.-THIN  SECTION   OF  SEDIMENTARY  BED,   FROM   CLARKSBURG  FORMATION,   SHOWING  SECONDARY   HORNBLENDE  CRYSTALS. 

Fig.    2.— thin  SECTION  OF  FRAQMENTAL  ROCK,   FROM   NEAR   BASE  OF  CLARKSBURG 

Fig.   3.     THIN  SECTION   OF  BANDED  TUFF,   FROM  CLARKSBURG   FORMATION. 

Fig.  .1.     THIN   SECTION   OF  GREENSTONE,    FROM   REPUBLIC,   SHOWING  <-ECOMDARY   H: 


THE   CLARKSBURG   FORMATION.  471 

in  contiguous  bands  are  due  mainly  to  differences  in  the  quantity  of 
magnetite,  biotite,  and  certain  indefinite  dust  particles  present.  In  all  these 
rocks  the  plagioclase  is  altered,  and  among  its  alteration  products  are 
found  biotite,  quartz,  and  occasionally  a  little  muscovite. 

The  greater  number  of  the  sedimentary  beds  are  characterized  by  the 
presence  of  amphibole.  They  comprise  rocks  whose  difference  in  degree 
of  crystallization  is  dependent  apparently  upon  the  proportion  of  amphibole 
present  in  them.  This  amphibole  is  certainly  not  a  product  of  the  decom- 
position of  the  usual  constituents  of  a  sedimentary  rock;  on  the  other 
hand,  it  is  the  most  common  product  of  the  alteration  of  the  igneous  rocks 
associated  with  the  fragmental  ones.  For  this  reason  largely,  and  because 
certain  rocks  interleaved  with  the  sediments  are  composed  almost  exclusively 
of  amphibole,  while  at  the  same  time  they  are  unquestionably  marked  by 
bedding  lines,  and  further,  because  the  hornblende  rocks,  by  the  gradual 
loss  of  iheir  hornblende,  pass  into  the  sediments,  the  material  from  which 
the  liornjjlende  in  the  sediments  was  formed  is  believed  to  have  been 
tufifaeeous. 

On  the  weathered  surface  the  majority  of  the  hornblendic  sediments 
present  a  very  rough  aspect,  a  consequence  of  the  projection  of  the  horn- 
blende crystals  beyond  the  general  surface  of  weathering.  The  rocks  are 
dark-green  on  a  fresh  fracture,  where  they  look  like  massive  crystallines. 

The  thin  sections  show  a  groundmass  which  surrounds  the  hornblendes. 
The  amphibole  itself  is  in  large,  cellular,  green  plates,  filled  with  inclusions 
of  the  rock's  components.  In  cross-section  many  of  the  plates  have  the 
idiomorphic  outlines  of  crystals,  but  in  longitudinal  section  they  are  more 
or  less  irregularly  shaped  (see  PI.  XXXII,  fig.  1),  the  ends  especially 
fraying  out  into  long  needles  which  penetrate  the  groundmass. 

The  groundmass  is  composed  of  quartz,  altered  plagioclase,  biotite, 
chlorite,  and  magnetite,  with,  sometimes,  light-colored  garnets  in  little 
dodecahedi'a.  Where  the  rock  is  massive  the  garnets  are  less  plentiful 
than  they  are  in  the  schistose  phases. 

This  groundmass  has  a  coarse  and  a  fine  part,  the  latter  serving  as  a 
matrix  to  the  former.  The  coarser  part  is  composed  of  quartz  and  biotite. 
Some  of  the  quartz   grains  show  the  rock  to  be  fragmental,  since  a  few 


472  THE  MARQUETTE  IKON  BEARING  DISTRICT. 

rounded  ones  are  observed  that  have  been  built  out  by  the  addition  of 
quartz,  and  so  have  preserved  the  proof  of  their  clastic  origin.  Other 
gi-ains  resemble  oi'dinary  sand  grains  until  they  are  examined  in  polarized 
light,  when  they  break  up  into  many  interlocking  areas,  so  that  often  a 
section  which  in  ordinary  light  has  the  typical  clastic  structure  (PI.  XXXII, 
fig.  2)  presents  the  appearance  of  a  typical  schist  between  crossed  nicols. 
The  foliation  of  the  rock  is  due  principally  to  the  arrangement  of  the 
biotite  in  laminae  that  wind  in  and  out  between  the  larger  quartz  grains. 
The  biotite  appears  to  have  originated  mainly  in  the  sedimentary  material 
between  the  larger  quartz  grains.  The  large  plates  of  amphibole  that  lie 
in  the  gi'oundmass,  on  the  other  hand,  were  formed  after  the  rock  became 
schistose.  They  probably  originated  in  greater  part  from  the  basic  material 
added  to  the  sediments  by  the  tuffs,  either  directly  or  through  solutions 
passing  into  the  sediments  from  the  tuff  beds  interstratified  with  them. 

The  fine-grained  material  of  the  groundmass  is  composed  principally 
of  quartz,  chlorite,  altered  plagioclase,  and  magnetite  dust.  It  has  been 
so  completely  recrystalhzed  that  all  traces  of  its  original  structure  have 
disappeared.  It  was  probably  originally  the  finer-grained  matrix  between 
the  coarser  components  of  a  graywacke  or  slate. 

The  most  hornblendic  of  the  fragmental  rocks  have  already  been 
referred  to  as  looking  like  massive  crystalline  rocks.  Upon  close  examina- 
tion, however,  it  is  observed  that  even  in  the  most  crystalline  of  the  beds 
there  is  a  distinct  banding,  which  is  emphasized  by  the  different  quantities 
of  biotite,  magnetite,  and  hornblende  in  the  different  layers.  Where  the 
hornblende  is  in  great  excess  the  bands  look  very  much  like  an  amphibolite. 
Where  the  other  components  exceed  the  amphibole  in  quantity  the  bands 
resemble  more  closely  basic  tuffs.  A  few  bands  are  actinolitic.  These 
contain  fairly  large  plates  and  groups  of  actinolite  needles,  identical  with 
the  griinerite  in  the  griinerite-slates  of  the  Negaunee  formation,  scattered 
through  a  groundmass  of  fragmental  quartz  grains  and  occasional  garnets 
embedded  in  a  matrix  of  chlorite,  hornblende,  biotite,  magnetite,  and  limon- 
ite,  closely  aggi-egated  and  without  any  Avell-defined  structure. 

Gradation  varieties  between  sediments  and  tuffs. BctWCeU     tllC     I'Ocks     jUSt    deSCribcd 

as  consisting  largely  of  sedimentary  material  and  others  that  are  composed 


THE    CLARKSBURG   FORMATION.  473 

almost  exclusively  of  tuft'aeeous  material  there  exists  a  large  series  of  iiit-er- 
mediate  rocks  that  are  mixtures  of  sedimentary  and  pyroclastic  material  in 
varying  pro^iortions.  On  the  one  hand  they  pass  directly  into  well-defined 
sedimentaries,  and  on  the  other  hand  into  typical  tuffs.  In  macroscopic 
appearance  they  resemble  the  amphibole-l)earing  elastics  described  in  the 
last  paragraph. 

In  thin  section  the  gradation  varieties  differ  from  the  hornblendic  frag- 
mental  rocks  simply  in  the  absence  of  any  great  quantity  of  quartz  grains. 
Only  occasionally  is  the  tuffaceous  character  of  some  of  their  constituents 
revealed  by  their  structure.  Usually  the  rocks  have  been  recry stall ized,  so 
that  their  original  components  have  entirely  disappeared.  Now  and  then 
a  quartz  grain  is  observable  in  the  midst  of  an  aggregate  of  green  horn- 
blende and  bi'own  biotite,  but  the  characteristic  clastic  structure,  as  well  as 
the  characteristic  tuffaceous  structure,  is  wanting. 

The  tuffs. — The  tuffs,  like  the  mixed  tuffs  and  sediments,  are  usually 
largely  recrystallized.  Quite  frequently,  however,  beds  of  them  are  found 
in  which  the  tuffaceous  structure  has  been  well  preserved.  In  the  hand 
specimens  these  rocks,  where  fresh,  present  the  usual  aspect  of  a  hornblendic 
tuff;  where  altered  they  closely  resemble  the  "  Schalsteius."  In  almost  all 
of  them  more  or  less  calcite  may  be  observed.  In  a  few,  as,  for  instance, 
in  the  rock  at  the  crossing  of  the  Chicago  and  Northwestern  and  the 
Milwaukee  and  Northern  railroads,  just  east  of  Champion,  the  calcite  is 
present  in  such  large  quantities  that  the  rock  becomes  practically  a  lime- 
stone. Even  in  these  much  altered  forms,  however,  the  tuffaceous  character 
of  the  beds  may  often  be  recognized  in  the  hand  specimen,  as  fragments  of 
minerals  and  rocks  of  all  shapes  may  be  seen  crowding  the  calcareous  mass 
in  which  they  lie.  Many  of  the  fragments  have  been  completely  altered  to 
calcite,  which  nevei'theless  has  preserved  their  outlines  by  forming  from 
them  perfect  j^seudomorphs. 

The  less  altered  tuffs  are  more  interesting  from  a  ]jetrogra})hieal  stand- 
point, since  they  often  present  excellent  proofs  of  their  original  character. 
In  general  the  fine-grained,  pure  tuffs  are  more  frequently  found  interbedded 
with  volcanic  conglomerates  and  breccias  than  witli  sedimentary  layers, 
although  alternations  of  the  tuffs  with  the  sediments  are  not  uncommon. 


474  THE  MARQUETTE   IRON-BEARING   DISTRICT. 

AIL  of  them  are  well  banded,  even  where  their  substance  has  been  entirely 
recrystallized,  and  most  of  them  are  schistose. 

In  thin  section  the  crystallized  tuffs  are  seen  to  be  made  up  of  a 
crystalline  aggregate  of  quartz,  cloudy  feldspar,  small  greenish-yellow 
biotite  flakes,  occasional  anhedra  of  green  hornblende,  and  little  irregular 
grains  of  magnetite.  The  biotite  is  so  arranged  as  to  give  a  foliated  struc- 
ture to  the  section,  while  at  the  same  time  it  is  more  abundant  in  certain 
bands  than  elsewhere,  thus  producing  the  banding  noticed  in  the  hand 
specimen.  In  the  midst  of  this  groundmass  there  are  often  embedded 
broken  fragments  of  an  altered  plagioclase,  or  even  entire  crystals  of  this 
mineral.  (PI.  XXXII,  fig.  3.)  These  rocks  still  show  evidence  of  their  tuffa- 
ceous  character,  though  some  of  them  contain  small  quantities  of  sediments. 
Their  decomposition,  recrystalHzation,  and  the  changes  that  have  been 
effected  in  them  by  dynamic  processes  have  effaced  most  of  the  marks  of 
their  original  nature,  but  here  and  there,  where  a  larger  fragment  or  a  com- 
plete crystal  of  plagioclase  has  resisted  alteration,  the  marks  of  tuffaceous 
origin  are  still  clearly  legible.  Most  of  the  quartz  and  of  the  biotite  in  the 
rocks  was  derived  most  probably  from  a  strongly  feldspathic,  tuffaceous 
dust,  though  a  small  quantity  of  the  former  mineral  may  originally  have 
been  present  as  a  sediment. 

In  the  nonschistose  tuffs  the  tuffaceous  structure  is  too  evident  to  be 
mistaken.  In  these  rocks  large  anhedra  of  green  amphibole  are  scattered 
through  a  matrix  which  is  composed  of  broken  crystals  of  plagioclase  in  a 
groundmass  of  smaller  fragments  of  the  same  mineral,  little  wisps  of  brown 
biotite,  nests  of  chlorite  and  calcite,  and  a  still  finer  matrix  of  the  same 
substances,  cemented  together  by  a  mass  of  crypto-crystalline  quartz.  The 
biotite  seems  to  have  come  from  chlorite  on  the  one  hand  and  from  plagio- 
clase on  the  other.  The  chlorite,  in  turn,  appears  to  have  come  from  a 
basic  glass,  or  possibly  from  augite,  since  it  often  contains  within  its  mass 
"divergent  radial"  plagioclase  microlites. 

In  addition  to  the  fragments  of  feldspar  mentioned  above,  there  may 
be  noticed  in  a  few  specinens  an  occasional  fragment  that  resembles  a  piece 
of  an  altered  glassy  rock.  These  fragments  now  consist  of  little  flakes  of 
muscovite,  a  very  few  of  biotite,  and  grains  and  crystals  of  magnetite,  all 


THE    CLARKSBURG   FORMATION.  475 

embedded  in  a  faintly  polarizing  aggi'egate  that  is  apparently  made  up  of 
quartz  and  plagioclase  in  very  fine  grains,  like  the  aggregate  produced  by 
the  devitrification  of  a  glass.  Under  high  powers,  in  addition  to  the  mag- 
netite grains  there  are  detected  certain  small  purplish  2:)lates  resembling 
those  formerly  so  well  known  under  the  name  of  "  Eisenglimmer."  The 
magnetite  of  the  fragments  is  nontitaniferous,  while  that  in  the  inclosing 
rock  is  often  strongly  titaniferous,  if  we  may  judge  from  the  great  quan- 
tities of  leucoxene  and  sphene  in  the  latter  and  its  absence  from  the  former. 

In  the  above  descriptions  detailed  reference  is  made  only  to  the  sedi- 
ments and  to  the  tuffs,  as  though  these  rocks  were  the  more  imjjortant 
members  of  the  Clarksburg  formation.  As  a  matter  of  fact,  the  well-defined 
sediments  and  the  typical  tuffs  constitute  a  much  smaller  aggregate  in  the 
series  than  do  the  mixed  sediments  and  tuffs.  They  are  described  in  some 
detail  because  they  have  retained  their  original  characteristics  better  than 
have  the  mixed  rocks,  and  so  afford  better  evidence  as  to  the  nature  of  the 
formation  than  do  the  latter.  The  variety  of  the  mixed  rocks  is  great. 
They  well  deserve  close  study;  but  to  describe  them  in  detail  would 
unduly  enlarge  this  monograph.  It  is  enough  for  the  present  to  reiterate 
the  statement  that  the  mixed  rocks  are  intermediate  in  their  characters 
between  the  types  of  rocks  that  are  above  described. 

From  the  facts  already  related  it  is  clear  that  we  have  in  the  Clarks- 
burg formation  a  series  of  typical  tuffs,  together  with  a  series  of  mixed 
sediments  and  tuffs,  formed  by  the  accumulation  of  volcanic  dust  and  ashes 
in  a  basin  in  other  parts  of  which  the  deposition  of  ordinary  land  debris 
was  going  on.  The  variation  of  the  quantities  of  quartz,  amphibole,  and 
biotite  in  the  alternate  beds  is  easily  accounted  for  in  the  safe  assumption 
of  a  variation  in  the  volcanic  activity.  The  alteration  and  crystallization  of 
the  beds  are  ascribed  partly  to  contact  action  and  partly  to  mashing.  The 
former  was  due,  no  doubt,  more  to  the  chemical  effects  of  the  solutions 
passing  between  lavas,  tuffs,  and  sediments  than  to  heat  alone.  The 
mashing  resulted  in  the  contortion  of  all  the  beds  of  the  formation. 

THE    HORNBLENDE-SCHISTS. 

The  processes  which  changed  the  mixed  sediments  and  tuffs  of  the 
Clarksburg  formation  must  have  been  very  similar  to  those  which  produced 


476  THE    MAEQUETTE   IRON-BEARING   DISTRICT. 

the  o-reen  schists  of  the  Moua  formation  in  the  Basement  Complex.  The 
uhiraate  products  in  the  two  cases  are  practically  identical,  except  that  the 
Mona  schists  are,  as  a  rule,  more  weathered  than  the  Clarksburg  rocks.  The 
final,  most  crystalline  phases  of  the  Mona  rocks  were  described  by  Williams 
as  schistose  "diorites."  In  this  volume  they  are  regarded  as  the  most 
highly  metamorphosed  phases  of  tuffs.  Rocks  analogous  to  these  exist 
also  in  the  Clarksburg  series.  They  are  very  lustrous,  foliated  rocks, 
resembling  in  their  hand  specimens  typical  hornblende-schists.  The  only 
differences  noted  between  these  schists  and  the  amphibolic  sediments  and 
tuffs  described  above  are  in  the  greater  schistosity  of  the  former  and  in 
the  greater  abundance  of  l)iotite  and  amphibole  in  them. 

In  the  most  typical  of  the  hornblende-schists  a  small  quantity  of 
quai'tz  is  present,  with  tlie  rounded  outlines  of  clastic  grains.  The  horn- 
blende is  idiomorphic  in  cross-section,  and  is  often  actinolitic  in  habit.  In 
a  few  specimens  well-defined  crystals  of  actinolite  are  surrounded  by 
zones  of  chloritoid,  with  the  deep  bluish-green  pleochroism  of  this  mineral. 
In  others  a  light-green  amphibole  is  surrounded  by  hornblende  with  the 
properties  of  uralite.  In  the  majority  of  specimens,  however,  only  one 
amphibole  occurs,  and  this  is  usiially  the  uralitic  variety  Moreover,  it 
is  often  cellular,  including  many  grains  of  magnetite,  and  in  some  cases 
even  grains  of  sedimentary  quartz.  In  origin  this  hornblende  is  evidently 
secondary,  and  the  schists  themselves  are  consequently  secondary  rocks. 
They  are  interbanded  with  schistose  sediments  in  which  the  sedimentary 
structure  can  still  be.  made  out.  This  latter  fact,  together  with  the  micro- 
scopic structure  of  the  schists,  makes  it  seem  very  probable,  that  the 
amphibole-schists  are  highly  foliated,  recrystallized  phases  of  mixed  sedi- 
ments and  tuffs  in  which  the  tuffaceous  material  predominates  over  that  of 
sedimentary  origin. 

If  this  view  of  the  origin  of  the  schists  is  correct,  these  rocks  throw 
considerable  light  on  the  origin  of  the  hornblende-schists  of  the  Mona 
series,  and  add  considerable  weight  to  the  statement  made  in  a  former 
chapter  to  the  effect  that  these  rocks  are  mashed  tuffs. 

THE   BRECCIAS   AND   CONGLOMERATES. 

In  the  discussion  of  the  Clarksburg  formation  the  breccias  and  con- 
glomerates should  be  distributed  between  the  sediments,  gradation  varieties, 


THE   CLAEKSBURG    FOllMATIOK  477 

and  tuffs  just  described;  but  as  this  is  impracticable,  the}'  are  considei'ed  as 
a  division  by  themselves. 

In  this  discussion  but  little  distinction  is  made  between  those  rocks 
•with  a  conglomeratic  habit  and  those  that  are  more  properly  brecc-ias, 
since  both  contain  rounded  and  angular  fragments  of  preexisting  rocks. 
The  breccias  contain,  in  addition,  angular  fragments  of  the  same  compo- 
sition as  the  matrix  inclosing  them.  In  the  following  pages  the  two  terms 
are  used  indiscriminatel}'. 

In  describing  the  essential  features  of  the  conglomerates  it  is  almost 
impossible  to  avoid  repetition  of  facts  already  stated  with  respect  to  the 
tuifs  and  sediments  of  the  formation,  since  the  matrices  of  the  conglomerates 
are  frequently  identical  with  the  material  of  these  rocks.  Sometimes  the 
material  is  sedimentary  in  character,  sometimes  tuifaceous,  and  sometimes 
crystalline.  The  sedimentary  and  tuffaceous  matrices  do  not  differ  in  any 
essential  features  from  the  sediments  and  tuffs  already  described,  while  the 
crystalline  matrices  are  often  similar  to  mashed  greenstones  or  amphibole- 
schists. 

The  conglomerate-schists,  when  vieAved  in  the  ledge,  often  ^^I'esent  tne 
appearance  of  a  black  biotite-schist  or  hornblende-schist,  containing  frag- 
ments of  quartz  and  feldspar,  of  granite,  of  griinerite-schist,  and  of  a  light- 
colored  sandy  rock,  and  very  large  irregular  pieces  of  a  lustrous  black  rock 
of  nearly  the  same  character  as  the  schist  itself  On  the  weathered  surface 
the  fragments  stand  out  plainly,  but  on  the  fresh  fracture  only  the  quartz 
and  the  sandy  rock  become  visible,  the  rock  as  a  whole  resembling  a  well- 
crystallized  "  augen-schist." 

In  other  cases  the  mati-ix  of  the  conglomerate  is  a  tuif  that  differs 
very  slightly,  if  at  all,  from  the  tuffs  already  described.  Its  fragments  are 
large  pieces  of  biotite-schist,  and  smaller  ones  of  the  sandy  rock.  These 
latter  in  many  instances  are  banded,  when  they  are  identified  as  fragments 
of  the  sedimentary  beds  interstratified  with  the  conglomerates.  Grains  of 
quartz  are  also  noticeable  scattered  among  the  feldspar  fragments  and  crj^s- 
tals  that  help  to  make  up  the  tuffaceous  groundmass,  and  occasionally 
fairly  large  pebbles  of  the  same  minerals  are  met  with. 

A  third  class  of  the  conglomerates  is  characterized  by  the  sedimentary 
nature  of  its  groundmass.     These  rocks  resemble  true  conglomerates  and 


478  THE   MARQUETTE   IRON-BEAEmG   DISTRICT. 

breccias  more  closely  than  do  those  with  the  schistose  or  those  with  the 
tufiaceous  matrix.  Their  groundmass  is  identical  in  structure  and  composi- 
tion with  the  rocks  constituting-  the  sedimentary  beds  of  the  formation.  In 
this  are  embedded  tuff  fragments,  fragments  of  biotite-schist,  occasionally 
large  pieces  of  an  amphibole  rock  that  resembles  a  slightly  foliated  green- 
stone, pebbles  of  iron  ore,  and  others  of  a  fragmental  rock  similar  to  that 
forming  the  matrix.  Usually  the  fragments  are  not  so  abundant  but  that 
the  character  of  the  matrix  is  easily  recognized.  In  a  few  cases,  however, 
they  are  present  in  such  large  quantities  that  the  existence  of  any  matrix 
can  with  difficulty  be  detected. 

The  conglomerates  of  all  classes  are  more  or  less  clearly  banded, 
and  all  are  foliated  in  some  degree  at  least.  The  most  schistose  phases 
are  those  whose  matrix  resembles  a  biotite-schist,  and  the  least  schistose  are 
those  with  a  sedimentary  groundmass.  Garnets  are  common  in  all  of  these 
rocks.  They  are  apparently  most  abundant  in  those  that  contain  the  great- 
est amounts  of  basic  components.  In  the  conglomerates  with  the  schist 
matrix  they  are  most  common.  Here  the  garnets  exist  as  pink  granular 
lines  marking  the  bedding  planes  of  the  original  rock. 

The  microscopical  examination  of  the  conglomerates  and  breccias  adds 
little  to  the  knowledge  concerning  their  nature  which  is  gained  from  obser- 
vation of  the  rocks  in  the  field.  The  matrices,  as  has  repeatedly  been 
stated,  are  similar  to  the  materials  of  the  sediments  and  tuffs  that  constitute 
such  an  impoi-tant  portion  of  the  entire  Clarksburg  formation,  although 
in  no  case  are  they  purely  sedimentary  or  tuffaceous,  as  are  some  of  the 
non conglomeratic  sediments  or  tuffs.  Usually  there  is  an  admixture  of 
sedimentary  and  volcanic  material  in  the  groundmass  of  these  rocks  as 
well  as  in  the  pebbles  embedded  in  it. 

The  schist-conglomerates  have  a  matrix  composed  of  quartz,  biotite, 
hornblende,  magnetite,  occasionally  a  little  altered  plagioclase,  and  some- 
times a  few  crystals  of  tourmaline.  The  quartz  and  biotite  are  arranged 
to  form  a  foliated  groundmass,  through  which  the  other  components  are 
scattered.  In  this  schistose  groundmass  are  also  the  small  fragments  of 
quartz,  graywacke,  quartzite,  iron  ore,  and  greenstone  already  referred  to 
above.     The  quartz  is  often  in  rounded  grains,  as  though  waterworn.     The 


THE   CLARKSBLTEG   FORMATION.  479 

Other  fragments  are  also  more  or  less  rounded  at  times,  but  more  frequently 
they  are  sharply  angular. 

The  hornblende  is  of  the  green  variety  common  to  the  beds  already 
described.  It  occurs  in  the  usual  large  grains,  which  sometimes  are  idio- 
morphic  in  cross-section,  sometimes  irregular  in  shape,  and  always  more 
or  less  cellular.  The  grains  occur  independently,  lying  in  all  azimuths 
in  the  schistose  matrix,  or  they  are  grouped  together  into  little  sheaf-like 
bundles.  They  are  nuich  more  abundant  in  some  bands  than  in  others, 
often  occurring  so  thickly  as  to  exclude  from  them  all  biotite.  In  other 
bands  no  amphibole  occurs,  and  in  these  biotite  is  abundant.  Moreover, 
in  these  bands  the  quartz  grains  are  much  more  fragmental-looking  than 
those  in  the  hornblendic  bands,  and  besides  there  exists  between  them  a 
very  fine  grained  aggregate  of  quartz  and  plagioclase,  mainly  the  former. 

The  fragments  in  the  schist-conglomerates  require  no  special  mention. 
They  are  pieces  of  the  tuffs  and  sediments,  interstratified  with  the  conglom- 
erates, or  of  the  ores  and  quartzites  of  the  Marquette  series  below  the 
Clarksburg  formation,  or  of  greenstones  that  may  have  been  portions  of 
interleaved  lava  flows,  or  perhaps  portions  of  dikes  occurring  in  the  pre- 
Clarksburg  beds,  or,  finally,  fragments  from  the  Basement  Complex.  Some 
of  the  fragments  are  waterwom,  while  others  are  sharply  angular. 

The  explanation  of  the  schistose  conglomerates  is  that  they  were  orig- 
inally beds  made  up  of  alternating  layers  of  sediments,  tuffs,  and  mixtures 
of  these,  in  which  were  embedded  bowlders  and  pebbles  of  preexisting 
rocks  and  in-egular  fragments  ejected  from  the  volcanic  vent.  Some  of 
these  fragments  must  have  been  portions  of  the  walls  of  the  orifice  through 
which  the  eruption  took  place,  for  they  are  certainly  pieces  of  the  rocks 
that  constitute  the  Clarksburg  formation.  True  volcanic  bombs  have  not 
yet  been  recognized,  though  it  is  possible,  and,  indeed,  probable,  that  some 
of  the  lustrous  black  fragments  embedded  in  these  conglomerates  are  of 
this  character.  These  beds  were  rapidly  hardened  and  afterward  made 
schistose  by  mashing.  Since  the  biotite  flakes  wind  about  the  garnets,  it 
is  concluded  that  these  minerals  formed  before  or  during  the  mashing. 
After  this,  contact  action  or  later  metasomatic  change  resulted  in  the 
production  of  the  amphibole.     This  is  shown  by  the  fact  that  the  small 


480  THE   MARQUETTE   lEON-BEAKING   DISTRICT. 

crystals  lie  in  all  azimuths,  their  longer  axes  cutting  the  plane  of  schis- 
tosity.  The  contact  effects  were  probably  the  result  not  so  much  of  the 
beat  alone  to  which  the  beds  were  subjected  as  to  the  hot  solutions  that 
passed  between  the  basic  tuffaceous  beds  and  the  acid  sedimentary  ones, 
and  between  the  basic  and  acid  components  of  the  mixed  beds. 

The  conglomerates  with  a  tuffaceous  groundmass  present  in  the  thin 
section  nearly  the  same  appearance  as  the  hornblendic  bands  of  the  sedi- 
mentary rocks  described  in  the  last  few  paragraphs.  Very  rarely  is  a  typical 
tuffaceous  structure  observable,  although  on  the  sides  of  the  weathered 
ledges  this  structure  is  very  plain.  Biotite  is  present  in  small  quantity 
only,  while  green  hornblende  is  abundant.  In  nearly  all  cases  some  sedi- 
mentary material  can  be  detected  as  a  fine-grained,  almost  dusty  aggregate 
between  the  large  amphibole  grains,  but  in  no  case  is  it  in  any  large 
quantity.  In  these  conglomerates  the  most  interesting  fragments  are  those 
that  are  sinjilar  to  the  sedimentary  rocks  of  the  formation.  Many  of  them 
are  large,  white,  rounded  pebbles,  which  in  thin  section  are  found  to  possess 
a  well-preserved  fragmental  structure.  They  consist  of  quartz  and  altered 
feldspar  grains,  the  former  predominating,  sericite  and  biotite  in  very  small 
quantities,  and  magnetite  in  dust  grains.  Scattered  here  and  there  through 
the  mass  are  delicate  plumose  groups  of  green  hornblende  that  are  evidently 
much  younger  than  the  clastic  grains.  In  cross-section  the  amphiboles  are 
idiomorphic.  In  all  its  essential  features  the  rock  of  these  pebbles  is  iden- 
tical with  that  of  the  sedimentary  beds  interstratified  with  the  tviffs,  even 
to  the  presence  in  it  of  the  introduced  idiomorphic  amphiboles.  The  other 
fragments  occurring  in  these  rocks  need  no  description. 

The  only  other  class  of  conglomerates  distinguished  is  that  in  which 
the  rocks  possess  a  sedijnentary  groundmass.  This  is  composed  of  a  frag- 
mental aggregate  of  quartz  and  a  little  feldspar,  large  qviantities  of  brown 
biotite,  a  small  quantity  of  magnetite,  and  the  usvial  spicules  and  crystal- 
loids of  the  green  amphibole,  occurring  sometimes  in  single  grains  and 
sometimes  in  plumose  or  sheaf-like  bundles. 

CONCLUSIONS. 

In  the  petrogi'aphical  study  of  the  rocks  we  find  abundant  confirmation 
of  the  accurac}'  of  our  conclusions  regarding  the  origin  of  the  Clarksburg 


THE   CLARKSBUECr  FORMATION.  481 

formation.  This  is  unquestionably  a  set  of  sediments,  tuffs,  lavas,  and 
volcanic  and  sedimentary  conglomerates  that  were  deposited  beneath  the 
surface  of  some  body  of  water.  The  volcanic  contributions  to  the  series 
probably  exceeded  in  volume  those  contributed  by  aqueous  agencies, 
although  these  latter  were  by  no  means  small  in  amount.  Of  the  volcanic 
contributions  the  larger  portion  was  in  the  form  of  volcanic  cinders,  ashes, 
etc.,  a  much  smaller  portion  being  in  the  form  of  lavas.  This  fact  would 
indicate  that  the  eruptions  were  violent,  like  the  type  represented  by 
Vesuvius  at  present,  rather  than  quiet,  like  the  Hawaiian  volcanoes.  That 
they  were  intermittent  is  proved  by  the  numerous  alternations  of  tuffs  with 
sedimentary  layers.  The  conglomerates  that  occur  in  the  formation  are 
simply  tuffs  or  sediments  containing  large  fragments  of  preexisting  rocks, 
sometimes  waterworn  and  sometimes  angular.  The  former  were  worn  from 
rocks  that  were  exposed  to  the  action  of  the  waves  when  the  deposits  in 
Avhich  they  are  found  were  being  laid  down.  The  latter  were  torn  from 
the  throat  of  volcanic  vents  or  were  produced  by  the  shattering  of  rock 
beds  already  existing,  or  perhaps,  in  the  case  of  some  tuff  fragments,  by 
the  breaking  of  the  rock  beds  actually  in  process  of  formation  at  the 
time  when  the  conglomerates  containing  them  were  being  built  up. 

All  forms  of  volcanic  products  are  recognized  among  the  beds  compris- 
ing the  series  except  volcai^ic  bombs  and  perhaps  those  peculiar  breccias 
produced  by  the  breaking  of  a  lava's  crust  and  the  cementing  of  the 
fragments  thus  formed  into  a  solid  rock  by  the  cooling  of  the  liquid  mass 
in  which  they  became  embedded.  The  bombs  may  possibly  be  represented 
by  some  of  the  altered  greenstone  bowlders  occasionally  met  with  in  the  con- 
glomerates, and  the  lava  breccias  may  be  represented  by  some  of  the  schis- 
tose conglomerates  In  which  irregularly  shaped  schistose  fragments  are 
embedded.     If  so,  however,  there  are  no  positive  proofs  of  the  facts. 

INTERESTING    LOCALITIES. 

Good  exposures  of  the  rocks  of  the  Clarksburg  series  are  found  along 
the  north  and  west  lines  of  the  NW.  i  sec.  4,  T.  47  N.,  R.  29  W.  (Atlas  Sheet 
XIII),  near  Champion.  The  hill  immediately  north  of  the  west  quarter  post 
of  the  section  is  made  up  in  large  part  of  well-bedded  conglomeratic  rocks 
with  a  green  schistose  matrix.  The  beds  strike  a  little  north  of  west  and  dip 
MON  xxviii 31 


482  THE  MAKQUETTE  lEON-BEAKING  DISTEICT. 

uniformly  to  the  north  at  varying  angles.  These  conglomerates  are  inter- 
bedded  with  wide  or  narrow  bands  of  a  fine  or  coarse  grained  graywacke 
or  quartzitic  rock,  sometimes  the  conglomerate  and  sometimes  the  sedi- 
mentary rock  being  in  excess.  The  matrix  of  the  conglomerate  is  often 
coarsely  crystalline,  with  a  very  rough,  dark-green,  weathered  surface. 
The  roughness  is  caused  by  the  projection  of  numerous  hornblende  crystals 
beyond  the  general  surface  of  weathering.  The  latter  is  often  dark-gra}^, 
like  that  of  the  interbedded  graywacke.  Often  bands  of  the  graywacke 
and  bands  of  the  amphibole-bearing  rock  alternate,  forming  together  the 
matrix  in  which  the  fragments  are  embedded.  At  other  times  the  matrix  is 
a  brilliantly  black  hornblende-schist  or  biotite-scliist.  The  pebbles  and 
bowlders  embedded  in  this  matrix  are  sometimes  waterworn,  but  oftener 
they  are  sharply  angular.  The  rounded  fragments  are  principally  quartz- 
ites  and  griinerite-schists,  and  the  angular  ones  are  similar  in  composition 
to  the  graywacke  interbedded  with  the  greenstone-conglomerates.  On  the 
weathered  surfaces  the  contact  between  the  pebbles  and  the  matrix  is  sharp 
and  clear,  but  on  the  fresh  fracture  the  materials  of  pebble  and  matrix  appear 
to  grade  into  each  other.  On  the  north  side  of  the  hill  the  interbanding  of 
the  light-weathering  graywacke  and  a  dark-weathering  "greenstone"  is  well 
shown.  The  dark  rock  is  composed  almost  exclusively  of  hornblende  and 
garnets.  In  the  hand  specimens  it  appears  thoroughly  crystalline,  but  in 
thin  section  there  are  seen  numerous  grains  of  quartz  which  appear  to  be 
of  clastic  origin.  Both  the  graywacke  and  the  greenstone  become  con- 
glomeratic at  times,  the  former  containing  pebbles  and  large  bowlders  of 
the  latter  rock,  and  this  in  turn  containing  bowlders  of  the  graywacke. 
All  the  beds  at  this  place  are  much  contorted,  and  often  they  are  crossed 
by  numerous  faults  with  small  throws. 

On  the  top  and  along  the  south  side  of  the  ridge  in  the  northeni  part 
of  the  section  the  conglomeratic  rocks  are  beautifully  exposed.  Here  great 
flat  surfaces  exhibit  a  strikingly  handsome  brecciated  structure.  The  inter- 
banding of  the  graywacke  and  the  dark  rock  is  not  so  plain  here,  although 
fragments  showing  the  interbanded  rocks  are  met  with  embedded  in  other 
rocks.  Large  irregular  fragments  of  a  black  schistose  rock,  like  the  matrix 
of  the  conglomerate  near  the  west  quarter  post  of  the  section,  are  found 


THE   CLARKSBURG  FOEMATION.  483 

inclosed  in  a  plainly  fragmeutal  greenstone  resembling  a  tuff,  and,  on 
the  other  hand,  equally  as  large  fragments  of  tuff  are  found  in  the  schist. 

The  impression  made  on  the  mind  by  the  confused  association  of  these 
different  rocks  is  not  cleared  up  until  the  nature  of  the  rocks  is  revealed 
through  the  microscope.  It  then  seems  plain  that  we  have  here  a  series  of 
tuffs,  sediments,  and  lavas.  The  tuffs  contain  a  great  deal  of  sedimentary 
material,  and  the  sediments  much  tuffaceous  material.  The  two  are  inter- 
bedded  and  grade  into  each  other.  The  dark  lustrous  rock  is  an  almost 
pure  tuff  in  some  cases,  and  in  others  a  lava.  A  lava  flow  caught  up  frag- 
ments fi-om  the  tuffs  and  sediment.  Later  this  lava  contributed  fragments 
to  subsequent  tuffs  and  sediments. 

The  little  group  of  hills  east  of  Champion,  in  the  SE.  ^  sec.  32,  T.  48  N., 
R.  29  W.  (Atlas  Sheet  XII),  affords  other  excellent  exposures  of  the  conglom- 
erates. Here  the  black,  lustrous,  schistose  groundmass  is  usually  full  of 
little  garnets.  Large  bowlders  and  shai-p-edged  fragments  of  griinerite- 
schist,  granite,  and  quartzite  are  crowded  into  the  schist  in  great  numbers. 
The  conglomerate  appears  also  to  be  interbedded  with  narrow  seams  of  a 
fine-grained  quartzite.  The  rocks  are  all  very  much  contorted,  the  bedding 
planes  of  the  black  schist  being  marked  by  rows  of  garnets. 

The  hills  north  of  the  railroad  track,  in  sec.  13,  T.  47  N.,  R.  28  W. 
(Atlas  Sheet  XXII),  present  a  somewhat  different  aspect.  They  are  built 
up  of  alternating  conglomeratic  and  nonconglomeratic  beds  striking  about 
N.  20°  W.  and  dipping  50°  NW.  The  conglomeratic  beds  are  composed 
principally  of  tuffaceous  material,  and  the  nonconglomeratic  ones  mainly 
of  sedimentary  substance.  All  the  rocks  are  schistose,  with  the  foliation 
inclined  to  the  bedding,  dipping  in  the  same  direction  as  the  latter,  but  at 
a  smaller  angle.  At  the  base  of  the  hill,  on  the  south  side,  are  true  slates 
or  fine-grained  graywackes  interbedded  with  the  tuffs,  and  a  little  to  the 
northwest,  across  a  north-south  valley,  are  some  small  sheets  of  the  tuffs 
interbedded  with  massive  crystalline  greenstones. 

In  all  the  instances  described  the  major  portion  of  the  volcanic  part  of 
the  Clarksburg  rocks  consists  of  tuffs  and  lavas,  the  former  predominating. 
Occasionally  dikes  and  small  knobs  of  massive  greenstone  are  associated 
with  these,  but  they  are  rare.     In  sees.  7,  17,  and  18,  T.  47  N.,  R.  28  W. 


484  THE    MARQUETTE   IKOX  BEAKIXCx   DISTPJCT. 

(Atlas  Sheet  XIX),  soutlieast  of  the  village  of  Clarksburg,  however,  the 
case  is  different.  Here  coarse  greenstones,  in  the  form  of  knobs,  are 
the  predominant  rocks.  These  are  associated  with  a  few  amygdaloids  and 
occasionally  with  tuffs.  Sedimentary  beds  are  present  in  small  quantity  in 
the  interior  of  the  area,  and  when  present  the  sediments  are  freely  inter- 
mingled with  tuffaceous  materials.  Toward  the  northern  and  southern 
borders  of  the  belt  sediments  are  more  abundant,  but  the  transition  from 
well-marked  tuffs  into  typical  sedimentary  rocks  is  more  sudden  here  than 
elsewhere  along  the  belt. 

Because  of  the  great  abvmdance  of  coarse  greenstones  at  this  place, 
the  location  of  the  principal  vents  for  the  volcanic  portions  of  the  formation 
are  supposed  to  have  been  here.  The  knobs  are  taken  to  be  volcanic  plugs 
or  portions  of  thick  flows  that  have  escaped  erosion.  The  amygdaloids  are 
lava  flows.  Tuffs  may  have  been  present  in  large  quantity  in  the  valleys 
between  the  knobs,  but  if  so  they  have  been  almost  entirely  removed  by 
denuding  agencies. 

SUMMARY. 

The  Clarksburg  formation  is  a  set  of  interbedded  tuffs,  lavas,  sedi- 
mentary and  volcanic  conglomerates  and  breccias,  and  other  sediments,  cut 
through  and  through  by  dikes  and  bosses  of  an  altered  diabase  or  basalt  that 
is  similar  in  composition  to  the  older  greenstones  intrusive  in  the  pre-Clarks- 
burg  beds  of  the  Marquette  series.  The  eruptive  materials  are  basic.  They 
are  in  all  probability  the  surface  facies  of  the  greenstones  above  mentioned 
as  intrusive  in  the  Marquette  series. 

From  its  relations  to  the  Goodrich  quartzites  and  the  Michigamme 
slates  it  is  learned  that  the  period  of  deposition  of  the  volcanic  series 
embraced  the  closing  stages  of  Ishpeming  time  and  the  opening  stages  of 
Michigamme  time. 

All  the  rocks  of  the  Clarksburg  series  except  the  greenstones  and  the 
lavas  are  banded  and  bedded.  Most  of  them  are  foliated,  and  nearly  all  are 
more  or  less  completely  recrystallized.  Although  originally  approximately 
horizontal,  the  beds  are  now  contorted  and  folded  so  intricately  that  no 
accurate  estimate  of  the  thickness  of  the  formation  can  be  made. 


THE   OLARKSBUEG   FORMATION.  485 

The  bosses  of  greenstone  mark  the  sites  of  the  old  volcanic  vents  from 
which  the  materials  of  the  tuffs  were  erupted.  The  most  prominent  of  these 
were  situated  a  few  miles  to  the  southeast  of  the  village  of  Clarksburg, 
though  others  were  opened  from  time  to  time  to  the  eastward  and  the  west- 
ward of  this  center. 

The  ashes  and  lavas  sent  from  these  vents  fell  into  water  and  were 
interbedded  with  sediments.  The  pyroclastic  material  became  consolidated 
into  tuffs  and  the  sediments  modified  into  slates,  schists,  and  graywackes. 
The  former  mark  the  periods  of  volcanic  activity  and  the  latter  mark  periods 
of  rest.  Mixed  tuffs  and  sediments  were  formed  during  the  less  violent  stages 
of  the  eruptions.  After  deposition  the  beds  were  hardened  by  alteration  and 
by  the  formation  of  new  products  resulting  from  the  decomposition  of  the 
constituents  already  existing  in  the  beds,  with  the  addition,  perhaps,  from 
extraneous  sources,  of  a  little  quartz. 

The  conglomerates  and  breccias  interstratified  with  the  sediments  and 
tuffs  are  simply  these  rocks  with  the  addition  to  them  of  bowlders  and  frag- 
ments, mainly  of  preexisting  rocks  cast  out  through  the  volcanic  vents  or 
broken  from  ledges  by  the  action  of  the  waves,  but  occasionally  of  por- 
tions of  lavas  and  tuffs  shattered  in  the  process  of  solidifying.  In  the  latter 
case  the  fragments  are  very  similar  to  the  rock  masses  in  which  they  are 
embedded. 

The  lavas  associated  with  the  fragmental  rocks  are  rare.  They  consist 
of  altered  diabasic  or  basaltic  amygdaloids  that  have  lost  nearly  all  of  their 
original  structural  features. 

All  the  evidence  obtained  through  the  microscope  confirms  the  conclu- 
sion of  the  field  study,  viz:  That  the  Clarksburg  series  consists  of  an 
accumulation  of  the  ordinary  deposits  of  Ishpeming  and  Michigamme 
sediments,  with  interbedded  pyroclastic  material  erupted  by  a  volcano  whose 
principal  vents  are  located  by  the  greenstone  knobs  in  the  vicinity  of 
Clarksburg  village. 

In  their  present  forms  the  greenstones  are  altered  diabasic  porphyrites; 
the  lavas,  basalts  or  diabases;  the  pure  sediments,  graywackes  or  slates; 
and  the  tuffs,  "Schalsteins,"  where  much  weathered,  and  where  but  slightly 
weathered,  aggregates  of  amphibole,  biotite,  altered  plagioclase,  magnetite, 


486  THE   MAEQUETTE  IRON-BEARING  DISTRICT. 

garnet,  and  quartz.  The  garnets  and  amphibole  are  unquestionably  new 
products.  Both  are  idiomorphic,  and  both  occur  in  large  crystals.  The 
biotite  is  a  red-brown  variety  that  apparently  resulted  from  the  decom- 
position of  portions  of  the  original  materials  of  the  rock,  especially  where 
these  contained  some  proportion  of  sedimentary  material.  Where  these 
new  products  are  present  in  large  quantity  the  original  structure  of  the 
tuffs  has  nearly,  if  not  quite,  disappeared.  Where  they  are  scarce  the  rock 
retains  its  tuffaceous  character.  In  this  latter  case  fragments  of  plagioclase 
and  crystals  of  this  mineral  are  found  to  be  embedded  in  an  aggregate  of 
small  fragments  of  the  same  substance,  flakes  of  biotite,  spicules  and  plates 
of  chlorite,  fibers  of  green  hornblende,  grains  of  magnetite,  and  an  inter- 
stitial aggregate  of  cryptocrystalline  quartz.  Where  the  tuffaceous  material 
is  mixed  with  sedimentary  substances  there  are  found  also,  in  the  aggregate, 
rounded  grains  of  quartz.  In  the  foliated  phases  of  the  tuffs  and  mixed 
tuffs  and  sediments  the  biotite  exists  in  very  large  quantity,  so  large,  indeed, 
that  these  phases  often  resemble  biotite-schists.  In  them  a  few  quartz 
grains  are  observable,  and  a  fine-grained  matrix  that  appears  to  have  been 
derived  from  a  fine-grained,  clastic,  interstitial  filling  between  the  larger 
grains  of  the  original  rock. 


CHAPTER    V. 

THE  IGNEOUS  ROCKS. 

By  W.  S.  Bayley. 


The  igneous  i-ocks  associated  with  the  Marquette  sediments  and  ores, 
while  varying  in  their  present  character,  were  originally  nearly  uniform  in 
their  mineralogical  composition.  Although  occurring  as  bosses,  dikes,  inter- 
leaved sheets,  surface  flows,  and  tuffaceous  beds,  which  have  suffered  a 
greater  or  less  amount  of  alteration  into  products  which  are  now  not  a  little 
unlike  one  another,  they  were  all,  so  far  as  has  been  determined,  originally 
basic  rocks  of  the  composition  of  diabases.  The  variety  at  present  exhibited 
by  them  is  due  almost  exclusively  to  subsequent  alterations. 

Most  of  the  rocks  here  considered,  including  the  "diorites,"  "diorite- 
schists,"  "chlorite-schists,"  "magnesian-schists,"  "soapstones,"  and  "paint- 
rocks,"  have  been  regarded  by  some  geologists  as  metamorphosed  sedi- 
mentary fragmentals.  As  we  shall  discover  later,  there  is  not  a  particle  of 
evidence  for  this  assumption.  Even  the  most  schistose  of  these  rocks,  with 
the  possible  exception  of  some  of  the  "soapstones"  and  "paint-rocks,"  are 
certainly  of  igneous  origin.  The  pyroclastic  beds,  so  abundantly  developed 
in  the  western  portion  of  the  district,  and  constituting  a  large  portion  of  the 
Clarksburg  formation  of  the  Upper  Marquette  series,  are,  of  course,  frag- 
mental,  but  they  are  of  volcanic  and  not  of  sedimentary  origin. 

For  convenience  of  discussion  the  igneous  rocks  are  separated  into  two 
classes,  in  the  first  of  which  are  placed  those  associated  exclusively  with  the 
beds  below  the  Clarksbm-g  formation,  and  in  the  other  those  cutting  also 
the  beds  above  this  terrane.     The  latter  are  evidently  younger  than  the 

487 


488  THE    MARQUETTE   IKON-EEARmG   DISTRICT. 

Clarksburg  rocks,  while  the  former  are  believed  to  be  mainly  of  Clarksburg- 
age,  and  to  be  the  lower  portions  of  the  rock  masses  whose  surface  facies  are 
represented  in  part  by  the  flows  and  tuffs  that  constitute  the  main  mass 
of  the  Clarksburg  formation.  It  has  been  suggested  by  Lane  that  the 
younger  intrusives  cutting  the  Michigamme  formation  may  in  a  similar 
manner  be  dikes  from  the  volcanoes  that  yielded  the  Keweenawan  laA'as. 

The  Clarksburg  "greenstones"  are  discussed  in  connection  with  the 
other  rocks  of  the  Clarksburg  series. 

SECTION  I.— THE   PRE-CLABKSBUEG  GREENSTONES. 

The  igneous  rocks  associated  with  the  beds  older  than  the  Clarksburg 
formation,  and  especially  those  in  the  iron-bearing  formation,  have  been 
very  thoroughly  discussed  by  the  different  geologists  who  have  studied  the 
Marquette  district,  while  the  dikes  of  later  age  have  scarcely  been  men- 
tioned. Practically  all  the  references  to  "diorites,"  "greenstones,"  "  dio- 
ritic  schists,"  and  "  chloritic  schists"  that  are  met  with  in  the  literature  of 
the  district  apply  to  the  "  greenstones "  and  schists  in  the  iron  formation, 
and  these,  as  has  already  been  seen,  were  usually  regarded  as  interleaved 
sheets,  or  as  beds  of  metamorphosed  sediments. 

Structurally  the  pre-Clarksburg  eruptives  occur  ^principally  as  dike- 
like bosses  or  as  dikes,  although  sheets  and  tuff"  beds  are  also  known  to 
exist. 

The  dike  and  the  boss  masses  are  very  much  more  common  than  the 
other  structural  forms  of  the  greenstones,  and  are  those  that  have  hitherto 
been  studied  most  carefully.  They  may  be  divided  into  two  classes — 
those  occurring  as  typical  dikes,  and  those  forming  bosses  or  boss-like 
dikes.  There  are  no  essential  differences  between  the  rocks  of  the  two 
classes,  except  that  the  dike  masses  have  been  much  more  completely 
altered  than  the  boss  masses. 

The  boss  masses  form  the  large  knobs  of  "greenstone"  or  "diorite" 
that  are  so  prominent  a  feature  of  the  topography  in  the  neighborhood  of 
Ishpeming  and  Negaunee.  (Atlas  Sheets  XXVIII  and  XXIX.)  Some  of 
these  knobs  may  be  regarded  as  parts  of  very  large  dikes,  as,  for  instance, 
the  knobs  in  the  northwestern  portion  of  Negaunee,  which  together  form  a 


TUE   IGNEOUS   ItOOKS.  4^9 

high  ridge  some  2i  miles  in  length  and  not  more  than  an  eighth  of  a  mile  in 
width.  Other  assemblages  of  knobs  cover  irregular  but  neai'ly  equidimen- 
sional  areas.  There  can  be  detected  no  striking  differentiation  of  their 
parts.  They  are  nearly  uniform  in  composition  and  structure  throughout, 
and  hence  they  have  features  that  ally  them  with  boss  masses.  In  one 
instance  a  laccolitic  character  is  plainly  discernible  in  a  mass  of  greenstone 
that  has  raised  into  a  dome  the  griinerite-schists  which  cover  it.  The  place 
in  question  is  in  sec.  12,  T.  47  N.,  R.  29  W.  (Atlas  Sheet  XVI),  where  the 
relations  between  the  greenstone  and  schists  are  as  indicated  in  fig.  17, 
p.  330.     (See  also  PI.  XL) 

Geographically  considered,  the  knobs  are  not  found  east  of  the  center 
of  R.  26  W.  From  this  point  west  to  the  extreme  limit  of  the  mapped  area 
they  occur  in  greater  or  fewer  numbers,  being  most  abundant  in  the 
Ishpeming-Negaunee  mining  area  and  in  that  north  of  Michigamme  Lake 
and  along  Michigamme  River.  In  this  western  area  they  form  long,  naiTow 
ridges  rather  than  irregularly  shaped  knobs.  Moreover,  the  rocks  in  these 
ridges  have  a  composition  different  from  that  of  the  rocks  forming  the 
knobs  farther  east.  They  seem  to  have  suffered  dynamic  metamorphism 
to  a  greater  degree  than  the  eastern  rocks,  while  the  latter  have  suffered 
more  severely  from  the  effects  of  weathering.     (See  also  pp.  329-330.) 

THE    BOSSES. 
THE  EASTERN   KNOBS. 

RELATIONS    TO    MAKQUETTE    SEDIMENTS. 

The  relations  of  the  eastern  greenstones  to  the  rocks  with  which  they 
are  associated  prove  conclusively  that  they  are  intrusive  in  them,  and  are 
neither  interleaved  flows,  as  they  have  so  frequently  been  stated  to  be,  nor 
areas  of  the  Basement  Complex  from  which  the  Marquette  beds  have  been 
eroded,  as  was  supjjosed  by  N.  H.  Winchell.  That  they  are  intrusive  is 
shown  by  the  peripheral  dikes  extending  from  some  of  them  into  the  sur- 
rounding sedimentaries  and  by  the  nature  of  the  disturbances  created  in  the 
bedding  of  the  intruded  rocks  near  the  contacts  with  the  greenstones.  At 
the  south  of  the  hard-ore  open  pit  of  the  Lake  Superior  mine  in  Ishpeming 
(Atlas  Sheet  XXVIII)  a  number  of  small  dikes  may  be  seen  in  the  jaspers 


490  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

and  ores,  sometimes  being  parallel  to  the  bedding  of  these  rocks  and  some- 
times cutting  across  it.  In  the  underground  working  some  of  these  dikes 
may  be  traced  continuously  into  the  large  greenstone  bluflf  south  of  the 
mine,  and  others  into  that  east  of  the  pit.  A  few  of  the  dikes  still  preserve 
their  diabasic  structure,  but  most  of  them  have  become  "chlorite-schists"  or 
"  soapstones."  In  composition  they  are  identical  with  the  peripheral  schis- 
tose portions  of  large  knobs.  The  disturbance  created  in  the  bedding  of 
the  sediments  contiguous  to  many  of  the  boss  masses,  moreover,  is  of  such 
a  nature  that  it  admits  of  but  one  interpretation,  viz,  that  the  knob  green- 
stones are  irruptive  into  the  Marquette,  not  as  interleaved  sheets,  but  as 
true  bosses,  in  some  places  with  laccolitic  features.  In  the  majority  of 
cases  the  bedded  rocks  dip  away  from  the  contacts,  and  at  much  higher 
angles  near  the  greenstones  than  at  a  distance  from  them.  Besides,  wherever 
small  areas  of  the  bedded  rocks  lie  between  the  arms  of  an  irregularly  out- 
lined knob  the  Ijeds  are  usually  bent  into  little  folds,  with  axes  pitching 
away  from  the  greenstone.  Further,  a  glance  at  the  detailed  maps  of  the 
area  around  Ishpeming  and  Negaunee  (Atlas  Sheets  XXVIII,  XXIX,  and 
XXXI)  will  show  that  the  exposures  of  the  greenstones,  in  this  area  at 
any  rate,  are  so  irregularly  distributed  throughout  the  iron-bearing  forma- 
tion as  to  indicate  that  the  crystalline  rocks  are  bosses  and  not  interleaved 
sheets.  An  accurate  mapping  of  the  greenstones,  wherever  undertaken, 
effectually  disposes  of  this  latter  idea,  even  in  the  absence  of  the  intrusive 
phenomena  described  above.  It  would  seem  that  the  intrusive  relation  of 
the  knob  greenstones  to  the  bedded  rocks  is  settled  beyond  reasonable 
doubt. 

The  greenstone  knobs  are  very  much  more  abundant  in  the  iron- 
bearing  formation  than  in  any  other,  though  they  are  by  no  means  limited 
to  it.  They  occur  in  all  the  formations  of  the  Lower  Marquette  and  in  the 
lower  members  of  the  Upper  Marquette,  but  their  number  in  these  other 
formations  is  inconsiderable  and  their  size  small. 

PETROGRAPHICAL   CHARACTER. 

The  material  of  the  eastern  knobs  differs  in  no  essential  respects  from 
much  of  the  basic  material  intrusive  in  the  Basement  Complex.  All  the  rocks 
comprising  them  are  altered  diabases,  sometimes  coarse-grained,  sometimes 


THE   IGNEOUS   EOCKS.  491 

medium-grained,  and  rarely  fiue-grained.  In  many  of  them  the  diabasic 
structure  is  still  very  plainly  visible,  Avhile  in  others  it  has  been  lost  through 
alteration  and  through  mashing.  The  more  massive  of  the  greenstones, 
more  particularly  those  occurring  in  the  knobs,  are  the  miners'  "diorites." 

Macroscopically  the  material  of  the  eastern  knol)s  is  a  light  or  dark 
grayish-green,  mediumly  coarse  grained  rock,  that  is  rarely  massive.  Usu- 
ally some  trace  of  foliation  may  be  distinguished  in  the  hand  specimens. 
Frequently  the  schistosity  is  so  slight  in  amount  that  it  is  recognizable 
only  in  the  ledge.  In  other  cases  the  rocks  are  highly  schistose,  when  they 
merit  the  name  of  greenstone-schists.  While  the  schistosity  of  many  of  the 
knobs  is  more  pronounced  ai'ound  their  peripheries  and  along  joint  planes 
than  elsewhere  within  their  masses,  the  schistose  greenstone  may  occur 
anywhere  within  a  greenstone  area,  even  in  the  midst  of  great  areas  of 
perfectly  massive  rock.  The  intimate  relations  existing  between  the  schis- 
tose and  massive  greenstones  indicate  conclusively  that  both  are  phases  of 
the  same  rock  mass,  which  yielded  here  and  there  to  some  force,  Avitli  the 
result  that  motion  was  set  up  between  its  parts,  which  have,  as  a  conse- 
quence, become  schistose.  Along  joint  planes  are  often  shear  zones,  and  at 
these  places  the  rocks  are  as  completely  schistose  as  are  chlorite-schists. 

In  nearly  all  specimens  of  the  eastern  knobs,  even  in  some  of  those 
that  are  schistose,  the  diabasic  structure  may  be  detected.  When  not 
apparent  in  the  hand  specimen,  it  can  nearly  always  be  observed  in  the 
thin  section,  although  in  most  cases  all  traces  of  the  original  components 
of  the  rocks  have  disappeared  and  their  places  have  been  taken  by 
secondary  substances.  In  the  freshest  of  the  rocks,  which  usually  come 
from  the  centers  of  the  knobs,  cores  of  pale-violet  or  almost  colorless 
augite,  surrounded  by  rims  of  green  hornblende,  altered  plagioclase,  chlo- 
rite, epidote,  and  often  calcite,  are  arranged  in  the  typical  ophitic  manner. 
The  plagioclase,  by  its  alteration,  has  given  rise  to  the  chlorite,  ei^idote, 
and  calcite,  and  often  to  a  sei-icitic  substance,  which  in  some  instances 
appears  to  be  genuine  muscovite. 

A  typical  knob,  from  the  petrographical  standpoint,  and  one  which 
exhibits  all  the  varieties  met  with  in  the  eastern  knobs,  is  that  which  extends 
from  Gunpowder  Lake,  at  the  east  quarter  post  of  sec.  11,  T.  47  N.,  R.  27  W. 


492 


THE   MARQUETTE   IRON-BEARING   DISTRICT. 


(Atlas  Sheet  XXVIII),  southeastward  to  a  httle  beyond  the  east  quarter  post 
of  sec.  13  in  the  same  township.  Only  that  portion  of  the  knob  that  lies 
in  sec.  12  has  been  sectioned,  althougli  the  entire  knob  has  been  carefully 
sampled  (fig.  27).  Of  the  eleven  sections  made  from  this  portion  of  the 
knob,  three  contain  remnants  of  augite.  The  balance  of  the  specimens 
are  amphibole  rocks. 


1 
1 
.L 

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• 

"^ 

..^^ 

/ 

• 

•^^/60 

^^^/e6 

•?SI70 

i--... 

'""■■•••.. 

\  • 

^e^iya 

\   » 

,.•■••■ 

'■■..* 

'Bzisa 

*ee/69 

iBel73 

■■■    • 

" '  i 

, 

*s^i6e 

* 

1       I 

: 

■-... 

* 

Fig.  27.— Position  of  specimens  of  greenstone  from  south  half  of  sec.  12,  T.  47  N.,  E.  27  "W. 

The  commonest  phase  of  the  greenstone  is  a  dense  granular  rock 
showing  compact  hornblende.  Under  the  microscope  this  latter  mineral  is 
typical  uralite.  It  is  strongly  pleochroic  in  light  yellowish-green  and  dark 
bluish-green  tints,  and  is  often  bordered  by  dirty-brown  biotite.  The  horn- 
blende is  often  twinned,  and  often  it  possesses  idiomorphic  outHnes.  The 
plagioclase  is  not  very  abundant.  The  crystals  are  shattered.  Fibrous 
hornblende  of  the  same  nature  as  the  larger,  more  compact  grains,  together 
with  chlorite,  has  filled  the  fissures  thus  jiroduced.     Some  biotite  occurs  in 


THE   IGKEOUS  KOCKS.  493 

the  midst  of  even  the  most  compact  amphibole,  and  brown  ocher  colors 
portions  of  most  of  the  feldspars.  In  spite  of  the  fact  tliat  this  rock  has 
been  so  changed,  its  diabasic  structure  remains,  the  compact  hornblende 
occupying  the  place  of  the  original  augite,  while  the  altered  plagioclase,  the 
filjrous  hornblende,  and  most  of  the  biotite  and  of  the  chlorite  occupy  the 
place  of  the  original  feldspars. 

Some  of  the  rocks  from  the  interior  of  the  knob  (Specimens  22168 
and  22173)  are  dark-gray,  rather  dense-looking,  and  perfectly  massive, 
exhibiting  very  decided  luster-mottling,  and  showing  plainly  in  the  hand 
specimen  the  diabasic  structure.  Under  the  microscope  the  augite  that 
remains  is  found  to  be  limited  almost  exclusively  to  the  mottled  areas,  and  to 
be  the  mineral  to  whose  presence  the  latter  is  due.  The  pyroxene  is  almost 
colorless.  It  occurs  as  very  raggedly  irregular  cores  surrounded  by  light- 
green  hornblende.  It  is  penetrated  by  what  were  originally  plagioclase 
laths  but  are  now  largely  an  aggregate  of  small  saussurite  crystals, 
between  which  are  here  and  there  patches  of  feldspar.  The  green  horn- 
blende is  very  slightly  pleochroic.  It  borders  the  augite  mottlings,  and 
elsewhere  in  the  sections  it  occupies  wedge-shaped  areas,  penetrated,  like 
the  augite  areas,  by  laths  of  plagioclase.  Chlorite  is  also  present  in  small 
quantities,  and  it  likewise  is  in  ophitic  wedges.  The  plagioclase  has  nearly 
all  disappeared  into  its  decomposition  products.  Leucoxene  and  epidote  are 
the  only  other  important  minerals  present.  The  latter  is  in  small  yellow 
grains,  often  in  the  midst  of  chlorite,  and  the  former  in  fine  pseudomorphs 
after  crystals  of  ilmenite,  cores  of  which  occasionally  remain  in  the  centers 
of  the  leucoxene  masses. 

Other  varieties  of  the  rock  (Specimens  22159,  221G0,  and  221C9), 
from  well  within  the  rock  mass,  resemble  very  closely  the  augitic  phases 
described  above,  except  that  they  appear  a  little  more  altered,  and  their 
plagioclase  is  in  places  reddened.  Under  the  microscope  they  are  aggre- 
gates of  light-green  hornblende  and  chlorite,  often  stained  with  ocher,  in 
a  mass  of  altered  plagioclase  whose  principal  decomposition  products  are 
epidote,  saussurite,  and  chlorite.  The  epidote  is  embedded  in  chlorite, 
either  as  pale-yellow  plates  or  as  little  grains,  often  with  crystal  outlines. 
Occasionally  a  large  crystal  of   andesine,  or  of   some  plagioclase   whose 


494  THE   MAEQUETTE  IKON  BEARING  DISTRICT. 

chemical  composition  is  in  the  neighborhood  of  this  mineral,  is  embedded 
in  the  mass  and  so  gives  a  porphyritic  habit  to  the  sections.  Leucoxene  is 
abundant.  The  original  structure  of  the  rock  is  obscured  by  the  abundant 
secondary  substances  occumng  in  it,  but  the  ojihitic  texture  is  clearly 
apparent.  In  one  phase  (Specimen  22169)  the  amphibole  is  more  fibrous 
than  in  the  others,  but  in  the  center  of  an  amphibole  cluster  a  small  core 
of  augite  was  noticed. 

The  rocks  nearer  the  periphery  of  the  mass  (Specimens  22166,  22170, 
and  22172)  show  glistening  areas  of  hornblende  and  small  brilliant  laths 
of  plagioclase  in  a  dull-green  groundmass.  In  thin  section  they  are  appar- 
ently porphyritic,  for  large  crystals  of  feldspar  are  embedded  among  the 
small  laths  of  this  mineral  and  the  amphibole  which  together  make  up  the 
larger  portion  of  the  rocks.  None  of  these  rocks  differ  essentially  from 
those  above  mentioned.  Chlorite  is  more  abundant  in  them,  but  the 
amphibole  is  of  the  same  light-green  color,  and  is  present  in  the  same 
ophitic  areas.  The  plagioclase  is  more  altered  than  in  any  of  the  other 
rocks  mentioned,  but  its  original  outlines  can  still  be  recognized. 

At  the  very  edge  of  the  knob  is  a  dark-green  schistose  phase  of  the 
rock.  The  hand  specimen  is  made  up  of  small,  dark,  glistening  areas 
resembling  those  of  amphibole.  The  thin  section  is  a  nearly  uniform  aggre- 
gate of  tiny  chlorite  and  small  brown  biotite  flakes,  grains  of  magnetite 
or  ilmenite,  and  very  small  grains,  sometimes  laths,  of  clear  plagioclase. 
This  uniformity  is  broken  in  places  by  lenses  of  altered  plagioclase,  in 
which  badly  defined  feldspathic  substance  is  cut  by  spicules  of  chlorite. 

In  the  study  of  these  sections  from  a  single  rock  mass  we  become 
acquainted  with  the  different  forms  which  may  be  assumed  by  a  mediumly 
coarse  grained  diabase  under  the  influence  of  processes  that  are  mainly 
weathering  and  metasomatic  phenomena  but  partially  phenomena  due  to 
dynamic  agencies.  The  original  rock  was  a  coarse  diabase.  This  has  given 
rise  to  epidiorites  containing  secondary  fibrous  hornblende,  to  "diorites"  in 
which  the  amphibole  is  a  compact  and  sometimes  an  idioraorphic  variety, 
and  to  uralite-diabases.  Where  mashing  has  taken  place  in  addition  to  the 
weathering,  chlorite-schist  has  been  produced. 

There  is  not  a  particle  of  evidence  in  these  sections  that  the  schist  or 
any  of  the  forms  of  the  massive  greenstone  were  ever  fragmental  rocks. 


THE   IGNEOUS  ROCKS. 


495 


The  composition  and  structure  of  even  the  more  altered  forms  are  proof  that 
all  have  originated  by  ordinary  processes  from  an  igneous  magma. 

In  order  to  determine  the  chemical  differences  that  exist  between  the 
augitic  phases  of  the  greenstone  and  those  phases  in  which  all  the  augite 
has  been  changed  to  amphibole,  analyses  of  two  specimens  were  made  by 
George  Steiger  in  the  Survey  laboratory.     His  results  are  as  follows: 

Analyses  of  greenstones. 


I. 

11. 

SiOj 

48.44 

.74 

18.84 

9.56 

48.85 
1.28 

15.83 
2.50 

10.79 
.11 
6.20 

5.82 
1.31 
2.79 

3.77 
.06 

.22 

TiC 

A1"0, 

FeiO-5 

FeO 

CaO 

8.89 

MfO 

3.79 

P.O.. 



99.89 

I.— No.  22168.  From  SW.  i  sec.  12,  T.  47  N.,  R.  27  W.  Massive  greenstone  contain  ing  pale  augite 
in  fairly  large  quantity. 

II.— No.  22159.  From  a  point  near  No.  22168.  Typical  nonaugitio  greenstone,  like  the  greater 
portion  of  the  miners'  "diorites." 

Evidently  there  is  no  great  difference  in  the  composition  of  these  two 
rocks.  The  more  altered  one  contains  less  alumina  and  lime  than  the  less 
altered  one,  and  a  little  more  iron  oxide  and  magnesia;  otherwise  the  two 
are  similar.  These  analyses  afford  no  evidence  confirmatory  of  the  view 
that  in  the  formation  of  amphibolites  from  diabases  iron  oxides  are  removed 
and  condensed  into  ore  bodies.' 

The  rocks  of  the  other  eastern  knobs  present  few  features  different 
from  those  described  above.  From  six  different  knobs  specimens  were  col- 
lected whose  shdes  show  remnants  of  nearly  colorless  augite  when  examined 


I  Report  of  State  Board  of  Geological  Survey  for  1891  and  1892,  Lansing  [Mich.], 


3,  p.  180. 


496  THE   MAEQUETTE   lEON-BEARlNGl   DISTRICT. 

under  the  microscope.  In  most  of  the  rocks,  however,  no  traces  of  augite 
remain.  Hornblende  or  chlorite  has  taken  its  place,  and  the  rocks  are  now 
either  uralitic  diabases,  in  which  the  hornblende  is  compact  and  pseudomor- 
phic  after  the  augite,  or  epidiorites,  in  which  the  amphibole  is  fibrous,  and 
in  which  the  ophitic  structure  of  the  diabase  has  In  some  cases  entirely 
disappeared.  The  rocks  of  both  these  varieties  are  dark-green  in  color, 
fibrous  in  texture,  and  often  schistose  in  structiire. 

Under  the  microscope  they  are  found  to  be  composed  most  largely  of 
light-green  amphibole,  remnants  of  altered  plagioclase,  large  plates  of  epi- 
dote,  much  chlorite,  large  masses  of  very  beautiful  leucoxene  surrounding 
ilmenite,  and  nests  of  calcite.  In  the  freshest  of  the  uralitic  diabases  the 
augite  has  given  rise  to  compact,  dark-green  uralite,  and  the  plagioclase  to 
epidote  and  calcite.  In  the  epidiorites  the  amphibole  has  become  fibrous. 
Not  only  are  areas  formerly  occupied  by  the  augite  now  filled  with  fibrous 
amphibole,  but  long,  slender  needles  of  the  mineral  extend  far  out  into  the 
surrounding  rock  materials. 

The  plagioclase  in  these  rocks  has  suffered  extreme  alteration.  Its 
twinning  bars  have  nearly  disappeared  and  its  material  has  been  changed 
to  kaolin,  chloi'ite,  saussurite,  epidote,  and  calcite.  Sometimes  one  and 
sometimes  the  other  of  these  substances  predominates,  and  not  infrequently 
there  occur  scattered  through  the  slides  perfect  calcite  pseudomorphs  of 
lath-shaped  crystals  of  plagioclase  that  preserve  even  the  twinning  of  the 
original  feldspar. 

The  epidote  is  in  the  usual  green  plates.  While  often  an  alteration 
product  of  the  plagioclase,  it  is  sometimes  a  result  of  the  decomposition  of 
the  augite,  when  it  is  intex'mingled  more  or  less  freely  with  chlorite  and 
calcite.  "The  chlorite  is  in  little  nests  scattered  between  the  other  minerals, 
and  in  groups  of  fibers  pseudomorphing  feldspar.  It  is  an  alteration  of  the 
amphibole  as  well  as  of  the  plagioclase. 

Further  alteration  of  the  uralitic  diabases  and  the  epidiorites  gives  rise 
to  chloritic  rocks  in  which  chlorite  has  replaced  the  hornblende.  The 
chlorite  here  is  in  pale-green,  very  weakly  doubly  refracting  fibers  that 
form  pseudomorphs  of  the  amphibole  and  preserve  the  ophitic  texture  of 
the  original  rock.     Epidote,  calcite,  and  leucoxene  are  abundant  in  these 


THE   IGNEOUS   ROCKS.  497 

rocks,  and  sometimes  there  are  present  small  nests  of  secondary  quartz. 
All  plagioclase  has  entirely  disappeared.  The  forms  of  its  crystals  are 
preserved  by  calcite  and  epidote,  or  perhaps  by  calcite  alone,  which  pseudo- 
morphs  the  feldspar. 

Where  rendered  schistose  by  mashing,  as  happens  on  the  peripheries 
of  most  of  the  knobs,  the  alteration  of  the  greenstones  is  far  advanced. 
Chlorite  and  calcite,  with  a  little  magnetite,  quartz,  and  other  minerals  in 
small  quantity,  sometimes  constitute  the  entire  rock,  which  is  then  a  typical 
chlorite-schist.  In  no  cases  observed  are  the  greenstone-schists  enveloping 
the  more  massive  greenstones  of  sedimentary  origin,  as  might  be  inferred  to 
be  the  case  from  Wadswoi-th's  ^  earlier  statements.  In  all  cases  the  schistose 
rocks  differ  from  the  more  massive  ones  in  being  more  highly  foliated  through 
mashing,  which  Avas  naturally  more  easily  possible  on  the  peripheries  of  the 
rock  masses  than  elsewhere.  The  schists  often  exhibit  traces  of  the  diabasic 
structure,  even  when  greatly  altered.  The  degree  of  alteration  to  which 
they  have  been  subjected  appears,  however,  to  have  increased  with  the 
degree  of  the  foliation,  so  that  the  most  schistose  of  the  peripheral  rocks 
have  lost  all  traces  of  their  origin.  It  is  principally  through  their  gradation 
into  less  highly  foliated  jjhases  that  their  true  nature  is  made  out. 

Under  the  microscope  the  foliation  of  the  schists  is  plainly  seen  to  be 
an  effect  of  motion  in  a  solid  rock  mass.  Broken  crystals  of  plagioclase, 
crystals  faulted  along  cracks,  others  crushed  into  powder  on  their  borders, 
and  others  fissured,  with  their  cracks  filled  with  a  secondary  mosaic  like 
that  of  the  rock's  groundrnass,  all  bear  strong  evidence  that  the  rocks  in 
which  these  phenomena  are  found  have  been  at  some  time  subjected  to 
great  stresses.  The  foliation  is  due  to  the  arrangement  of  the  chlorite  and 
amphibole  in  j^arallel  fibers,  and,  since  the  direction  of  the  parallelism  cor- 
responds with  that  along  which  the  particles  of  the  broken  crystals  have 
been  moved,  it  is  concluded  that  the  schistosity  is  also  an  effect  of  pressure. 

Wliile  the  rocks  described  above  are  the  jjredomiuant  ones  in  the 
eastern  knobs,  there  is  another  type  that  should  be  mentioned.  A  consid- 
erable number  of  the  greenstones  are  dark-green  in  color  and  coarse- 
grained.    On  a  freshly  fractured  surface  brilliant  black  columnar  crystals  of 

'  Bull.  Mu8.  Comp.  Zool.  Harvard  Coll.,  Geol.  Series,  No.  1,  1880,  p.  41. 
MON   XXVIII 32 


498  THE   MARQUETTE   lEON-BEAEING  DISTEICT. 

hornblende  are  seen  lying  in  a  greenish-gray  groundmass,  and  producing  in 
places  a  well-defined  luster-mottling.  The  hornblende  of  these  rocks  is  a 
compact  blue-green  variety,  in  long  columnar  crystals  that  are  idiomorphic 
in  the  prismatic  zone.  Occasionally  they  are  aggregated  into  areas  resem- 
bling those  of  ophitic  augite,  but  usually  they  appear  to  be  scattered  indis- 
crimiuately  through  the  rock  mass,  which  consists  largely  of  chlorite, 
epidote,  and  the  remnants  of  very  much  decomjiosed  plagioclase.  The 
amphibole  has  undoubtedly  been  formed  at  the  expense  of  previoiisly 
existing  pyroxene,  for  the  hornblende  rocks  are  in  many  instances  but  local 
phases  of  well-defined  altered  diabases.  The  hornblende  has  grown  until 
it  has  extended  beyond  the  areas  formerly  occupied  by  augite  into  the 
matrix  produced  by  the  decomposition  of  the  plagioclase.  This  hornblende 
is  always  compact,  and  its  crystals  are  often  twinned.  The  rocks  containing 
them  are  "diorites"  in  structure  as  well  as  in  composition,  though,  of  course, 
they  are  not  diorites  which  have  crystallized  directly  from  a  magma. 

In  another  class  of  the  diorites,  represented  by  the  knob  in  the  center 
of  sec.  12,  T.  47  N.,  R  27  W.  (Atlas  Sheet  XXVIII),  the  compact,  apparently 
idiomorphic  amphibole  is  evidently  a  pseudomorph  of  augite.  Remnants 
of  pink  augite  may  still  be  detected  in  the  individual  hornblendes,  and 
occasionally  nearly  complete  crystals  of  the  minerals  may  be  observed. 
Rocks  of  this  kind  were  originally  augite-porphyiites. 

One  other  exposure  deserves  to  be  mentioned,  on  account  of  its  pecul- 
iar appearance.  It  is  on  the  north  side  of  the  Duluth,  South  Shore  and 
Atlantic  Railway  track,  in  the  garden  of  a  house  built  on  a  hill  about  half 
a  mile  east  of  the  Ishpeming  station.  The  rock  exposed  is  a  coarse-grained, 
slightly  foliated  one,  with  a  smooth,  glaciated  surface,  marked  by  concentric 
or  spiral  lines,  resembling  on  a  large  scale  the  perlitic  cracks  in  glassy 
rocks.  When  broken  the  fresh  fracture  of  the  rock  presents  no  unusual 
features.  The  phenomenon  noticed  on  its  exposed  surface  is  apparently 
that  of  spheroidal  weathering,  for  the  partings  which  produce  the  lines  do 
not  extend  any  considerable  distance  below  the  surface. 

Contact  phenomena  around  the  eastern  greenstones  are  rarely  seen. 
The  only  evidence  of  endomorphous  contact  action  noticed  in  any  of  the 
eastern  knobs  was  observed  in  that  forming  the  foot-wall  of  the  open  pit 


THK   IGNEOUS  EOCKS.  499 

near  the  NW.  corner  of  sec.  10,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVIII). 
In  other  phice.s  some  of  the  greenstones,  where  not  schistose,  are  slightly 
finer  grained  on  their  borders  than  in  the  interiors  of  the  knobs,  but  in  this 
case  the  greenstone,  near  its  contact  with  the  rocks  of  the  iron  formation, 
is  highly  charged  with  magnetite.  Some  of  the  magnetite  is  certainly  titan- 
iferous,  like  the  most  of  the  magnetite  of  the  normal  greenstone,  but  the 
greater  portion  of  it  is  nontitaniferous.  The  rock  consists  mainly  of  almost 
amorphous  chlorite.  Scattered  through  this  are  large  plates  of  a  colorless 
lamellar  mineral  that  appears  to  be  muscovite,  and  some  grains  of  quartz. 
A  few  long,  columnar  crystals  of  tourmaline,  pleoclu'oic  in  pink  and  very 
dark  Ijluish-green  tints,  are  also  scattered  here  and  there  among  the  other 
components,  but  its  presence  is  only  doubtfully  refeiTed  to  contact  action, 
as  toui'maline  has  been  found  in  small  quantities  in  other  greenstones  both 
of  the  eastern  and  of  the  western  knobs. 

THE   WESTERN   KNOBS. 
RELATIONS  TO   MARQUETTE   SEDIMEKIS. 

As  topographical  features,  the  western  knobs  differ  from  the  eastern 
ones  in  that  they  are  linear  and  dike-like  in  shape  rather  than  irregular  in 
outline.  The  most  typical  of  these  knobs  are  in  the  area  directly  north  of 
Lake  Michigamme,  where  they  constitute  well-defined  hills  rising  boldly 
as  bare  knolls  above  the  general  level  of  the  surrounding  country.  Sim- 
ilar greenstones  occur  also  in  the  Republic  trough,  being  best  know  n  at 
Republic  Mountain,  where  they  are  associated  with  the  jaspers  and  schists 
of  the  iron-bearing  formation.  At  this  place  they  are  not  higher  than  the 
iron-bearing  beds;  hence  they  appear  at  first  sight  as  interleaved  sheets  that 
have  been  planed  down  by  erosion  equally  with  the  iron-formation  rocks. 

That  the  linear  masses  are  intrusive  rather  than  eff"uslve  is  shown  by 
the  following  facts:  The  knobs,  while  ari-anged  in  approximately  straight 
lines,  are  not  continuous,  but  are  separated  from  one  another  by  little 
valleys  of  sedimentary  rocks;  occasionally  the  individual  knobs  are  not 
confined  to  a  definite  horizon  in  the  Marquette  series,  but  cut  across  the 
beds  of  a  formation,  or  even  cross  the  line  between  two  contiguous  forma- 
tions and  traverse  parts  of  each,  as  is  the  case  with  the  knob  in  sec.  21, 


500  THE   MAEQUETTE    IRON  BEARmG   DISTRICT. 

T.  48  N.,  R.  30  W.  (Atlas  Sheet  V);  further,  numerous  inclusions  of 
the  griinerite-magnetite-schists  are  found  at  one  place  within  the  green- 
stone (PI.  XII). 

"With  respect  to  the  Republic  greenstones,  Rorainger'  well  says: 

The  whole  slope  of  the  hillside  is  fomposed  of  an  endless  successiou  of  banded 
ferrugino-siliceous,  actinolite  schists,  united  into  bulky,  compact  masses,  which  are  here 
and  there  interrupted  by  intrusive  diorite  belts  of  short,  local  extension,  and  not,  as 
represented  by  Major  Brooks,  in  regular  continuous  bands  encircling  the  whole  side 
of  the  mountain. 

Although  it  is  clear  that  all  the  greenstones  of  the  western  knobs  and 
those  of  Republic  Mountain  are  inti-usive,  it  is  not  known  in  all 
whether  they  possess  the  features  of  dikes,  sheets,  or  bosses.  In  some 
as  at  the  Spurr  mine,  the  greenstones  are  apparently  interbedded  with  the 
sedimentary  formations ;  but  even  here  the  supposed  beds  are  of  short  linear 
extent,  and  may  be  dikes  that  happen  to  coincide  in  direction  with  the 
strike  of  the  bedded  rocks  at  the  position  of  the  present  plane  of  erosion. 

From  the  general  relations  of  the  western  greenstones,  exclusive  of  the 
smaller  dikes,  it  would  appear  that  most  of  them  are  boss-like  dikes  whose 
courses  on  the  surface  are  approximately  j)arallel  to  the  strike  of  the  sedi- 
mentary beds  intruded  by  them.  A  few  may  be  in  the  form  of  sheets,  but 
if  so  they  are  intrusive  and  not  surface  sheets,  and  hence  are  not  available 
for  working  out  the  structure  of  the  Marquette  series.  They  do  not  con- 
stitute well-defined  beds  at  definite  horizons  in  the  series,  as  was  supposed 
by  Brooks. 

PETROGRAPHICAL    CHARACTER. 

The  rocks  of  the  western  knobs  differ  materially  from  those  of  the 
eastern  knobs  in  their  microscopic  features,  although  some  of  them  are  very 
like  the  latter  macroscopically.  Originally  there  M'as  probably  no  difference 
between  the  two  types  of  rock.  In  the  eastern  greenstones,  however,  the 
alteration  was  degradational  in  its  nature.  The  diabases  have  passed 
through  epidiorites  and  chlorite-schists  into  aggregates  of  calcite,  chlorite, 
epidote,  etc.,  all  of  which  may  be  regarded  as  final  stages  in  the  weathering 
of  plagioclase  and  pyroxene.     In  the  western  greenstones  dynamic  processes 

'  Geological  Survey  of  Michigan,  Vol.  IV,  p.  101. 


THE    IGNEOUS   KOCKS.  501 

appear  to  have  played  the  more  important  role.  In  these  rocks  the  amphi- 
bole  is  dark-green  and  compact.  It  is  never  pale-gi'een  and  fibrous.  More- 
over, chlorite  is  rare,  except  locally,  while  fresh  brown  biotite  and  grains  of 
quartz  are  abundant,  and  in  many  cases  there  has  been  produced  a  mosaic 
of  the  latter  mineral  and  albite,  as  in  the  case  of  the  hornblendic  schists  of 
the  Basement  Complex. 

These  differences  between  the  eastern  and  the  western  greenstones 
may  be  explained  by  the  differences  in  form  and  in  the  geological  conditions 
under  which  the  rocks  are  found.  The  eastern  knobs  are  irregular  in 
shape  and  are  boss-like  in  their  features,  while  the  western  knobs  are  linear 
in  shape  and  dike-like  in  their  general  features.  The  former  were  able  to 
withstand  stress  more  successfully  than  the  latter,  and  so  have  suffered  less 
dynamic  metamorphism  than  these.  Moreover,  in  the  western  part  of  the 
Marquette  district  the  folding  and  mashing  of  the  formations  were  more  severe 
than  they  were  in  the  Ishpeming-Negaunee  area.  (See  Metamoi-phism, 
Chapter  VII,  pp.  573-575.)  Consequently,  as  a  rule,  the  western  green- 
stones are  more  schistose  than  the  eastern  ones.  They  are  darker  in  color, 
fresher  in  appearance,  and  more  crystalline-looking-.  Often  large,  brilliant, 
dark-green  or  black  amphiboles  lie  in  a  dark-green  groundmass,  through 
which  small,  sparkling  crystals  of  the  same  mineral  are  thickly  strewn,  with 
their  longer  axes  in  the  planes  of  schistosity.  In  their  macroscopic  features 
these  rocks  resemble  very  closely  schistose  diorites  and  camptonites. 

Under  the  microscope  all  their  sections  are  fresh-looking.  The  weath- 
ering products  so  noticeable  in  the  eastern  greenstones  are  rarely  observed. 
Biotite  is  their  characteristic  component.  It  occu.rs  as  large  and  small 
plates  of  a  deep  reddish-brown  color,  like  that  in  the  micaceous  schists  of 
the  Basement  Complex.  It  is  derived  very  largely  from  the  plagioclase. 
In  those  specimens  in  whicli  the  plagioclastic  nature  of  the  altered  feldspar 
is  still  clearly  apparent,  small  brown  biotite  flakes,  little  spicules  of  horn- 
blende, and  granules  of  epidote  are  scattered  in  large  quantities  through 
the  feldspathic  substance,  and  here  and  there  quartz  also  is  present.  Upon 
further  alteration  of  the  plagioclase  the  quartz  becomes  more  and  more 
abundant,  and  sometimes  secondary  albite  is  formed.  In  the  final  stage  of 
the  change  all  the  plagioclase  has  been  replaced  by  an  aggregate  of  biotite, 


502  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

quartz,  some  hornblende,  and  a  little  epidote,  with  the  addition  in  many- 
cases  of  a  quartz-albite  mosaic  between  these  components. 

The  hornblende,  as  already  mentioned,  is  a  dark-green,  compact  variety 
that  is  often  idiomorphic  in  cross-section  and  is  often  twinned.  Its  crys- 
tals lie  in  a  matrix  having  the  structure  and  composition  of  the  aggregate 
described  above  as  the  final  alteration  product  of  the  plagioclase.  In  some 
cases  the  aggregate  is  rich  in  biotite,  while  in  other  instances  biotite  is 
present  only  in  very  small  quantity,  and  the  aggregate  is  practically  a 
plagioclase-quartz  mosaic. 

The  larger  hornblendes,  which  appear  as  phenocrysts  in  the  hand  speci- 
men, differ  from  the  smaller  columnar  crystals  mainly  in  size.  Their  rela- 
tions to  the  aggregate  are  the  same.  In  both  cases  the  mineral  shows  its 
secondary  nature  by  the  frayed  ends  of  its  ciystals,  and  by  the  fact  that 
large  areas  of  almost  pure  hornblende  are  made  up  of  bundles  of  small, 
compact,  columnar  crystals.  Another  form  of  amphibole  is  frequently 
encountered.  In  this  many  individual  crystals  are  bound  together  in  sheaf- 
like bundles,  with  their  ends  extending  far  out  into  the  surrounding  ground- 
mass.  The  rocks  in  which  the  amphiboles  are  of  this  kind  resemble  very 
closely  the  "diorites"  and  altered  tuffs  of  the  Mona  formation.^ 

In  the  thin  section  the  schistosity  of  these  rocks  is  very  striking.  All 
their  constituents  are  arranged  with  their  longer  directions  approximately 
parallel,  and  lenses  of  mosaic,  with  their  major  axes  running  the  same 
way,  wind  in  and  out  among  the  other  components.  In  spite  of  their 
great  alteration  the  diabasic  structure  can  still  be  detected  in  some  of  the 
specimens,  especially  when  their  thin  sections  are  viewed  against  a  white 
background.  Under  the  microscope  this  structure  can  rarely  be  recognized, 
since  the  hornblendes  in  their  growth  have  extended  beyond  the  areas 
originally  occupied  by  the  augite.  Sometimes  hornblende  crystals,  biotite, 
and  chlorite  fill  ophitic  spaces  between  the  decomposition  products  of 
plagioclase,  bvit  these  cases  are  rare. 

As  the  schistosity  of  the  greenstones  increase  in  amount  their  material 
appears  to  become  better  crystallized,  except  where  the  foliated  phases  are  in 
contact  with  other  rocks,  in  which  case  they  have  suffered  not  only  extreme 

'  Cf.  fls.  1,  PI.  XVI,  Bull.  U.  S.  Geol.  Survey  No.  62,  by  G.  H.  Williams. 


THE    IGNEOUS   ROCKS.  503 

mashing-  but  great  chemical  changes  as  well,  for  they  are  now  often  j^ure 
chlorite-schists,  composed  of  a  solid  mesh  of  small  chlorite  fibers,  between 
which  are  occasionally  small  areas  of  quartz  mosaic,  tiny  grains  of  magnet- 
ite, biotite  flakes,  and  pyrite  crystals.  In  some  cases  quartz  mosaics  pseudo- 
morpli  the  original  plagioclase  grains,  and  in  others  large  garnets  occur 
scattered  indiscriminately  through  the  rocks.  These  latter  are  well  seen  at 
the  Michigamme  and  Spurr  mines  and  on  the  bordei's  of  some  of  the  boss- 
like greenstone  masses  in  Humboldt  Mountain,  where  they  have  been 
regarded  as  possibly  the  result  of  contact  action,  since  they  are  often  as 
well  developed  in  the  sedimentary  beds  contiguous  to  the  greenstones  as  they 
are  in  the  greenstones  themselves  The  garnets  in  the  greenstones  exhibit 
no  anomalies,  so  far  as  seen.  They  are  almost  colorless,  isotropic  bodies, 
crossed  by  irregular  cracks  and  containing  as  inclusions  a  great  many 
irregular  gi-ains  of  magnetite  and  very  irregularly  outlined  colorless  masses 
that  under  the  crossed  nicols  appear  to  be  quartz  and  plagioclase.  At  the 
Spurr  mine  many  of  the  garnets  are  more  or  less  completely  changed  to 
chlorite,  as  described  by  Pumpelly  in  1875. 

The  very  schistose  phases  of  the  western  greenstones,  where  they  are 
not  on  the  contact  with  the  sedimentary  rocks,  are  almost  typical  horn- 
blende-schists. This  is  especially  true  of  those  greenstones  in  the  Republic 
trough.  As  we  pass  .southward  from  the  Magnetic  mine,  in  sec.  20,  T.  47  N., 
R.  30  W.  (Atlas  Sheet  VII),  it  is  noticed  that  the  greenstones  become  more 
and  more  schistose  and  at  the  same  time  more  crystalline.  Their  quartz- 
ose  component  increases  in  quantity  until  in  some  of  the  rocks,  especially 
those  in  the  vicinity  of  Republic  (Atlas  Sheet  XI),  it  makes  up  a  large 
proportion  of  the  rock  masses.  Many  of  these  rocks  are  composed  of 
crystals  of  bright-green  amphibole,  often  with  idiomorphic  cross-sections, 
large  lenticular  grains  of  quartz  and  plates  of  epidote,  and,  between 
these,  masses  of  altered  plagioclase,  consisting  largely  of  kaolin,  epidote 
or  saussurite,  and  biotite.  Leucoxene  and  magnetite  also  occur  in  the 
schists,  the  former  mineral  with  the  habit  of  sphene  and  the  latter  with 
very  irregular  outlines.  Others  of  the  schists  contain  large  quantities  of  a 
bright-green  hornblende  with  extremely  ragged  contours  and  groups  of  this 
mineral  composed  of  numbers  of  small  grains  and  spicules  of  amphibole 


504  THE  MARQUETTE    IEON-BEARINCt   DISTRICT. 

variously  orientated,  several  together  often  forming  small  prismatic  crystals 
idiomorphic  in  the  prismatic  zone,  and  the  whole  forming  a  very  irregular 
area.  Nearly  all  of  the  areas  are  cellular.  Their  component  parts  are 
filled  with  inclusions  of  the  quartz,  feldspar,  epidote,  etc.,  that  make  up 
the  body  of  the  rock.  The  hornblende  in  these  instances  is  certainly 
secondary,  and  was  probably  the  last  of  the  principal  rock  constituents 
to  form.  Fig.  4  of  PI.  XXXII  illustrates  the  structure  of  one  of  the  clus- 
ters in  a  greenstone  from  Republic  Mountain. 

The  other  components  of  these  rocks  are  identical  in  form  and  nature 
with  those  of  the  predominant  schists. 

The  greater  schistosity  of  the  Republic  greenstones,  as  compared  with 
those  near  the  Michigamme  and  Spurr  mines,  and  their  more  crystalline 
character,  are  accounted  for  in  the  same  manner  as  are  the  greater  schistosity 
and  higher  degree  of  crystallization  of  the  western  greenstones  in  general  as 
compared  with  the  eastern  greenstones,  viz,  by  the  fact  that  in  the  Republic 
trough  the  mashing  of  the  igneous  rocks,  along  with  the  sedimentary  beds, 
was  greater  than  anywhere  else  in  the  Marquette  district  with  the  exception 
of  the  Western  trough. 

The  general  features  of  various  rocks  of  the  western  greenstone  knobs 
and  dike  masses  have  been  described.  Descriptions  of  the  special  features 
of  the  different  exposures,  so  vai'ied  are  they,  would  consume  more  space 
than  would  be  justifiable  in  a  discussion  which  is  not  purely  petrographical. 
A  brief  description  of  one  or  two  exceptional  phases,  however,  will  be  made. 

Some  of  the  greenstone-schists  deserve  mention  for  the  beautiful  clilo- 
ritoid  found  in  them.  This  mineral  has  all  the  properties  of  true  chloritoid. 
It  forms  large  plates  that  are  pleochroicin  greenish-blue  and  greenish-yellow 
tints,  and  has  an  extinction  of  1°  to  3°,  and  often  more.  Lane,  Keller,  and 
Hobbs  (see  Chapter  I,  pp.  129, 148)  have  described  this  chloritoid  very  fully. 
But  Lane  and  Keller  state  that  "all  the  Michigan  chloritoids,  so  far  as  yet 
known,  occur  in  altered  arkoses  or  similar  rocks."  In  the  present  instance, 
and  in  some  others  to  be  mentioned  later,  the  rocks  in  which  the  chloritoid 
exists  are  quite  certainly  mashed  eruptives.  They  consist  of  brownish-green 
biotite,  large  flakes  of  the  chloritoid  mentioned,  crystals  of  clear  and  almost 
colorless  epidote,  and  small  grains  of  magnetite,  forming  areas  between 


THE   IGNEOUS  KOCKS.  505 

which  are  other  Ughter  areas  composed  of  cjuartz,  satissurite,  calcite,  and 
large  irregular  cellular  garnets. 

A  singular  set  of  knobs  is  north  of  Lake  Corning,  in  the  SW.  ^  sec.  5, 
T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXV).  The  main  mass  of  these  knobs  is  a 
coarse-grained  uralitic  diabase  resembling  a  camptonite,  with  lath-shaped 
feldspars  and  hornblende  crystals  scattered  through  a  fine-grained,  grayish- 
green  matrix  (Specimen  19547).  On  the  top  of  the  hill  the  rock  is  coarser, 
and  its  feldspathic  constituent  is  in  patches  and  is  of  a  pinkish  tinge  (Specimen 
19.548).  The  hornblende  is  in  acicular  crystals.  Farther  east  the  feldspar 
crystals  are  larger  and  redder,  while  the  hornblendes  are  not  acicular 
(Specimen  19549).  Certain  patches  in  the  rock  were  taken  for  inclusions. 
They  are  pink  in  color  and  are  very  much  finer  in  texture  than  the  main 
mass  of  the  rock,  and  around  their  edges  they  are  bordered  by  bands  of 
green,  as  though  they  had  been  afi"ected  by  contact  action.  Other  patches 
are  epidotized  throughout.  In  both  cases  the  "inclusions"  resemble  fine- 
grained granites  that  have  been  altered  by  the  greenstone. 

In  thin  section  the  main  mass  of  rock  is  seen  to  have  a  coarse  diabasic 
structure.  The  hornblende  is  always  found  in  ophitic  areas,  either  alone 
or  with  chlorite  and  epidote.  The  plagioclase  is  changed  to  a  mixture  of 
saussurite,  calcite,  and  epidote,  which  is  cut  through  and  through  by  horn- 
blende spicules.  The  rock  is  thus  a  uralitic  diabase,  like  so  juany  others 
of  the  knob  greenstones,  and  not  a  camptonite,  as  it  appears  to  he  from  its 
macroscopic  habit. 

The  porphyritic  phases  on  the  top  of  the  hill  (Specimen  19549)  are 
composed  of  large,  partially  idiomorphic,  altered,  and  often  crushed  and 
reddened  plagioclases  in  a  matrix  of  small  laths  of  the  same  mineral 
and  small  triangular  masses  of  chlorite  that  have  evidently  been  derived 
from  hornblende.  Leucoxene  is  also  present  in  large  quantity.  The 
supposed  inclusions  are  fine-grained  diabases.  They  contain  only  small 
quantities  of  the  chloritic  interstitial  substance,  Avliile  large  quantities  of 
calcite  and  leucoxene  are  present  in  them.  The  epidotized  inclusions  are 
of  the  same  nature,  except  that  they  contain  an  abundance  of  epidote. 

The  "inclusions"  may  be  fragments  of  a  preexisting,  fine-grained  dia- 
base, caught  up  by  the  coarse  diabase  in  its  upward  passage,  or  they  may 


506  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

be  but  local  phases  of  the  latter  rock.  There  is  no  evidence  of  contact 
action  in  them.  The  epidotization  noticed  around  their  edges  is  but  one 
exhibition  of  the  general  epidotization  which  much  of  the  mass  of  the  knob 
has  suffered.  Nests  and  veins  of  epidote  are  common  at  the  eastern  end  of 
the  westernmost  knob,  and,  so  far  as  known,  they  bear  no  definite  relation 
to  the  occun-ence  of  the  "inclusions." 

THE  DIKES. 

The  typical  dikes  intrusive  in  the  pre-Clarksburg  rocks  can  not  be 
sharply  distinguished  from  the  boss-like  dikes  that  have  already  been 
described.  They  vary  in  width  from  a  few  inches  to  60  feet,  and  so  are 
distinguished  from  the  larger  masses  in  size.  Moreover,  their  walls  are  par- 
allel and  their  courses  usually  straight,  and  these  features  again  distinguish 
them  from  the  rocks  that  constitute  the  knobs.  However,  the  small  dikes 
often  are  but  apophyses  of  the  boss-like  dikes,  and  therefore  they  are  but 
portions  of  the  latter,  from  which  they  do  not  differ  in  any  essential  respects. 
The  rocks  that  originally  composed  them  were  of  the  same  nature  as  the 
materials  of  the  larger  masses;  at  present  they  differ  from  the  latter  to  a 
slight  degree  in  consequence  of  their  greater  proneness  to  alteration. 

While  many  of  the  small  dikes  are  composed  of  greenstone  identical 
in  composition  with  the  materials  of  the  knobs,  the  majority  consist  of  the 
highly  schistose  and  much  altered  rocks  which  constitute  the  "diorite- 
schists,"  "chlorite-schists,"  "soapstones,"  and  "paint-rocks"  of  the  miners. 
The  dikes  of  this  kind  are  sometimes  offshoots  of  the  great  knobs  of  green- 
stone; at  other  times  they  appear  as  isolated  bodies,  which,  however,  in  all 
probability  are  connected  underground  with  the  bosses  or  with  their  down- 
ward extensions. 

The  dike  rocks  are  in  all  respects  so  similar  to  the  boss  rocks  that  no 
doubt  could  arise  as  to  their  intrusive  nature  even  were  their  field  relations 
not  clearly  those  of  intrusives.  Every  gradation  exists  between  the  most 
schistose  dikes  and  those  which  still  preserve,  faintly  it  is  true,  the  diabasic 
structure.  These  obscure  diabases  are  identical  Avith  similar  rocks  forming 
the  knobs,  and  tlie  latter  may  be  clearly  traced  into  true  diabases,  in  which 
augite  may  still  be  detected.     None  of  the  chlorite-schists  and  none  of  the 


THE    IGNEOUS    ROCKS.  507 

schistose  greenstones  associated  with  the  iron-bearing  rocks,  so  far  as  known, 
are  of  fragmental  origin.     All  of  them  are  metamorphosed  igneous  rocks. 

In  some  cases  the  smaller  dikes  are  fresh,  dark  olivine-diabase  or  basalt. 
These  appear  to  be  independent  bodies  that  are  younger  than  the  schistose 
dikes  and  the  bosses,  for  they  traverse  the  latter  as  well  as  the  fragmental 
beds.  One  of  them  may  be  seen  on  the  side  of  the  cliff  overlooking  the 
lake  .shaft  of  the  Cleveland  mine.  Most  of  these  dikes  are  the  quartz- 
diabase  of  Lane,  and  are  identical  in  nature  with  the  fresh  diabases  cutting 
the  Archean  rocks  below  the  Lower  Marquette  series,  and  with  those  occur- 
ring in  the  upper  portion  of  the  Upper  Marquette  series.  They  are  therefore 
of  post-Clarksburg  age,  and  are  discussed  with  the  younger  dikes. 

The  small  greenstone  dikes,  like  the  boss  greenstones,  are  found  to  cut 
all  the  rocks  below  the  top  of  the  Clarksburg  formation,  but  also,  like  the 
boss  masses,  they  are  confined  principally  to  the  iron-bearing  formation. 
They  are  very  frequently  met  with  in  traversing  the  country  underlain  by 
this  formation,  but  are  even  more  numerous  than  they  appear  to  be.  Many 
of  their  exposures  are  small  and  badly  decomposed,  so  that  they  often 
escape  notice  even  where  not  completely  covered  by  loose  material.  In  the 
mine  workings,  however,  their  lower  extensions  are  brought  to  light,  when 
their  abundance  is  better  appreciated.  In  Chapter  III,  Section  VI,  they 
are  shown  to  be  closely  connected  with  the  accumulation  of  ore  bodies.  In 
those  areas  where  the  alteration  of  the  dikes  and  the  inclosing  schists  was 
not  carried  so  far,  as,  for  instance,  at  Mount  Humboldt  (Atlas  Sheet  XVI), 
where  the  iron  formation  is  chiefly  represented  by  griineritic  schists  rather 
than  by  jaspers,  ores,  and  ferruginous  schists,  the  dikes  may  easily  be  recog- 
nized on  the  surface,  and  their  number  is  fully  realized.  The  sketch  map 
(PI.  XXXIII)  of  the  W.  J  of  sec.  12  and  the  E.  J  of  sec.  11,  T.  47  N.,  R.  29 
W.,  shows  approximately  the  number  of  dikes  met  with  in  traversing  at 
intervals  of  one-eighth  mile  a  square  mile  of  territory. 

It  is  possible  that  some  of  the  greenstones  that  are  considered  as  dikes 
may  be  interleaved  sheets,  since  some  of  the  latter  are  chlorite-schists  that 
are  identical  with  the  chlorite-schists  of  undoubted  dikes.  The  number  of 
the  sheets,  however,  is  probably  not  great,  and  it  is  not  important  to  di.stin- 
guish  between  them  and  the  dikes,  especially  since  many  of  the  former  are 
undoubtedly  intrusive,  just  as  are  the  dikes  themselves. 


508  THE    MARQLTETTE    IRON-BEARING   DISTRICT. 


Many  of  the  dike  rocks  ai-e  so  similar  to  tlie  altered  forms  of  diabase 
characterizing-  the  knobs  of  the  eastern  portion  of  the  Marquette  district 
that  detailed  descriptions  of  them  are  unnecessary.  The  extremely  schistose 
varieties  of  the  greenstones  are  especially  prominent  in  the  smaller  dikes, 
including-  as  they  do  the  chlorite-schists  and  talc-schists,  the  "soapstones" 
and  the  "paint-rocks"  of  the  miners.  The  former  occur  throug-hout  the 
entire  district,  but  are  of  course  much  more  common  in  those  areas  Avhere 
there  has  been  a  great  amount  of  mashing-. 

The  great  majority  of  the  less  schistose  dikes  contain  large  quantities 
of  fibrous  amphibole  and  the  remnants  of  altered  plagioclase  Sometimes 
the  feldspar  is  rejjresented  by  saussurite,  quartz,  or  by  these  minerals,  calcite, 
and  chlorite;  at  other  times  by  a  sericitic  mineral  and  quartz;  and  again, 
very  frequently,  by  a  mosaic  of  quartz  and  fresh  feldspar  Biotite  is  also 
present  in  some  of  the  schists,  especially  those  containing  large  quantities  of 
the  quartz-feldspar  mosaic.  The  principal  bisilicate  constituent  is  always 
either  amphibole  or  amphibole  and  its  alteration  product,  chlorite.  Occa- 
sionally, even  in  the  schistose  rocks,  these  minerals  occur  in  ophitic  ai*eas, 
but  usually  the  mashing  has  been  so  great  that  the  darker-colored  com- 
ponents of  the  dike  rocks,  as  well  as  the  lighter  ones,  are  in  narrow  lenticular 
bodies.  The  cause  of  the  foliation  of  the  dike  masses,  like  that  of  the  boss 
masses,  is  clearly  seen  to  be  dynamic.  Not  only  are  the  components  nearly 
always  in  the  lenticular  forms  referred  to,  but  very  frequently  the  altered 
plagioclases  are  crushed  and  broken  and  their  fragments  moved  apart  in  the 
direction  of  the  foliation.  Moreover,  on  their  edges  the  larger  grains  are 
not  infrequently  granulated. 

The  petrographical  varieties  of  these  dike  rocks  are  very  numer- 
ous, but  their  features  are  in  general  not  different  from  those  of  the  boss 
masses.  A  few  varieties,  however,  should  be  mentioned  as  peculiar.  In 
one  or  two  instances  (as  in  Specimen  16180,  from  a  dike  8  feet  wide  in 
one  of  the  pits  in  the  NW.  ^  sec.  12,  T.  47  N.,  R  29  W.,  and  in  Specimen 
17505,  from  a  large  ledge  in  the  NW.  \  sec.  30,  T.  47  N.,  R.  30  W.)  the 
rocks  were  originally  porphyrites,  or,  perhaps,  basaltic  phases  of  diabase. 
The  first  of  these  rocks  now  shows  plagioclase  crystals,  that  are  more  or 


f     Mine    I  Pits     Sig^       || 

m 

juliiw'"'^"- 


.eOOfOm 


SKETCH   MAP  OF   THE    DIKES  OF   MOUNT    HUMBOLDT. 


THE   IGNEOUS   ROCKS.  509 

less  chloritized,  in  an  altered  groundmass  filled  with  skeleton  crystals  of 
magnetite.  Apparently  this  groundmass  was  originally  glassy.  It  now  con- 
sists of  chlorite  and  certain  indefinite,  brownish -green,  fibrous  substances 
in  a  light-colored  matrix  that  polarizes  feebly  in  places,  like  an  altered 
feldspar,  and  in  other  places  acts  like  an  isotropic  substance.  The  second 
rock  has  been  entirely  recrystallized.  It  now  consists  of  large,  fresh  plagio- 
clases  and  small  areas  of  quartz  mosaic  in  a  groundmass  cut  through  and 
through  by  light-green  amphibole  crystals  resembling  actinolite.  The 
interstitial  substance  between  these  is  a  mosaic  of  quartz  and  plagioclase. 
There  are  among  the  dike  rocks  a  few  otlier  porphyrites,  but  they  are  coarse- 
grained, and  differ  from  the  nonporphyritic  phases  merely  in  that  they 
contain  phenocrysts  of  plagioclase. 

The  most  schistose  phases  of  the  dike  rocks  include  the  chlorite-schists, 
the  talc-schists,  and  the  kaolin-schists. 

In  the  chlorite-schists  chlorite  predominates  over  all  other  components. 
The  rock  of  the  dike  at  the  old  Gilmore  mine  (Atlas  Sheet  XXXV),  near 
the  north  quarter  post  of  sec.  26,  T.  47  N.,  R.  26  W.,  for  instance,  is  an 
aggregate  of  bundles  of  a  fibrous  green  chlorite,  through  which  are 
scattered  laths  of  plagioclase.  The  latter  can  be  seen  only  in  polarized 
light,  since  in  natural  light  the  green  color  of  the  chlorite  obscures  them. 
A  little  limonite  stains  the  chlorite  here  and  there,  and  a  few  magnetites 
are  besprinkled  through  it.  Other  chloritic  schists  are  very  similar  to  this 
one,  though  not  many  of  them  are  so  nearly  devoid  of  components  other 
than  chlorite. 

In  the  rock  of  two  dikes,  one  at  Humboldt  and  the  other  835  steps  W., 
700  steps  N.,  of  the  SE.  corner  of  sec.  24,  T.  48  N.,  R.  31  W.,  are  great 
plates  of  chloritoid  of  the  same  character  as  that  in  the  western  knob 
greenstones  described  on  page  604.  In  the  first  dike  (Specimen  14777)  the 
chloritoid  is  in  beautiful  fresh  columnar  crystals,  having  an  extinction 
varying  between  0°  and  16°  and  twinned  parallel  to  their  longer  axes, 
and  in  large  plates  with  the  cellular  structure  of  secondary  minerals, 
which  plates  are  probably  basal  sections  of  the  columnar  crystals.  The 
chloritoid,  with  fibrous  chlorite,  forms  a  network,  in  whose  meshes  ai-e 
flakes  of  colorless  muscovite,  little  grains  of  epidote,  and  grains  of  quartz. 


510  THE    MAEQUETTE   IRON -BEARING   DISTRICT. 

Magnetite  grains  are  scattered  everywhere  throughout  the  section.  The 
rock  of  the  second  dike  differs  from  that  of  the  first  one  in  being  coarser- 
grained.  The  chloritoid  is  developed  in  large  plates  whose  pleochi-oism 
is  very  marked  in  greenish-blue  and  yellowish-green  tints.  Biotite,  musco- 
vite,  magnetite,  and  quartz  are  included  in  small  quantity  in  most  of  the 
plates,  though  some  of  them  are  entirely  free  from  inclusions  of  all  kinds. 
Between  the  plates  are  little  nests  of  calcite  and  large  irregular  areas  of 
plagioclase  and  its  alteration  products,  chlorite,  kaolin,  quartz,  biotite,  and 
needles  of  araphibole.  The  biotite  is  also  present  in  large  flakes  of  the 
usual  color,  and  the  quartz  in  large,  clear  grains,  the  former  mineral  being 
usually  in  the  feldspar  and  at  its  contact  with  the  chlorite,  and  the  quartz 
occurring  especially  between  chloritoid  plates. 

The  talcose-schists  are  rare.  They  are  limited  almost  exclusively  to 
the  ore  horizons,  where  the  processes  that  have  resulted  in  the  accumulation 
of  the  ores  have  at  the  same  time  leached  the  iron  salts  from  schistose 
diabases  and  made  them  talcose-schists  instead  of  chlorite-schists.  By 
further  leaching  magnesium  is  removed  and  the  schists  become  kaolin- 
schists.  Some  of  the  iron  salts  abstracted  from  the  diabases  may  have 
aided  in  the  formation  of  the  ore  bodies,  but  they  certainly  did  not  con- 
tribute the  main  bulk  of  the  ore  deposits. 

The  talcose-schists,  " soapstones,"  and  "paint-rocks"  are  varieties  of 
the  same  rock.  They  are  all  talcose-schists,  which  differ  from  the  chlorite- 
schists  in  the  fact  that  their  magnesian  component  is  talc  rather  than  a 
chlorite.  The  "soapstones"  are  the  almost  pure,  light-colored  phases  of 
this  rock,  while  the  "paint-rocks"  are  varieties  that  have  been  colored  red 
or  brown  by  the  infiltration  of  red  or  brown  ocher.  Many  of  these  rocks 
are  so  much  decomposed  that  little  remains  to  tell  their  history.  The 
"soapstones"  are  composed  largely  of  quartz,  talc,  probably  a  little  seri- 
cite,  and  calcite.  Chlorite  is  also  present  in  most  specimens.  In  some  it 
occurs  in  but  small  quantity;  in  others  it  is  more  abundant;  wliile  in  still 
others  it  is  present  in  such  large  quantities  that  these  rocks  must  be  regarded 
as  linking  the  purer  talcose-schists  with  the  typical  chlorite-schists.  In 
other  words,  there  is  an  unbroken  gradation  between  the  talcose-schists  and 
the  chlorite-schists.     Since  the  latter  are  altered  diabase  dikes,  the  former 


THE   IGNEOUS   ROCKS.  511 

are  probably  likewise  altered  diabases.  The  alteration  in  the  two  cases  is 
somewhat  different,  since  in  the  talcose-schists  all  of  the  original  iron,  which 
in  the  case  of  the  chlorite-schists  was  largely  retained  in  the  chlorite,  has 
been  lost.  The  talcose-schist  dikes  are  confined  mainh'  to  the  vicinity  of 
the  ore  deposits  in  the  soft-oi'e  mines,  while  the  chlorite-schists  are  less 
common  here  than  they  are  in  the  hard-ore  mines,  althovigh  they  are  prac- 
tically universal  in  their  distribution.  Evidently  the  talcose-schists  may  be 
looked  upon  as  chlorite-schists  from  which  the  iron  has  been  leached  by 
the  same  process  that  secreted  the  ore  bodies  in  their  neighborhood. 

The  "paint-rocks"  require  but  little  special  mention.  The  ocherous 
coloring  matter  coats  the  talc  fibers  and  the  grains  of  the  other  components 
in  the  schists,  into  which  it  has  evidently  been  introduced  since  the  rocks 
were  transformed  into  schists.  In  a  few  of  the  "paint-rocks"  the  coloring 
matter  is  magnetite  or  martife  rather  than  ocher  or  limonite.  The  oxide  in 
these  rocks  occurs  as  little  octahedi-a  embedded  in  the  talc  and  quartz. 
It  was  introduced  after  the  schist  became  foliated,  for  its  grains  are  scat- 
tered through  the  rock  mass  without  respect  to  its  foliation,  and,  so  far  as 
has  been  noticed,  they  have  produced  no  effect  upon  the  disposition  of  the 
talc  fibers  in  their  vicinity.  There  is  no  bending  of  the  fibers  around 
the  larger  crystals,  as  would  be  the  case  were  these  present  when  the  rock 
became  foliated,  but  they  are  cut  off  abruptly  by  the  opaque  iron  oxide, 
as  though  they  passed  directly  through  its  crystals.  Although  the  greater 
portion  of  the  magnetite  and  martite  crystals  are  opaque,  on  their  borders 
they  are  often  changed  to  hematite,  and  little  plates  of  this  substance  also 
occur  as  elongated  lenses  in  the  body  of  the  rock.  These  latter  plates 
appear  to  be  altered  forms  of  a  preexisting  magnetite.  They  probably 
represent  little  grains  of  this  substance  that  were  present  in  the  original 
diabase. 

There  are  a  few  other  light-colored  schistose  rocks  that  are  some- 
times met  with  as  dikes  cutting  the  iron-bearing  formation.  They  are  not 
usually  distinguishable  from  the  talcose-schists  in  the  hand  specimen, 
and  so  they  have  generally  been  confused  with  the  latter  rocks,  and  have 
been  called  by  the  same  names  as  these,  viz,  soapstones  when  light- 
colored,  and  "paint-rock"  when  stained.     Under  the  microscope  the  fibrous 


512  THE   MAEQUETTE    IRON-BEARING   DISTRICT. 

component  of  these  schists  is  discovered  to  be  kaoUn  instead  of  talc.  Patton^ 
has  ah'eady  described  one  of  these  dike  masses ;  so  we  quote  his  statements 
concerning-  it: 

The  iron  ores  in  the  region  around  Ishpeming  are  frequently  cut  by  (lilies  of 
diabase  so  thoroughly  altered  as  to  be  no  longer  recognizable  except  by  their  dike 
form.     *     *     * 

Under  the  microscope  this  kaolin-like  rock  shows  very  well  preserved  the  diabase 
structure  *  *  *  ^  but  in  place  of  the  plagioclase  laths  and  the  augite  grains  there 
is  a  white,  earthy  substance  with  interspersed  colorless  quartz  grains,  as  well  as  black 
magnetite.  The  magnetite  presents  about  the  same  appearance  as  in  the  unaltered 
rock,  and  is  evidently  the  only  ingredient  that  has  not  been  altered.  A  chemical 
analysis  of  this  rock,  made  by  Mr.  Fred.  F.  Sharpless,  shows  that  it  is  composed  of 
about  79  per  cent  kaolin  and  20  per  cent  free  quartz  (magnetite  and  impurities 
amounting  to  1  per  cent).  It  appears  that  in  the  process  of  alteration  Na,  Ca,  Mg, 
and  Fe,  as  well  as  SO2,  have  been  removed  and  HjO  has  largely  increased. 

Patton's  description  of  this  kaohn  rock  appHes  as  well  to  some  of  our 
specimens  collected  in  the  district.  The  rocks  are  evidently  in  the  form 
of  dikes.  They  represent  the  extreme  phase  of  alteration  to  which  the 
diabases  in  a  few  instances  have  been  subjected 

It  should  be  remarked,  before  leaving  the  discussion  of  the  talc-bearing 
and  other  light-colored  schists,  that  there  are  certain  other  rocks,  found 
associated  with  the  ores  in  some  of  the  mines,  that  resemble  very  closely 
the  schists  that  are  derived  from  diabases.  These  rocks  are  included 
by  the  miners  with  the  "soapstones"  that  occur  as  dikes.  They  are  white 
schists,  often  interbedded  with  quartzite  and  sometimes  immediately  over- 
lying the  hard-ore  deposit  at  the  base  of  the  Upper  Marquette  series.  In 
the  field  these  schists  may  occasionally  be  traced  into  well-characterized 
quartzites.  Under  the  microscope  they  are  found  to  be  fragmental  in  struc- 
ture and  to  consist  of  altered  and  mashed  feldspathic  quartzites,  not  very 
different  from  some  of  the  finer-grained  phases  of  the  Palmer  gneisses. 
Many  of  them  contain  considerable  quantities  of  sericite,  so  that  in  the  hand 
specimen  they  possess  the  same  soapy  feeling  that  is  possessed  by  the  talc- 
schists.  While  very  similar  in  appearance  to  the  true  soapstones  occvu*- 
ring  in  dikes,  these  rocks  are  not  soapstones.     They  are  sericite-schists, 

'  Microscopic  study  of  some  Michigan  rocks,  by  H.  B.  Patton :  Report  of  the  State  Board  of 
Ceological  Survey  for  1891  and  1892,  Lansing,  1893,  pp.  185-186. 


TIIK    KINEOUS   HOCKS.  513 

■which  may  often  be  distinguished  from  the  true  soapstones  by  the  coarse 
grit  in  their  powder,  due  to  the  presence  of  the  fragmental  quartz  grains. 

A  small  number  of  the  dark-colored  dike  greenstones  of  the  district  are 
abnormal  in  that  they  contain  no  amphibole.  Their  original  nature  has  not 
been  made  out.  As  at  present  constituted  they  ai'e  composed  of  altered 
feldspar  grains  in  a  mosaic  matrix  of  quartz  and  plagioclase,  containing 
reddish-brown  biotite  of  the  usual  kind.  Calcite,  sericite,  kaolin,  chlorite, 
and  epidote  are  mingled  with  the  other  components  in  large  quantities,  so 
that  the  rocks  as  they  now  exist  consist  exclusively  of  secondary  products. 
These  rocks  remind  one  strongly  of  the  less  highly  foliated  varieties  of  the 
micaceous  schists  in  the  Basement  Complex. 

CONTACT  EFFECTS. 

In  the  field  work  on  the  dike  rocks  extensive  and  careful  search  was 
made  for  contact  effects  in  the  intinided  rocks,  but  the  only  evidences  that 
were  thought  possibly  to  indicate  contact  action  were  noted  on  Mount 
Humboldt,  upon  Republic  Mountain,  and  near  the  Magnetic  mine.  The 
Mount  Humboldt  occurrence  is  the  most  characteristic.  Here  we  find 
cutting  the  griineritic  schists  of  the  northwest  portion  of  the  hill  a  highly 
schistose,  dark-gi-een  chlorite-schist,  which  on  its  edges  is  thickly  besprinkled 
with  dodecahedral  garnet  crystals  measuring  about  2  mm.  in  diameter. 
The  griinerite-schists  on  the  other  side  of  the  contact  are  likewise  crowded 
with  similar  crystals.  Other  crystals  of  the  same  kind,  however,  are  often 
met  with  in  the  schists  at  long  distances  from  visible  contacts  with  dikes, 
but  never  so  abundantly.  Therefore  the  unusual  concentrations  of  garnets 
along  the  dikes  are  believed  to  be  due  to  chemical  interactions  between  the 
two  rocks  subsequent  to  the  intnisions. 

The  chlorite-schist  in  the  middle  of  the  dikes  is  not  essentially  different 
from  the  chlorite-schists  already  described.  Near  the  contacts  some  of 
the  schists  contain  groups  of  griinerite  needles  identical  with  those  in  the 
griineritic  schists.  Idiomorphic  hornblende  crystals  are  also  occasionally 
observed  in  them,  and  magnetite  is  everywhere  present.  Fibrous  green 
hornblende  is  a  prominent  component,  and  biotite  is  abundant,  intermingled 
with  the  amphibole  in  some  specimens  and  in  others  accumulated  around 
MON  xxviii 33 


514  THE   MAKQUETTE   IRON-BE AEING  DISTEIOT. 

the  garnets.  The  garuets  are  identical  in  nature  with  the  garnets  in  the 
gametiferous  knob  greenstones.  They  are  perfectly  isotropic.  In  color 
they  are  very  light  pink,  almost  colorless.  Their  principal  inclusions  are 
magnetite,  but  they  always  inclose  also  small  quantities  of  chlorite,  quartz, 
biotite,  altered  feldspar,  and  often  crystals  of  pyrite  and  needles  of 
actinolite.  In  all  cases  the  garnets  are  crossed  by  great  fracture  cracks, 
which  are  sometimes  so  coarse  that  they  give  the  crystals  a  granular 
appearance.  In  these  rocks  the  garnets,  although  idiomorphic,  appear  to 
be  the  youngest  component  present,  with  the  exception  of  the  biotite,  which 
smTounds  them  peripherally,  and  which  must  be  regarded  as  constituting  a 
reaction  rim. 

Other  gametiferous  dike  greenstones  are  more  like  mica-hornblende- 
schists  than  like  chlorite-schists.  They  contain  large  quantities  of  quartz, 
and  are  thus  similar  to  the  quartzitic  greenstones  of  the  Republic  and  Mag- 
netic mines,  from  which  they  differ  mainly  in  the  possession  of  garnets. 
In  these  latter  rocks  the  garnets  are  often  granulated,  as  though  the  rock 
in  which  they  occur  had  been  mashed  after  their  formation. 

From  the  above  description  it  is  noticed  that  the  gametiferous  schists 
differ  from  the  other  schistose  geeenstones  simply  in  the  possession  of  gar- 
net and  in  some  cases  of  griinerite,  both  of  which  minerals  are  found  as 
normal  constituents  in  the  griineritic  schists  through  which  the  greenstones 
cut.  The  inference  from  these  facts  is  that  the  gametiferous  rocks  are  not 
contact  rocks  in  the  usual  sense,  but  that  their  peculiar  features  are  due  to 
reactions  between  solutions  passing  between  the  intruded  and  intruding 
rocks  and  carrying  dissolved  salts  from  the  one  into  the  other.  Mechanical 
action  may  also  have  been  instrumental  in  the  formation  of  the  new  products 
in  the  schists,  but  it  is  not  believed  that  the  heat  of  the  dikes  was  at  all 
effective  in  their  production.  The  phenomena  are  believed  to  be  those  of 
metasomatism  and  of  dynamic  metamorjjhism  rather  than  of  contact  action. 

THE  SHEETS  AND  TUFFS. 

Only  a  few  undoubted  sheet  eruptives  and  a  small  number  of  areas  of 
volcanic  fragmental  rocks  have  been  positively  identified  in  the  beds  older 
than  the  Clarksbm'g  formation.     In  one  or  two  instances  the  sheet  rocks 


THE   IGNEOUS  EOCKS.  515 

are  thought  to  be  surface  flows,  while  of  course  the  tuffs  are  always  surface 
rocks.  In  other  cases  the  sheets  apjjear  to  be  intrusive,  where  they  are 
with  difficulty  distinguishable  from  the  dikes.  Indeed,  while  it  is  believed 
that  by  far  the  greater  number  of  the  narrow  bands  of  schistose  greenstone 
associated  with  the  pre-Clarksburg  sediments  are  true  dikes,  it  is  thought 
that  a  large  number  of  them  may  possibly  be  sheets.  The  distinction 
between  the  two,  however,  is  not  so  important  as  it  would  be  were  the 
sheets  effusive  ones. 

THE   SHEETS. 

The  undoubted  sheet  rocks  are  exposed  in  a  few  places  on  the  surfaces 
or  sides  of  ledges  as  a  series  of  bands  of  a  fine,  crystalline,  green  rock, 
sometimes  amygdaloidal  on  both  edges,  sometimes  on  one  edge  only. 
These  rocks  are  interbanded  with  genuine  sediments,  with  which  they 
conform  in  strike  and  dip.  The  best  exhibition  of  the  bands,  which  are 
believed  to  represent  old  volcanic  flows,  is  on  the  little  hillock  in  the  NE.  ^ 
sec.  28,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVI),  where  three  greenstone 
beds  are  interstratified  with  graywacke-like  quartzites. 

Again,  in  the  cut  on  the  Chicago  and  Northwestern  Railway  in  the 
NW.  1  sec.  8,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXI),  about  1^  miles 
southeast  of  Negaunee  station,  there  is  an  excellent  section  exposed 
through  seven  bands  of  chlorite-schists  that  are  interleaved  with  "flag  ores." 
Rominger  gives  a  sketch  of  the  cut  in  his  report,^  and  describes  it  as  follows: 

The  strata  in  the  cut  form  two  successive  anticlinal  arches,  which  are  in  two 
phices  transversely  intersected  by  wedge-like  masses  of  chloritic  schists  intruded  from 
below.  The  upper  portion  of  the  ledges  is  formed  of  the  banded  alternating  beds  of 
pale  brownish  jasper  and  of  siliceous  ore  seams  like  the  jasper-banded  rocks  of  the 
McOmber  mines.    *     •     * 

As  shown  by  fig.  18,  p.  332,  the  present  authors  observed  but  a  single 
anticline  in  the  schist  bands,  and  discovered  portions  of  seven  sheets  of  the 
chlorite-schists  instead  of  the  two  pictured  by  Rominger.  The  intersecting 
"wedge-hke  masses  of  chloritic  schists"  were  not  seen.  Only  two  of  the 
sheets  show  the  complete  fold,  but  the  others  are  so  regularly  disposed 
about  the  axis  of  this  fold  that  there  can  be  little  doubt  that  the  bands  on 


C.  Rominger,  Geol.  Surv.  of  Michigan,  Vol.  IV,  p.  79. 


516  THE    MARQUETTE   IROXBEAEING   DISTRICT. 

its  two  sides  were  continuous  over  the  apex  of  the  anticline  before  this  was 
removed  by  erosion.  "Whether  they  were  surface  flows  or  intrusive  sheets 
is  not  known,  since  the  rock  composing  them  is  a  chlorite-schist  which  has 
lost  all  traces  of  its  original  structure.  It  seems  probable  that  these  sheets 
are  intrusive,  while  the  amygdaloidal  ones  described  below  are  effusive. 

The  few  sheet-rocks  discovered,  like  the  dike-rocks  of  the  district,  were 
originally  diabases  or  basalts.  They  are  now  composed  largely  of  amphi- 
bole,  chlorite,  and  altered  plagioclase.  They  are  either  massive  or  schistose 
in  structure,  with  the  massive  phases  porphyritic  in  texture.  Some  of 
the  latter  are  holocrystalline,  while  others  originally  possessed  a  glassy 
gToundmass. 

The  rocks  of  the  little  hillock  already  mentioned  as  existing  in  the 
NE.  J  sec.  28,  T.  47  N.,  R  27  W.,  originally  included  both  glassy  and  crys- 
talline phases.  In  the  former  the  glass  which  was  formerly  present  in  the 
groundmass  has  entirely  disappeared.  It  has  undergone  change  into  a  pale- 
o-reen  chloritic  substance.  The  resulting  rock  is  a  dense,  green,  schistose 
one,  whose  mass  is  speckled  with  bright,  glistening  surfaces  of  plagioclase. 
In  thin  section  small  plagioclase  crystals  and  groups  of  crystals,  some  fresh 
and  others  changed  to  calcite,  and  small  flakes  of  brown  biotite,  are  seen 
in  a  groundmass  of  smaller  plagioclases,  biotite  wisps,  tiny  amphibole 
needles,  and  granules  of  leucoxene  embedded  in  a  fibrous  mass  of  chlorite 
that  is  supposed  to  represent  the  original  glass. 

The  other  sheet-rocks  occurring  in  the  same  hillock  differ  from  the 
one  just  described  mainly  in  the  structure  of  their  groundmasses.  All  of 
them,  whether  amygdaloidal  or  not,  possess  an  altered  diabasic  groundmass 
in  no  wise  differing  from  the  rocks  that  have  been  called  uralitic  diabases 
and  epidiorites.  The  amygdaloids  occurring  here  are  dark-green  porphy- 
rites,  with  phenocrysts  of  plagioclase  and  hornblende,  and  amygdules  filled 
with  magnetite  and  calcite  in  an  altered  diabasic  groundmass  speckled  with 
magnetite.  Under  the  microscope  the  feldspar  phenocrysts  are  discovered 
to  be  changed  into  calcite,  and  the  green  groundmass  is  found  to  be  com- 
posed of  small  plagioclases,  pseudomorphs  of  calcite  after  plagioclase,  green 
chlorite,  and  leucoxene,  with  here  and  there  some  epidote  and  a  few  grains 


THE   IGNEOUS   EOCKS.  517 

of  magnetite.  The  nonaiiiygdaloids  are  like  the  amygdaloids  except  that 
they  lack  amygdules. 

The  only  essential  difference  between  these  sheet  eruptives  and  those 
occurring  in  dikes  is  in  their  structure,  which  in  the  former  rocks  is  por- 
phyrltic  and  in  the  latter  ophitic. 

The  chlorite-schists  interbedded  with  the  iron-formation  rocks  at  the 
cut  in  the  Chicago  and  Northwestern  Railway  are  in  all  respects  like  the 
chlorite-schists  that  have  been  described  as  forming  dikes. 

TUB   TUFFS. 

The  tuff  deposits  among  the  pre-Clarksburg  rocks  are  not  widely 
spread.  So  far  as  known  they  occur  only  at  two  localities,  where  they 
are  very  closely  associated  with  knobs  of  greenstone.  One  of  these  is  the 
western  end  of  the  knob  near  the  east  quarter  post  of  sec.  20,  T,  47  N., 
R.  27  W.  (Atlas  Sheet  XXVI),  and  the  other  is  the  western  end  of  the 
northern  of  the  two  hills,  near  the  east  quarter  post  of  sec.  4  (Atlas  Sheet 
XXV),  in  the  same  township.  The  relations  of  the  tuffs  to  the  massive 
rocks  are  not  clear  at  either  of  these  places.  In  the  first  locality,  how- 
ever, the  tuffaceous  beds  are  cut  by  large  dikes  of  greenstone,  similar  to 
the  rock  that  forms  the  main  body  of  the  hill.  The  recognition  of  these 
rocks  as  fragmental  eruptives  is  due  to  microscopic  evidence  and  to  the 
conglomeratic  aspect  of  their  weathered  surfaces.  Light-gray,  pebble-like 
areas  appear  in  a  schistose  green  matrix  containing  small  fragments  of 
altered  feldspar  and  pieces  of  green  schists. 

The  tuffs  are  composed  of  exactly  the  same  minerals  as  are  found 
in  the  schistose  dike-rocks  and  other  greenstones,  but  their  structure  is 
obscurely  tuffaceous.  Under  the  microscope  fragments  of  plagioclase  may 
be  seen  distributed  thickly  among  the  other  rock  components,  and  large, 
rounded  and  irregular  fragments  of  a  fine-grained  diabase  may  be  detected 
here  and  there  embedded  in  a  schistose,  hornbleudic,  and  chloritic  ground- 
mass.  These  tuffs  are  not  unHke  those  occurring  in  the  Basement  Complex. 
They  differ  from  the  Kitchi  schists  mainly  in  the  manner  and  degi'ee  of 
their  alteration. 


518  THE  MARQUETTE   lEON-BEARraG  DISTRICT. 


SECTION   II.-THE   POST-CLARKSBURG   GREENSTONES. 

The  freshest  of  the  igneous  rocks  of  the  Marquette  district  are  those 
which  constitute  the  independent  dikes  that  have  ah-eady  been  mentioned 
as  being  in  all  probability  the  youngest  dikes  in  the  entire  area,  because 
they  cut  indifferently  the  larger  dikes  of  altered  diabase  and  all  the  forma- 
tions of  both  the  Upper  and  the  Lower  Marquette  series,  and  because  they 
are  only  locally  schistose. 

Most  of  these  younger  rocks  exist  as  well-defined  dikes  with  sharply 
marked  walls,  although  in  several  instances  they  occur  also  as  boss  masses. 
They  are  found  as  dikes  in  the  Basement  Complex  on  Light-House  Point 
(Atlas  Sheet  XXXVIII);  in  the  Lower  Marquette  series,  cutting  the 
quartzites  and  marbles  southwest  of  Lake  Mary,  in  T.  47  N.,  R  25  W. 
(Atlas  Sheet  XXXVII);  in  the  iron-bearing  formation  at  many  of  the 
mines;  and  in  the  Upper  Marquette  series  in  the  Ishpeming  and  Michi- 
gamme  formations,  especially  in  the  western  portion  of  the  area  mapped. 
They  are  even  more  abundant  still  farther  west,  in  the  slates  between 
Michigamme  and  L'Anse,  beyond  the  limits  of  the  map. 

The  boss  masses  of  these  younger  greenstones  are  rare  so  far  as  the 
area  under  discussion  is  concerned.  The  most  typical  occurrence  is  that 
of  the  knob  in  sees.  35  and  36,  T.  48  N.,  R.  30  W.  (Atlas  Sheet  VIII),  on 
the  shore  of  Lake  Michigamme. 

PETROGRAPHICAL   CHARACTEB. 

Petrographically  these  greenstones  are  aphanitic  to  medium-grained, 
dark-gray  or  black  basic  rocks,  in  which  secondary  products  may  be  abun- 
dant, but  in  which  the  original  structures  are  well  preserved.  As  has  already 
been  indicated,  these  rocks  are  scarcely  ever  schistose,  in  which  respect  they 
are  sharply  contrasted  with  the  greenstones  of  pre-Clarksburg  age.  They 
are  massive  rocks  that  resemble  strongly  some  of  the  dike  and  sheet  rocks 
of  the  Keweenawan  series  on  Keweenaw  Point,  and  hence  they  have  been 
thought  by  Lane  to  be  the  lower  portions  of  the  Keweenawan  eruptives,  in 
the  same  way  that  the  pre-Clarksburg  intrusives  are  regarded  as  the  lower 
portions  of  the  Clarksburg  greenstones. 


THE   IGNEOUS   HOCKS.  519 

The  principal  lithological  types  recognized  among  the  younger  green- 
stones are  diabases,  porphyrites,  and  basalts.  The  diabases  are  sometimes 
nonolivinitic,  but  more  frequently  they  contain  olivine  in  different  stages 
of  alteration,  and  often  small  quantities  of  quartz,  usually  in  micropeg- 
matitic  intergi'owths  with  feldspar.  The  porphyrites  are  mainly  diabase- 
porphyrites  in  which  plagioclase  is  the  principal  if  not  the  only  phenocryst. 
The  basalts  differ  from  the  diabases  in  the  possession  of  a  distinct  groundmass. 

QUARTZ-DIABASES. 

The  most  characteristic  of  the  younger  greenstones  are  those  that  have 
been  called  quartz-diabases  by  Lane.^  They  are  more  frequently  found 
west  of  the  area  discussed  in  this  monograph  than  within  it,  though  an 
excellent  representative  of  the  type  is  the  rock  constituting  the  mass  of 
the  knob  on  the  shore  of  Lake  Michigamme.  In  his  descriptions  of  these 
rocks  Lane  states  that  they— 

are  always  massive,  and  of  a  dark,  black,  or  brownish  gray  color  with  white  specks 
of  glassy,  more  or  less  lath-shaped  feldspar  that  shows  with  a  pocket  lens  twinning 
lines  on  the  cleavage  faces.  In  the  thin  section  we  see  that  the  other  components 
are  magnetic  iron  ore  and  a  brown  augite  that  has  more  or  less  of  a  violet  tinge.  AH 
these  components  have  at  times  their  own  crystalline  shape,  and  the  interstices 
between  them  I  have  called  acid  interstices.  They  are  similar  to  those  described  by 
A.  C.  Lawson  (American  Geologist,  1891,  Vol.  VII,  p.  153)  from  the  Rainy  Lake  region. 
Where  augite  comes  in  contact  with  these  interstices  it  is  coated  with  a  dark-brown 
hornblende  like  basaltic  hornblende  and  utterly  unlike  uralite.  When  feldspar 
adjoins  them  we  can  tell  by  its  optical  properties  that  from  being  at  the  center  a  lime- 
soda  feldspar,  like  labradorite,  the  soda  more  and  more  predominates  as  we  go  toward 
the  margin,  until  at  the  margin  we  often  have  growths  springing  out,  to  form  with  the 
quartz  what  is  known  as  micropegmatite.  These  growths  sometimes  fill  the  whole 
remaining  space.  At  other  times  there  is  some  quartz  in  compact  grains.  Another 
curious  feature  of  these  interstices  is  that  they  are  often  crossed  in  all  directions  by 
needles  of  apatite.  *  *  *  Folia  of  biotite  often  occur  in  the  interstices  near 
magnetite,  sometimes  evidently  derived  from  it. 

These  interstices  occur  in  their  most  characteristic  forms  in  the  freshet  rocks,  and 
their  structure  can  not  be  due  to  weathering  nor  to  pressure,  for  it  occurs  in  rocks 
which  show  no  trace  of  pressure.     It  seems    *     *     *     that  when  the  rock  consoli- 
dated there  were  left  interstices  filled  with  hot  alkaline  water  or  dissolved  water-glass, 
1  Report  of  the  State  Boanl  of  Geological  Survey  for  the  years  1891  and  1892,  Lansing,  1893,  p.  177. 


520  THE   MARQUETTE    IRON-BEARING   DISTRICT. 

which  was  probably  the  last  residue  of  the  lava.  *  *  *  into  these  interstices  the 
apatite  needles  grew,  and  the  alkaline  solution  attacked  the  augite  and  magnetite, 
turning  them  into  brown  hornblende  and  mica.  Finally  the  heated  solution  cooled, 
depositing  the  quartz  and  feldspar.    *     *     * 

*  *  *  Olivine,  or  its  alteration  product,  serpentine,  may  often  be  observed 
microscopically  as  an  occasional  accessory,  especially  in  marginal  forms,  but  is  seldom 
abundant  enough  really  to  characterize  the  rock,  and  certainly  is  not  characteristic  of 
the  group  as  a  whole. 

The  rock  of  the  knob  in  sees.  35  and  36,  T.  48  N.,  R.  30  W.  (Atlas 
Sheet  VIII),  corresponds  very  closely  in  the  main  with  Lane's  description 
except  that  no  olivine  has  been  detected  in  the  few  slides  made  from  it. 
The  augite,  moreover,  is  more  or  less  altered  into  green  amphibole  and 
chlorite,  and  the  cleavage  cracks  in  the  plagioclase  are  lined  with  the  latter 
mineral.  It  is  also  noticed  that  large  crystals  of  titaniferous  magnetite 
occur  amid  the  interstitial  substances  Since  this  mineral  is  one  of  the 
oldest,  if  not  the  oldest,  in  the  rock,  its  presence  among  the  interstitial 
substances  may  indicate  that  these  have  not  in  all  cases  the  origin  ascribed 
to  them  by  Lane. 

Another  variety  of  rock  from  this  knob  exhibits  a  different  structure 
from  that  described.  In  a  few  specimens  the  augite  is  in  idiomorphic 
grains  and  the  magnetite  in  large  skeleton  crystals.  In  more  altered  forms 
quartz  is  present  as  large  gi'ains  occupying  the  centers  of  the  interstitial 
spaces,  and  surrounding  them  are  beautiful  feathery  growths  of  gi-anophyric 
quartz  and  feldspar.  In  other  cases  the  granophyre  extends  from  a  nucleus 
of  plagioclase,  or  forms  a  zone  around  plagioclase  laths,  while  again  its 
areas  possess  the  outlines  of  feldspar  crystals.  If  the  granophyre  repre- 
sents an  acid  base,  then  the  interstices  in  these  specimens  occupied  a  greater 
volume  than  did  the  solid  portions  at  the  time  the  micropegmatite  began 
to  form.  It  would  seem  probable  that  some  of  the  granophyi-e  at  least  is 
secondary. 

OLIVINE-DIABASES. 

Some  of  the  quartz-diabases,  as  Lane  observes,  contain  olivine  in  small 
quantity.  There  is  another  class  of  dike-rocks,  however,  in  which  olivine 
is  an  important  component.  These  are  fresh,  heavy,  basaltic-looking 
rocks,  which  under  the  microscope  appear  as  very  fresh  olivine-diabases. 


THE   IGNEOUS   KOCKS.  521 

Occasionally  their  olivine  is  changed  more  or  less  completely  into  a  green, 
earthy  decomposition  product,  which  extends  out  from  the  olivine  grains 
and  fills  cracks  in  the  neighboring  plagioclases,  which  are  usually  quite 
fresh.  These  rocks  present  no  unusual  features.  They  are  typical  olivine- 
diabases.  The  best  example  of  the  type  is  found  on  Green  Island,  in  Lake 
Michigamme,  situated  in  the  SE.  i  sec.  27,  T.  48  N.,  R.  30  W.  (Atlas 
Sheet  VIII.) 

PORPHYUITES. 

The  porphyrites  differ  from  the  olivinitic  diabases  and  the  quartz- 
diabases  in  the  absence  of  quartz  and  olivine  and  in  the  presence  of  por- 
phyritic  plagioclases.  These  rocks  resemble  the  porphyritic  phases  of  the 
older  greenstones,  from  which,  however,  they  are  distinguished  by  their 
greater  freshness  and  by  the  retention  of  their  ty|3ical  porphyritic  structure. 

Although  all  the  porphyrites  are,  on  the  whole,  so  very  much  fresher 
than  the  pre-Clarksburg  greenstones,  they  nevertheless  show  some  altera- 
tion. Their  principal  alteration  products  are  those  characteristic  of  weather- 
ing processes.  Decomposition  products  resulting  from  dynamic  processes 
are  rare,  except  along  local  shear  zones  within  the  masses  of  the  dikes  and 
along  their  borders.  When  much  altered  the  rocks  do  not  differ  greatly 
from  the  older  greenstones  in  the  eastern  knobs,  except  that  their  original 
structure  is  so  much  better  preserved  that  it  can  nearly  always  be  observed 
in  the  hand  specimen. 

A  single  section  made  from  a  specimen  collected  from  a  dike  cutting 
the  rocks  of  the  iron  formation  in  one  of  the  pits  of  the  Jackson  mine,  in  the 
SE.  i  sec.  1,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXVIII),  is  enough  different 
from  the  types  described  above  to  deserve  mention.  The  rock  is  appar- 
ently a  porphyritic  diabase.  In  the  hand  specimen  it  shows  white  plagio- 
clase  groups  in  a  dark-gray  groundmass  that  is  cut  by  small,  almost  silky, 
white  fibers.  Under  the  microscope  the  section  shows  only  fresh  pla- 
gioclase,  magnetite,  and  olivine.  The  plagioclase  is  in  the  usual  lath-like 
forms,  which  are  very  small  in  some  portions  of  the  section,  and  in  others 
are  grouped  together  into  large  complex  aggregates.  These  latter  consti- 
tute the  larger  white  areas  seen  in  the  hand  specimen,  while  the  small 
isolated  crystals  are  the  tiny  fibers.     Many  of  the  feldspars  are  zonal,  with 


;522  THE  MARQUETTE  IRON  BEARING   DISTRICT. 

cores  of  altered  and  reddened  plagioclase,  surrounded  by  jjerfectly  fresh 
material.  The  magnetite  is  the  most  interesting  component.  It  occurs  in 
the  ophitic  spaces  between  the  plagioclases,  and  is  apparently  all,  or  nearly 
all,  a  decomposition  product  of  augite  or  of  olivine,  remnants  of  which 
have  been  left  in  its  mass.  No  traces  of  the  latter  mineral  have  been 
detected  in  the  section,  but  from  the  shapes  of  some  of  the  magnetite 
areas  it  is  thought  very  probable  that  pyroxene  was  once  present  in  the 
rock.  The  complete  substitution  of  augite  and  olivine  by  magnetite  is 
unusual  in  rocks  of  this  character.  The  processes  to  which  the  substitution 
is  due  in  the  present  instance  were  probably  related  to  the  processes  that 
gave  rise  to  the  ores. 

BASALTS. 

The  rocks  that  have  been  referred  to  as  basalts  are  rare.  They  consti- 
tute well-defined  dikes.  In  the  hand  specimen  they  are  dark,  dense  rocks, 
occasionally  dotted  here  and  there  with  small  white  spots  consisting  of 
plagioclase.  Under  the  microscope  plagioclase  laths,  magnetite  grains,  and 
small  spicules  of  augite  are  detected  in  a  glassy  groundmass,  which  is 
often  filled  with  green  alteration  products,  and  sometimes  almost  entirely 
replaced  by  these  substances. 

SUMMARY. 

The  igneous  rocks  occurring  in  the  Marquette  series  are  all  basic,  with 
the  composition  of  fresh  or  altered  diabases  of  various  kinds.  They  occur 
as  dikes,  bosses,  sheets,  and  tuff  beds.  An  association  of  sheets  and  tuff 
beds  is  found  to  constitute  a  well-defined  horizon  in  the  Upper  Marquette 
series.  These  rocks,  called  the  Clarksburg  formation,  have  consequently 
been  sepai'ated  in  the  discussion  from  the  other  igneous  rocks.  The 
remainder  of  the  igneous  rocks,  which  may  be  classed  together  under 
the  convenient  and  noncommittal  name  of  greenstones,  have  been  further 
divided  into  two  classes.  In  one  of  these  ai-e  placed  all  the  green- 
stones associated  only  with  rocks  older  than  the  Clarksburg  formation, 
and  in  the  other  those  associated  also  with  the  beds  younger  than  this 
formation. 

Among  the  jjre- Clarksburg  greenstones  dikes  and  boss  masses  are 
common,  while  sheets  exist  to  some  extent,  and  tuffs  are  rare.     The  dikes 


THE   IGNEOUS   ROOKS.  523 

and  boss  masses  are  very  siiuilar  in  the  nature  of  their  material.  Botli  com- 
prise schistose  forms  of  diabase,  in  which  all,  or  nearly  all,  of  the  augite 
has  been  changed  to  green  hornblende.  Along  the  peripheries  of  the  boss 
masses  and  throughout  many  of  the  dikes  the  rocks  are  very  schistose. 
They  have  likewise  suffered  great  changes  in  composition,  and  are  now 
often  chlorite-schists  or  talc-schists. 

In  the  eastern  portion  of  the  area  nearly  all  the  boss  greenstones  have 
the  characteristics  just  mentioned,  but  in  the  western  bosses  the  rocks 
have  suffered  a  different  change.  In  addition  to  the  green  hornblende, 
there  have  been  formed  considerable  quantities  of  quartz  and  not  a  little 
brown  biotite.  These  rocks  have  also  suffered  much  more  dynamic  meta- 
morphism  than  have  the  eastern  ones.  They  resemble  in  many  respects 
the  micaceous  hornblendic  schists  of  the  Basement  Complex,  and  thus 
furnish  additional  evidence  in  favor  of  the  view  that  these  rocks  are 
squeezed  eruptives. 

Of  the  sheet  greenstones,  a  very  few  seem  to  have  been  surface 
flows.  Others  were  intrusive  sills.  Only  a  few  instances  are  known  in 
which  the  existence  of  the  latter  forms  of  greenstone  may  be  shown  to  be 
probable,  although  it  is  believed  that  many  other  cases  of  intrusive  sills 
occur  in  the  Lower  Marquette  series.  They  have  escaped  detection,  how- 
ever, since  their  inaterial  is  similar  to  that  of  the  dikes,  and  in  a  district  of 
complicated  stratigraphy  it  is  almost  impossible  to  distinguish  between 
dikes  trending  witli  the  strike  of  the  sedimentary  beds  and  flows  interleaved 
with  these  beds. 

The  tuff  beds,  in  the  few  cases  noted,  are  associated  with  knobs  of 
nearly  massive  greenstone. 

Since  the  material  of  the  pre-Clarksburg  greenstones  is  similar  to  that 
of  the  Clarksburg  rocks,  and  since  the  former  do  not  occur  in  beds  younger 
than  the  Clarksburg  formation,  it  is  inferred  that  they  are  the  lower  portions 
of  the  flows,  tuffs,  and  associated  greenstones  that  constitute  the  Clarksburg 
formation. 

The  post-Clarksburg  greenstones  comprise  only  dikes  and  bosses. 
The  rocks,  while  more  or  less  altered,  are  all  much  fresher  than  the 
older  greenstones,  and  all  of  them  have  preserved  distinct  traces  of  their 
original  structure.     These  greenstones  cut  all  the  rocks  of  the  Basement 


524  THE  MARQUETTE   lEON-BEARING  DISTRICT. 

Complex  and  the  Marquette  series.  They  are  in  the  main  uonfoliated, 
even  where  the  rocks  into  which  they  are  intrusive  are  well-characterized 
schists.  For  these  reasons  they  are  separated  from  the  older  greenstones, 
and  because  they  are  very  similar  to  the  basic  rocks  of  the  Keweenawan 
series  on  Keweenaw  Point  they  are  thought  to  be  the  equivalents  of  some 
of  the  Keweenawan  erujitives. 

These  greenstones  comprise  olivine-diabases  and  quartz-diabases,  por- 
phyrites,  and  basalts.  Of  these  the  porphyrites  are  most  like  the  older 
greenstones,  from  which  they  are  distinguished  by  clear-cut  and  well- 
preserved  porphyritic  structure.  The  basalts  are  like  modern  basalts,  except 
that  their  groundmass  is  always  much  altered.  The  olivine-diabases  are 
typical  rocks  of  this  class.  The  quartz-diabases  are  peculiar  in  that  they 
contain  a  little  olivine  and  sometimes  a  large  quantity  of  quartz,  which 
occiTrs  in  micropegmatitic  intergrowths  with  plagioclase. 

None  of  the  larger  masses  of  the  greenstones  of  the  older  or  the 
younger  kinds  are  in  the  form  of  great  interbedded  sheets,  as  has  been 
stated  to  be  the  case  by  the  earlier  geologists.  The  sheets  that  do  occur 
are  thin,  and,  so  far  as  known,  they  are  not  continuous  for  long  distances, 
nor  do  they  appear  to  occupy  any  distinct  and  definite  horizons  in  the 
bedded  series  outside  of  the  Clarksburg  formation. 


CHAPTER    VI. 

THE  REPUBLIC  TROUGH. 


By  Henry  Lloyd  Smyth. 


i:iVTRODUCTIO>r. 


The  Republic  syncline  (Atlas  Sheets  IV,  VII,  X,  and  XI)  is  sharply 
marked  off  froni  the  rest  of  the  Marquette  district  by  the  simplicity  of  its 
structure  and  by  the  fact  that  the  folding  has  taken  place  about  an  axis 
which  strikes  northwest  and  southeast,  or  in  a  direction  considerably  inclined 
to  the  general  course  of  the  great  Marquette  synclinorium.  The  Republic 
syncline  is  thus  transitional  to  the  north-and-south  type  of  structure  that 
prevails  beyond  it  to  the  west  over  three  townships,  and  to  the  south  as 
far  as  the  Felch  Mountain  trough,  in  T.  42  N.  The  Republic  area  proper 
begins  near  the  south  end  of  Lake  Michigamrae  and  continues  southeast  to 
the  northwest  sections  of  T.  46  N.,  R.  29  W.  As  thus  defined,  it  is  a  simple 
syncline  in  Algonkian  rocks,  about  7  miles  in  length,  with  nearly  parallel 
sides  from  one-half  to  1  mile  apart;  on  both  sides  and  at  the  southeast 
end  it  is  inclosed  by  Archean  rocks,  while  at  the  northwest  end  it  rather 
suddenly  flares  out  into  the  main  Marquette  synclinorium. 

To  the  northeast  it  is  separated  from  the  southern  boundary  of  the 
main  Marquette  trough  by  an  area  of  Archean  granite  and  gneiss  about  6  J 
miles  broad.  To  the  west  and  southwest  about  half  this  distance,  over 
similar  Archean  rocks,  divides  it  from  the  next  naiTow  Algonkian  syncline. 
While  the  general  direction  of  the  main  Marquette  fold  is  nearly  east  and 
west,  the  fold  is  constricted  on  a  section  through  the  Champion  mine, 
where  it  is  only  2  miles  wide,  and  its  southern  boundary  has  a  northwest- 
ward trend,  to  which  the  Republic  fold  is  very  nearly  parallel. 


526  THE   MAKQITETTE   IRON  BEARING  DISTRICT. 

The  topography  is  as  simple  as  the  structure.  The  Michigamme 
River,  on  entering  the  syncline  about  1  mile  south  of  Lake  Michigamme, 
flows  through  the  trough  nearly  to  its  southeastern  end,  mainly  over  the 
upper  members  of  the  bedded  series.  The  river  valley  substantially 
coincides  with  the  bedded  rocks.  East  and  west  it  is  flanked  by  Archean 
uplands,  consisting  of  rounded  granite  knobs  of  characteristic  glacial  and 
disintegration  forms,  often  bare  or  covered  with  a  thin  drift  mantle.  In  the 
immediate  neighborhood  of  the  southeastern  termination  of  the  trough 
the  river  first  swings  to  the  east  into  the  eastern  granite  wall,  and  then 
returns  to  the  southwest,  occupying  a  large  part  of  the  interior  of  the 
trough  in  the  structurally  determined  expansion  of  Smiths  Bay,  and  finally 
leaving  it  on  the  western  side,  about  three-quarters  of  a  mile  northwest  of 
its  southeastern  end.  Within  the  general  topographic  depression  bounded 
by  the  Archean  areas  the  bedded  rocks  and  the  greenstone  intrusives 
within  them  occasionally  form  considerable  elevations,  none  of  which, 
except  Republic  Mountain  itself,  reaches  the  average  height  of  the  granite 
uplands. 

The  rocks  of  the  Repubhc  area  consist  of  (1)  granites,  gneisses,  and 
crystalline  schists,  which  form  the  basement  upon  which  the  iron-bearing 
series  were  laid  down;  (2)  quartzites,  mica-schists,  and  ferruginous  schists, 
of  both  Lower  and  Upper  Marquette  age;  and  (3)  later  igneous  intrusives. 

SECTION  I.— THE  ARCHEAN. 

The  granites,  gneisses,  and  crystalline  schists  here  constitute  the  unclas- 
sified Archean.  These  rocks  have  been  studied  only  incidentally  near  their 
contacts  with  the  iron-bearing  series,  and  chiefly  from  the  point  of  view  of 
their  structural  relations  with  the  latter.  It  appears  that  of  the  three  kinds 
of  rocks  into  which  the  Archean  may  be  divided  the  granites  are  by  far 
the  most  common.  These  are  usually  normal  granular  rocks,  made  up  of 
orthoclase  and  microcline,  plagioclase,  quartz,  light  and  dark  colored  mica, 
and  often  hornblende,  with  the  usual  accessory  minerals.  Often  the  ortho- 
clase is  present  in  large  porphyritic  Carlsbad  twins,  which  sometimes  attain 
a  length  of  2  inches.  This  coarse-grained  granite  is  the  prevailing  type  at 
Republic.  It  weathers  light-gray  or  white,  sometimes  with  a  marked  red 
tino-e.     The  constituent  minerals  show  no  parallel  arrangement. 


THE   KEPUBLIG  TKOUGH.  527 

Of  gneisses,  properly  so  called,  none  have  been  fonnd  in  the  Republic 
area  except  those  that  have  unmistakably  been  derived  from  the  normal 
granite  by  dynamic  metamorphism.  These  are  best  seen  in  the  immediate 
neighborhood  of  the  contacts  with  the  overlying  series,  and  they  are  so 
characteristic  of  these  contacts  that  where  gneissic  foliation  is  present  the 
contact  may  confidently  be  looked  for  close  at  hand.  This  gneissic  structure 
is  largely  due  to  the  development  of  mica,  usually  muscovite,  along  surfaces 
of  breaking  which,  while  individually  irregular  and  waving,  yet  in  the 
aggregate  are  distinctly  parallel  in  strike  and  dip  to  the  contact  surface  and 
to  the  bedding  planes  in  the  overlying  sediments. 

The  gneissic  structure  is  most  strongly  developed  at  the  contact.  In 
departing  from  the  contact  it  diminishes  by  degrees,  and  finally,  at  dis-^ 
tances  which  usually  do  not  exceed  200  feet,  it  disappears  altogether  or  is 
found  only  in  narrow,  irregular,  and  discontinuous  zones.  That  this  struc- 
ture is  really  due  to  the  processes  of  dynamic  metamorphism  acting  on  the 
normal  granite  is  evident  both  from  observation  in  the  field,  where  it  may 
be  seen  in  all  stages  of  development,  and  also  in  thin  sections,  where  it  is 
clearly  proved  to  result  from  granulation  of  the  original  quartz  and  feldspar, 
and  the  passing  over,  in  some  cases  partial  and  in  others  complete,  of  the 
latter  into  quartz  and  into  the  new  light-colored  micas,  which  are  orientated 
with  the  directions  of  fracture. 

In  the  Archean  areas  are  found  certain  dark-colored  hornblende-schists 
and  amphibolites.  These  occur  usually  in  narrow  bands  and  are  exceed- 
ingly variable  in  the  degree  of  schistosity  which  they  exhibit  and  in 
crystalline  character.  Some,  at  least,  are  without  question  old  dikes,  orig- 
inally diabase  or  diorite,  in  which  a  parallel  arrangement  of  new  minerals 
has,  with  more  or  less  completeness,  been  effected  by  dynamic  metamor- 
phism. In  many  cases  the  progress  of  these  changes  may  be  traced  from 
a  massive  crystalline  interior  into  nearly  perfectly  foliated  zones  at  the 
walls.  In  other  cases  the  schists  are  completely  crystalline  throughout, 
and  these  bear  no  evidence  of  their  igneous  character.  In  age  these 
schists  doubtless  vary  enormously.  Some  have  furnished  pebbles  to  the 
basal  conglomerate  of  the  Lower  Marquette  series,  and  these  pebbles  are 
as  thoroughly  crystalline  and  schistose  as  any  of  the  schist  bands  that  can 
now  be  found  in  the  Archean  areas.     Others  are  almost  certainly  younger 


528  THE   MAIIQUETTE   IRON-BEARING  DISTRICT. 

than  the  Upper  Marquette  sediments,  and  are  genetically  connected  with 
the  great  intrusions  of  diabases  which  are  found  abundantly  in  this  series. 
In  one  locality  south  of  the  Magnetic  mine  a  dike  of  fine-grained  red 
granite  was  found  cutting  the  ordinary  coarse  gray  granite.  No  other 
clear  proof  of  the  existence  of  younger  intrusive  granites  was  found  in  the 
Republic  area  proper. 

SECTION  II.— THE   LOWER  MARQUETTE   SERIES. 

The  bedded  rocks  of  the  Republic  area  belong  to  two  unconformable 
series  of  Algonkian  age.  The  lower  of  these,  to  which  the  name  Lower 
Marquette  series  has  been  applied,  consists  of  two  distinct  members,  a 
lower  fragmental  member  of  small  thickness,  probably  not  exceeding  100 
feet,  and  an  upper  iron-bearing  member,  which  in  its  maximum  develop- 
ment, including  intrusive  greenstones,  can  not  be  less  than  1,500  feet  in 
thickness.  The  lower  member,  from  its  usual  lithological  character,  is 
known  as  the  Ajibik  quartzite,  while  the  upper  member,  from  its  constant 
ferruginous  character,  may  be  distinguished  as  the  iron-bearing  member 
or  Negaunee  formation. 

THE    AJIBIK   QUARTZITE. 

The  lower  member  of  the  Lower  Marquette  series  in  this  part  of  the 
district  is  relatively  a  weak  rock,  and  as  its  thickness  is  small  it  rarely 
outcrops  above  the  glacial  mantle  in  the  Republic  area.  At  the  present 
time  but  seven  or  eight  localities  are  known.  These  are,  however,  so 
widely  distributed  over  the  area  that  it  is  very  probable  that  the  lower 
member  is  present  wherever  the  Lower  Marquette  series  is  represented  at 
all.  In  these  exposures  the  rock  usually  appears  as  a  white  quartzite, 
sometimes  vitreous,  but  often  of  an  opaque  white  color  from  the  large 
amounts  of  contained  muscovite  or  sericite.  The  mica  is  frequently  pres- 
ent in  such  abundance  that  the  rock  becomes  properly  a  mica-schist.  In 
only  one  known  locality,  in  which  it  is  found  to  rest  in  direct  contact 
upon  the  Archean,  does  it  appear  as  a  coarse  conglomerate  made  up  of 
recognizable  fragments  derived  from  the  underlying  granites  and  crystalline 
schists. 


THE  EEPUBLIO  TKOUGH.  529 

In  thin  sections  the  various  phases  of  this  member  are  seen  to  be 
eminently  crystalUne.  The  vitreous  varieties  consist  mainly  of  interiocking 
areas  of  quartz,  within  and  between  which  are  plates  of  light  and  dark 
mica  and,  less  often,  of  chlorite.  Magnetite  and  garnet  are  more  rarely  seen, 
and  neariy  complete  the  list  of  contained  minerals.  In  the  more  schistose 
varieties  the  mica  is  more  abundant,  and  occurs  in  long  plates  which  have 
a  parallel  physical  orientation.  In  none  of  the  many  slides  that  have  been 
studied  do  any  feldspar  grains  appear,  nor  has  a  trace  been  detected  of 
the  outlines  of  original  rolled  grains.  These  have  been  obliterated  in  the 
course  of  the  profound  changes  through  which  the  rock  has  passed  since  its 
deposition,  and  the  feldspar  of  the  original  granitic  ddbris  is  now  doubtless 
represented  by  the  light  micas  and  secondary  quartz. 

In  many  cases  the  larger  structures  of  the  original  rock  have  survived. 
Faint  color  banding  and  alternations  in  texture  and  composition  parallel  to 
the  original  deposition  planes  are  often  seen,  and  in  one  locaUty  a  beautiful 
false  bedding  can  be  distinctly  recognized. 

THE  NEGAUNEE  FORMATION. 

The  iron-bearing  formation  is  not  generally  exposed  in  the  Republic 
area,  except  at  the  extreme  southeastern  end  of  the  syncline,  where  mag- 
nificent outcrops  extend  from  the  old  Kloman  mine  in  the  SW.  J  sec.  6, 
T.  46  N.,  R.  29  W.,  almost  entirely  around  the  horseshoe,  through  a  large 
part  of  sec.  7  (Atlas  Sheet  XI).  Within  this  area  of  neariy  a  square  mile, 
which  comprises  Republic  Mountain,  there  are  small  portions  only  of  the 
interval  between  the  Ajibik  quartzite  beneath  and  the  Goodi-ich  quartzite 
above  that  are  not  somewhere  represented  by  outcrops.  The  rock  of  the 
iron  formation  has  many  phases,  but  consists  essentially  of  finely  crystalline 
quartz,  a  pale-green  radiating  amphibole  which  has  been  determined  to  be 
griinerite,  and  the  iron  oxides.  Within  this  area  the  iron-bearing  member 
has  been  divided  by  intrusive  masses  of  diorite  mainly  parallel  with  the 
stratification  planes^!  and  near  the  contacts  with  these  it  frequently  carries 
large  amounts  of  red  garnet.  The  three  chief  constituents  of  this  rock 
are  not  always  present.  Two,  or  even  one,  may  predominate  to  the  par- 
tial or  neariy  entire  exclusion  of  the  rest.     So  the  rock  is  found  in  certain 

MON    XXVIII 34 


530  THE   MAKQUETTE   IRON-BEARING  DISTRICT. 

phases  to  be  made  up  mainly  of  quartz  and  griinerite,  or  of  quartz  and 
iron  oxides,  or  of  griinerite  and  iron  oxides.  The  iron  oxides,  too,  may  be 
either  magnetite  or  hematite.  These  mineral  constituents  are  arranged 
in  very  distinct  narrow  bands  which  are  parallel  to  the  upper  and  lower 
bounding  surfaces  of  the  rock.  The  bands  are  not  wholly  regular,  nor 
ai-e  they  continuous  for  great  distances.  They  thicken  and  thin,  taper  out, 
and  break  joint.  It  is  certain  that  none  of  the  minerals  which  now  make 
up  the  rock  are  original,  and  that  the  parallel  banded  structure  signifies  that 
the  processes  of  metamorphism  through  which  it  has  reached  its  present 
constitution  were  controlled  by  a  primary  bedded  structure. 

The  variations  in  external  appearance  produced  by  these  considerable 
variations  in  composition  are  great.  These  vai'iations  are  not  wholly  irreg- 
ular, and  it  is  possible  to  distinguish  in  the  different  phases  a  definite 
distribution  through  the  iron-bearing  member,  which  holds  good  within  the 
limits  of  the  Republic  area.  The  lower  portion  of  the  formation  is,  on 
the  whole,  characterized  by  the  presence  of  griinerite  and  gray  or  dark- 
colored  quai-tz  with  magnetite,  while  the  higher  portion  is  characterized 
by  the  almost  complete  absence  of  griinerite  and  by  the  presence  of 
specular  hematite  and  red  quartz  or  jasper,  which  owes  its  color  to  the 
intimate  mixture  of  the  little  particles  of  hematite  with  the  quartz.  The 
study  of  the  western  portion  of  the  Marquette  area  alone  would  probably 
justify,  on  the  basis  of  difference  in  composition  and  external  appearance,  a 
division  of  the  iron-bearing  member  into  two  distinct  formations,  a  lower 
griinerite-magnetite-schist  member  and  an  upper  specular  jasper  member. 

In  the  Republic  area  magnetite  also  increases  in  amount  in  going  from 
lower  to  higher  horizons,  while  griinerite  decreases,  so  that  just  beneath  the 
specular  jasper  the  iron-bearing  member  is  mainly  made  iip  of  bands  of 
exceedingly  fine  grained  magnetite  alternating  with  bands  of  dark  to  black 
quartz,  the  color  of  which  is  due  to  the  presence  of  a  large  amount  of 
included  magnetite. 

Under  the  microscope  the  chief  interest  centers  in  the  question  of  the 
nature  of  the  quartz,  whether  it  is  partly  or  wholly  of  fragmental  origin. 

In  the  study  of  the  slides  no  evidence  has  yet  been  found  that  any  of 
the  quartz  is  fragmental.     Here   and  there  traces  are  seen  of  an  original 


THE   IIEPUBLIC  TROUGH.  531 

oolitic  structure,  such  as  is  so  beautifully  shown  in  the  more  modern  and 
less  altered  iron  formation  of  the  Mesabi  range.  In  the  Michigamme 
jasper  of  tlie  Menominee  district,  which  is  regarded,  on  stratigraphical 
grounds,  as  the  equivalent  of  the  Lower  Marquette  iron  formation,  an 
original  oolitic  and  concretionary  structure  is  common. 

The  question  of  the  nature  of  the  rock  from  which  the  iron-bearing 
member  has  been  derived  is  fully  discussed  by  Professor  Van  Hise  else- 
where in  this  memoir.  Whether,  as  seems  probable,  the  various  phases 
which  the  iron-beai'ing  member  now  presents  have  been  derived  from  a 
single  original  rock  of  sensibly  uniform  character  or  not,  it  is  very  evident 
that  much  of  the  differentiation  is  of  long  standing  and  occurred  before  the 
Upper  Marquette  transgression.  That  this  is  so  appears  from  the  presence 
of  pebbles  from  both  the  magnetite-griinerite-schists  and  the  specular 
jaspers  in  the  basal  conglomerate  of  the  Upper  Mai-quette  series.  In  the 
Republic  and  adjacent  areas  at  least,  the  specular  jaspers  occur  at  a  definite 
stratigraphical  position  in  the  highest  horizon  in  the  Lower  Marquette 
series.  They  are  present  only  in  those  places  where  large  thicknesses  of 
the  lower  series  remain,  as  at  Republic  Mountain  and  in  the  range  along  the 
northwest  side  of  Lake  Michigamme.  Where  the  lower  series  has  been 
more  deeply  eroded  before  the  deposition  of  the  Upper  Marquette  rocks  the 
specular  jaspers  are  far  less  continuous  and  of  less  common  occiu'rence  than 
the  magnetite-griinerite-schist  phases  of  the  iron-bearing  member.  These 
facts  appear  to  bear  strongly  against  the  view  that  the  specular  jaspers  are 
due  to  later  metamorphic  processes  which  acted  along  the  contact  with  the 
Upper  Marquette  quartzite  after  the  latest  folding,  while  they  are  what 
would  be  expected  if  these  two  chief  phases  existed  in  substantially  their 
present  condition  before  the  Upper  Marquette  series  was  laid  down. 

Another  fact  is  also  significant.  It  has  been  said  that  the  grlinerite, 
quartz,  and  iron  oxides  of  the  iron-bearing  member  have  a  very  distinct 
banded  arrangement  and  yet  are  not  original  minerals,  and  that  this  banding 
is  parallel  to  the  upper  and  lower  bovindaries  of  the  formation.  It  is  prob- 
able that  a  set  of  parallel  structural  planes  has  controlled  the  segregation 
of  the  present  constituent  minerals  during  the  changes  tlu'ough  which  the 
rock  has  passed,  and  that  these  planes  must  have  been  original  bedding 


532  THE   MAEQUETTE   lEOJf-BEAEING  DISTRICT. 

planes.  As  the  parallel  banding  is  confined  to  this  one  direction,  it  is  certain 
that  during  its  development  no  other  system  of  parallel  planes  existed  in  the 
rock.  The  last  severe  folding,  which  has  determined  the  larger  structural 
features  of  the  Marquette  district,  has  also  affected  the  rocks  in  a  more  inti- 
mate way.  In  certain  localities  strong  minor,  even  minute,  crenulations 
have  been  produced,  and  also  parallel  cleavage,  which  sometimes  traverses 
the  banding  of  the  rock  at  right  angles.  The  little  folds  are  often  broken 
and  faulted  and  the  siliceous  bands  reduced  to  fragments.  Along  the 
parallel  cleavage  planes  movement  has  often  taken  place,  as  is  shown  by 
the  displacement  of  a  particular  band  on  the  two  sides.  Along  this  sec- 
ondary cleavage,  which  dates  from  the  period  of  general  folding  after 
Upper  Marquette  time,  no  great  development  of  new  minerals,  except  the 
ii-on  oxides,  has  taken  place,  while  the  displacement  which  the  minute 
faulting  has  caused  in  the  banding  conclusively  proves  that  this  structure 
was  present  before  the  folding. 

From  these  various  lines  of  evidence,  from  the  apparently  definite 
stratigraphical  position  of  the  two  main  varieties  of  the  iron-bearing  member, 
from  the  presence  in  the  upper  conglomerate  of  pebbles  of  all  the  various 
kinds  of  rock  which  are  now  found  in  the  iron-bearing  member,  and  from 
the  mechanical  effects  which  the  last  folding  has  produced  in  the  banded 
structure,  it  seems  beyond  question  that  the  iron-bearing  formation  had 
essentially  its  present  character  at  the  time  when  the  Upper  Marquette  series 
was  laid  down. 

CONTACTS    BETWEEN  THE   LOWER    MARQUETTE   SERIES  AND   THE  ARCHEAN. 

It  lias  already  been  said  that  the  Ajibik  quartzite  has  been  foand  in 
only  a  few  places.  The  contact  between  this  rock  and  the  Archean  is 
almost  everywhere  drift-covered,  and  actual  juxtaposition  has  been  found 
in  but  two  localities.  The  evidence  at  one  of  these  as  to  the  relations 
between  the  two  series  is  very  clear  and  convincing. 

In  the  eastern  part  of  the  NW.  ^  of  the  NE.  i  sec.  18,  T.  46  N.,  R.  29  W. 
(Atlas  Sheet  XI),  is  a  large  outcrop  of  the  quartzite,  which  was  discov- 
ered bv  Pumpelly  and  Credner  in  1867.  The  locality  is  at  the  extreme 
southern  end  of  the  Republic  syncline.     A  short  distance  southwest  of  the 


THE  KEPUBUC  THOUGH.  533 

quartzite  is  a  ridge  running  northeast,  made  up  mainly  of  granite.  Near 
the  southwest  end  and  on  the  northwest  side  of  this  ridge,  which  has  a 
steep  northwest  slope,  is  found,  lying  upon  the  granite,  a  northwesterly 
dipping  fringe  of  conglomerate  which  extends  some  50  feet  along  the  strike 
as  a  continuous  rock  mass,  and  occurs  besides  in  occasional  disconnected 
patches  farther  north  on  the  sloping  face  of  the  hill.  The  granite  is  of  the 
usual  gray  variety,  and  carries  large  orthoclase  crystals  up  to  2  inches  in 
length. 

The  conglomerate  consists  of  pebbles  of  granite,  quartz,  and  black 
hornblende-schist  embedded  in  a  matrix  of  quartz  and  mica-schist.  The 
cement  is  distinctly  color-banded,  the  bands  being  parallel  to  the  contact 
stu'face.  They  are  thrown  into  small  folds  about  axes  which  pitch  north- 
westerly in  the  direction  of  the  dip  of  the  rock.  The  pitch  is  closely 
parallel  both  with  the  axis  of  the  main  Republic  fold  and  with  a  pronounced 
parallel  cleavage  which  affects  the  overlying  griinerite-magnetite-schists, 
the  conglomerate  cement,  and  also  the  underlying  granite  for  a  considera- 
ble distance  back  from  the  contact.  The  granite  pebbles  vary  in  size  fi'om 
a  fraction  of  an  inch  up  to  5  feet  in  diameter,  and  are  unmistakably  water- 
rounded.  The  larger  are  comparatively  thin  slabs,  lying  with  their  flat 
sides  in  the  bedding  of  the  matrix  which  often  follows  around  the  inclu- 
sions. The  granite  of  the  pebbles  is  lithologically  identical  with  that  of  the 
main  mass  on  which  the  conglomerate  rests. 

The  contact  itself  is  very  definite.  Between  the  undoubted  conglom- 
erate above  and  the  undoubted  granite  below  is  a  narrow  zone,  a  few  inches 
wide,  of  schistose  material,  which  probably  represents  a  shear  zone  aff"ecting 
both  rocks,  due  to  movement  along  the  contact  during  the  folding.  At  the 
north  end  of  the  main  outcrop  a  large  mass  of  granite  is  traversed  by  thin 
seams  of  the  conglomerate,  one  of  which  tapers  to  a  point  at  one  end  and 
connects  with  the  main  body  of  conglomerate  at  the  other.  It  is  impossible 
to  avoid  the  conclusion  that  this  represents  an  original  crack  in  the  some- 
what irregular  surface  upon  which  the  conglomerate  was  laid  down,  into 
which  the  finer  sand  and  pebbles  were  washed. 

The  facts  at  this  contact  can  only  be  interpreted  as  signifying  that  the 
gray  granite  upon  which  the  conglomerate  now  rests  existed  in  its  present 


534  THE   MARQUETTE   IRON-BEAEING   DISTRICT. 

condition  at  the  time  that  tlie  conglomerate  was  laid  down,  that  it  supplied 
a  large  part  of  the  materials  out  of  which  the  conglomerate  was  built,  and 
furnished  the  basement  upon  which  it  was  deposited.  In  short,  the  contact 
is  one  of  erosion,  the  conglomerate  is  a  basal  conglomerate,  and  the  facts 
indicate  an  important  time-break  at  its  base. 

At  the  other  locality,  in  sec.  7,  T.  47  N.,  R.  30  W.  (Atlas  Sheet  VII),  a 
short  distance  south  of  the  Magnetic  mine,  the  evidence  is  not  so  clear. 
Between  the  undoubted  granite  and  the  iron-bearing  member  is  a  consider- 
able interval  occupied  by  banded  gneisses  and  mica-schists,  which  certainly 
include  part  of  the  horizon  of  the  lower  quartzite,  but  how  much  it  is 
impossible  to  determine.  Some  of  the  gneisses  and  schists  have  evidently 
been  derived  in  place  from  the  gi-anite,  through  shearing  parallel  to  the 
contact;  others  seem  cleai'ly  to  be  metamorphosed  sediments  in  which  it  is 
possible  to  detect  here  and  there  traces  of  the  larger  quartz  pebbles.  But 
between  them  there  is  a  considerable  interval  of  somewhat  similar  gneisses 
and  schists  the  origin  of  which  is  wholly  indeterminate.  The  facts  here 
are  quite  in  harmony  with  the  view  that  the  contact  is  an  erosion  contact, 
although  they  do  not  give  it  direct  support. 

The  two  contacts,  therefore,  at  which  direct  juxtaposition  is  found 
justify  the  conclusion  that  the  relations  between  the  Lower  Marquette  series 
and  the  Archean  are  those  of  an  erosion  unconformity,  that  the  Archean 
in  its  present  form  is  the  older,  and  that  a  considerable  interval  of  time 
elapsed  between  the  formation  of  the  Archean  rocks  and  tlie  deposition  of 
the  Ajibik  quartzite. 

The  lithological  character  of  the  Ajibik  quartzite,  wherever  it  is  found, 
must  he  taken  as  corroborating  this  conclusion.  It  will  be  remembered 
that  this  rock  is  composed  of  quartz  with  variable  proportions  of  light- 
colored  mica,  muscovite,  or  sericite  as  essential  constituents.  These  micas 
have  probably  been  derived  from  the  alteration  of  original  orthoclase  or 
microcline,  feldspars  characteristic  of  the  Archean  granites,  of  which  the 
quartzite  otherwise  shows  now  no  traces.  The  quartzite  was  then  probably 
a  feldspathic  sandstone,  composed  of  granitic  debris  such  as  the  breaking 
down  of  the  adjacent  underlying  granite  would  unquestionably  furnish. 
The  persistence  of  its  lithological  character  and  the  fact  that  it  is  always 


THE   EEPUBLIC  TROUGH.  535 

found  ill  close  proximity  to  granite,  tlie  disintegration  of  which  would  have 
supplied  all  of  its  essential  constituent  minerals,  certainly  raise  a  strong 
presumption  that  such  actuall}'  has  been  its  origin. 

SECTION  III.— THE  UPPER  MARQUETTE   SERIES. 

In  the  Republic  area  proper  only  the  lower  member  of  the  Upper  Mar- 
quette series,  the  Goodrich  quartzite,  is  well  exposed.  This  is,  in  the  main, 
a  white  quai-tzite,  usually  massive  and  heavily  bedded  near  the  base  and 
passing  upward  into  the  mica-schist  of  the  Michigamme  formation.  At  the 
base,  conglomerate  layers  occur,  in  the  pebbles  of  which  all  the  underlying 
rocks  are  abundantly  found.  The  conglomerates,  while  usually  unimpor- 
tant, are  represented  in  great  volume  at  the  south  end  of  the  trouo-h,  where 
they  are  beautifully  exposed  by  the  extensive  mining  operations  about 
Republic  Mountain.  In  these  conglomerates  the  great  majority  of  the 
pebbles  have  a  local  origin,  being  derived  from  the  rocks  upon  which 
the  conglomerates  du-ectly  rest. 

Under  the  microscope  the  Goodrich  quartzites  and  quartz-schists  show 
a  decidedly  less  degree  of  metamorphism  than  do  the  Ajibilc  quartzites. 
Roundish  grains  of  feldspar,  usually  microcline,  derived  from  the  under- 
lying granites,  are  plentiful,  and  the  quartz  areas  often  exhibit  distinct 
indications  of  original  rolled  nuclei.  In  the  quartz-schist  and  mica-schist, 
into  which  the  more  massive  quai-tzite  usually  passes  upward,  certain  defi- 
nite layers  of  a  darker  color  are  often  distinguishable,  in  which  iron  oxides, 
usually  magnetite;  abundantly  occur.  In  these  layers  false  bedding  is 
often  strongly  brought  out,  and  it  is  believed  that  the  iron  oxides  are  in 
large  part  original  sediments. 

The  Goodrich  quartzite  is,  on  the  whole,  the  thickest  rock  in  the 
Republic  area,  and,  by  reason  of  its  A^olume  and  chai-acter,  that  which 
most  frequently  outci'ops. 

The  Michigamme  schist  occupies  the  center  of  the  tongue,  and,  because 
less  resistant  than  the  inferior  formations,  the  Michigamme  River  does  not 
wander  far  from  its  borders.  In  most  respects  this  schist  is  similar  to  the 
remainder  of  the  formation  elsewhere,  and  therefore  will  not  be  further 
considered  here. 


536  THE  MARQUETTE   lEON-BEAllING   DISTKICT. 

CONTACTS  OF  THE  GOODRICH  QUARTZITE  WITH  THE  LOWER  MARQUETTE 
SERIES  AND  W^ITH  THE  ARCHEAN. 

Direct  contacts  of  the  quartzite  which  forms  the  base  of  the  Upper 
Marquette  series  with  the  underlying  rocks  are  very  numei'ous,  and  the  evi- 
dence in  detail,  as  well  as  the  more  general  facts,  leaves  no  room  for  doubt 
that  this  quartzite  was  laid  down  on  a  deeply  eroded  surface  and  that  the 
relations  are  those  denoting  a  most  profound  time-break. 

The  detailed  facts,  which  may  be  observed  on  the  exceptionally  fine 
exposures  about  Republic  Mountain,  are  these:  (1)  There  is  a  sHght  but 
very  persistent  discordance  in  stratification — more  evident  at  any  single 
locality  in  dip  than  in  strike — between  the  Goodrich  quartzite  and  the  under- 
lying formation  of  the  lower  series.  This  diff'erence  in  dip  is  on  the  aver- 
age not  far  from  15°.  (2)  The  basal  conglomerate  of  the  upper  series  is 
crowded  with  fragments  of  the  iron-bearing  member  upon  which  it  lies. 
These  fragments  are  often  of  large  size  and  imperfectly  rounded,  and  evi- 
dently have  not  moved  far.  The  included  fragments  at  the  immediate 
contact  are  almost  wholly  from  the  subjacent  formation,  and  from  their 
often  irregular  shapes  and  great  preponderance  might  frequently  be  mis- 
taken for  the  products  of  brecciation,  if  it  were  not  for  the  sparse  presence 
in  the  conglomerate  cement  of  quartz  and  feldspar  derived  from  the  more 
distant  granites.  (3)  The  structural  details  of  the  contacts  prove  uncon- 
formity. The  layers  of  the  underlying  iron  formation  are  often  for  short 
distances  traversed  at  large  angles  by  the  contact  surface.  Extending  back 
into  the  mass  of  the  iron  formation  cracks  are  occasionally  found  into 
which  the  fine  material  of  the  conglomerate  cement  has  sifted.  Finally, 
in  the  conglomerates  in  the  lower  member  of  the  upper  series  pebbles  of  all 
the  underlying  rocks  are  seen,  from  the  Archean  to  the  top  of  the  Lower 
Marquette  series. 

The  more  general  facts  of  the  relation  of  the  Upper  Marquette  series 
to  the  underlying  rocks  may  be  summed  up  in  the  statement  that  within 
the  narrow  limits  of  the  Republic  area  the  upper  series  rest,  in  one  locality 
or  another,  on  each  of  the  older  formations  (Atlas  Sheets  IV,  VII,  X,  and 
XI).     The  maximum  thickness  of  the  Lower  Marquette  series  is  found  at 


THE   KEPUBLIC   TEOUGH.  537 

Republic  Mountain.  lu  going  north  from  Republic  Mountain  on  the  east 
side  of  the  fold  the  lower  series  is  progressively  and  rather  slowly  cut  out, 
so  that  at  the  old  Chippewa  exploration,  in  sec.  22,  T.  47  N.,  R.  30  W.,  the 
Goodi-ich  quartzite  rests  directly  upon  the  Archean.  Thence  northward, 
and  eastward,  almost  to  the  Champion  mine,  the  lower  series  probably  does 
not  again  emerge. 

On  the  west  side  of  the  fold  the  lower  series  is  entirely  gone  on 
the  west  side  of  the  river,  opposite  the  Republic  mine,  and  the  Good- 
rich quartzite  rests  directly  on  the  granite.  It  reappears  to  the  north 
only  in  patches,  once  at  the  Standard  location,  possibly  again  at  the  Metro- 
politan, and  again  at  the  Erie.  Beyond  the  Erie  it  appears  again  and 
continues  beyond  the  Magnetic  mine  and  the  limits  of  the  area  now 
described. 

The  evidence,  which  it  is  not  thought  necessary  to  present  here  in 
greater  detail,  is  thus  conclusive,  and  settles  beyond  the  possibility  of  ques- 
tion that  between  the  deposition  of  the  Lower  Marquette  series  and  that  of 
the  Upper  Marquette  series  an  interval  of  time  elapsed  dvu-ing  which  the 
lower  series  was  elevated,  folded,  probably  metamorphosed,  and  deeply 
denuded.  This  break  in  continuity  of  deposition  between  the  two  series 
lasted  sufficiently  long  to  permit  the  removal  in  many  places  of  the  entire 
Lower  Marquette  series  and  a  deep  gnawiiig  into  the  Archean.  The 
present  uneroded  thickness  of  the  Lower  Marquette  series  on  Republic 
Mountain  is  at  least  1,500  feet.  How  much  in  all  was  removed  by  erosion 
before  Upper  Marquette  time  there  is  no  means  of  knowing.  Fifteen 
hundred  feet  of  Lower  Marquette  strata,  with  an  unknown  thickness  of 
Archean,  is  the  minimum  amount  taken  away  in  the  RejDublic  area.  The 
time-break  in  the  Marquette  district  is  far  less  impressive  than  that  below 
the  upper  series  on  the  north  shore  of  Lake  Superior  (with  which,  indeed, 
we  do  not  know  that  it  was  conterminous),  because  the  earlier  folding  on 
the  south  shore  was  less  severe,  while  the  later  folding,  which  followed 
Upper  Marquette  time,  was  far  more  severe  than  on  the  north  shore;  and 
hence  the  structural  discordances  and  the  differences  in  degree  of  metamor- 
phism  between  the  two  series  are  less  pronounced.  But  the  conviction 
remains  that  this  is  one  of  the  great  breaks  in  the  geological  record. 


538  THE   MAEQUETTE  IRON-BEAIUNG  DISTRICT. 

SECTIOX  IV.— r-ATER  IGNEOUS  I:N^TRUSIVES, 

These  are  the  diorites  of  Brooks,  and  they  occur  in  great  abundance 
in  both  the  upper  and  lower  series.  They  are  dark-green  to  bhack,  often 
coarsely  crystalline  rocks,  composed  essentially  of  green  hornblende,  biotite, 
and  plagioclase,  and  doubtless  were  originally  diabases.  They  occur  in 
sheets  intruded  parallel  to  the  stratification  of  the  bedded  rocks,  in  dikes, 
and  in  irregular  bosses.  The  great  regularity  of  some  of  the  intruded 
sheets,  such  as  those  on  Republic  Mountain,  is  remarkable,  and  led  Brooks 
to  regard  them  as  regularly  interbedded  and  continuous  members  of  the 
stratified  series.  Close  examination,  however,  shows  that  even  here  they 
often  really  traverse  the  banding  of  the  iron-bearing  member  at  small 
angles  or  in  steps.  In  one  case  a  dike  several  feet  wide  was  found  to  leave 
the  main  sheet  and  to  cut  the  structural  planes  of  the  inclosing  jasper  at  an 
angle  of  45°.  In  the  immediate  neighborhood  of  the  ore  deposits  bodies 
of  so-called  soaprock  are  found,  Avhich  have  in  many  cases  intrusive  rela- 
tions to  the  iron-bearing  member.  At  Republic  it  was  not  possible  to  follow 
these  soaprock  bodies  in  any  instance  into  a  rock  which  retains  traces  of 
an  original  crystalline  structure,  but  at  the  Champion  mine  exactly  similar 
soaprock,  occurring  in  similar  relations  to  the  ore,  in  several  instances  was 
found  to  run  into  typical  diorite. 

In  age,  manv,  probably  most,  of  these  rocks  are  younger  than  the 
Upper  Marquette  sediments.  Some,  however,  penetrated  the  Lower  Mar- 
quette series  before  Upper  Marquette  time.  In  sec.  23,  T.  47  N.,  R.  31  W,, 
the  basal  conglomerate  of  the  Upper  Marquette  series  is  seen  to  rest  upon 
and  to  hold  numerous  fragments  of  an  old  diorite.  Within  the  Republic 
area  no  surface  eruptives  have  been  seen  in  either  the  upper  or  the  lower 
series. 

SECTIOK  v.— GENERAL  GEOLOGY. 

All  the  rocks  of  the  Upper  Marquette  and  Lower  Marquette  series 
have  been  closely  folded  in  the  Republic  area  into  a  syncline  the  axis  of 
which  runs  about  northwest  and  southeast.  The  present  fold  for  most 
of  its  length  is  sunk  deeply  into  the  Archean,  and  the  axis  is  practically 
horizontal.     Southeast  of  Smiths  Bay,  however,  the  axis  rises  with  a  pitch  of 


THE   EEPDBLIC   TROUGH.  539 

nearly  45°,  the  several  formations  swing  around  successively  in  horseshoe 
form  through  an  angle  of  180°  from  the  northeastern  to  the  southwestern 
side,  and  the  fold,  so  far  as  it  affects  the  Algonkian  rocks,  abruptly  termi- 
nates. Through  the  greater  part  of  the  length  of  the  trough  the  rocks  on 
the  two  sides  of  the  axial  plane  have  been  squeezed  nearly  into  parallelism. 
None  of  the  many  surface  observations  show  in  the  Ishpeming  quartzite  a 
dip  less  than  80°.  The  formations  in  the  underlying  Lower  Marquette 
series  dip  at  a  uniformly  higher  angle  on  the  eastern  side,  being  either 
vertical  or  slightly  overturned  toward  the  west,  while  on  the  western  side, 
owino-  to  the  absence  of  the  lower  series  for  much  of  the  way,  observations 
are  rare,  but  a  similar  divergence  in  dip  is  found  in  two  or  three  places. 

If  the  base  of  the  Goodrich  quartzite  be  developed  into  a  horizontal 
straight  line  along  any  cross-section  (thus  approximately  restoring  the 
conditions  to  what  they  were  before  the  last  folding),  it  will  be  seen  that 
the  rocks  of  the  underlying  lower  series  on  the  two  sides  of  the  trough 
dip  toward  each  other  (Atlas  Sheet  XI,  sees.  A'A'  and  B'B').  This  conver- 
gence in  dip  along  a  developed  section  points  clearly  to  the  existence  of  a 
gentle  syncline  in  the  Lower  Marquette  series  before  Upper  Marquette  time, . 
within  the  limits  of  the  rocks  included  in  the  present  fold.  The  very  slight 
discordance  between  the  strikes  of  the  members  of  the  two  series,  which, 
broadly  regarded,  is  measurable  in  feet  per  mile  rather  than  in  degrees, 
would  indicate  that  the  axis  of  the  later  fold  is  sensibly  parallel  to  that  of  the 
older,  while  the  greater  thickness  of  the  lower  series  remaining  on  the  east 
side  of  the  present  trough,  as  compared  with  that  remaining  on  the  west, 
gives  good  ground  for  the  inference  that  the  axis  of  the  old  syncline  lay 
somewhat  east  of  the  present  axis.  This  previously  existing  synclinal  axis 
doubtless  determined  in  the  later  folding  the  position  of  the  present  trough. 

It  has  been  said  that  the  trough  as  a  whole  pitches,  at  its  southeast  end, 
toward  the  northwest  at  an  angle  of  about  45°.  This  is  not  far  from  the 
average  pitch  at  the  surface.  With  depth  this  angle  slowly  diminishes, 
and  at  about  900  feet  below  the  surface  it  is  less  than  40°.  The  distance 
in  which  the  turn  is  made  at  the  southeast  end  of  the  trough  is  relatively 
very  short.  The  average  radius  of  the  generalized  curve  into  which  tlie 
base  of  the  Goodrich  quartzite  has  been  thrown  can  be  very  little  greater 


540  THE   MAEQUETTE   IRON-BEARING  DISTRICT. 

than  the  thickness  of  that  formation  (Atlas  Sheet  XI,  sees.  AA  and  BB). 
Field  study  shows  clearly  that  the  neutral  surface^  of  the  column  of  folded 
material  lay  below  the  base  of  the  Goodrich  quartzite,  and  included  a  con- 
siderable portion  of  the  Negaunee  formation.  This  is  proved  by  the  severely 
plicated  condition  of  the  thin-bedded  jaspers,  and  by  the  same  structure  on 
a  larger  scale  in  the  more  heavily  bedded  quartzite.  The  crowding  of  the 
rocks  above  the  neutral  surface  into  a  very  constricted  space  has  resulted  in 
the  formation  of  three  synclines  of  the  second  order  separated  by  two  anti- 
clines, all  subordinate  to  the  main  fold.  The  most  eastern  of  the  synclines 
occurs  at  the  great  open  pit  of  the  Republic  mine;  the  middle,  in  the 
ground  opened  by  the  Morgan,  Pascoe,  and  Ely  shafts,  and  the  western- 
most at  the  Swamp  shaft.  Upon  these  folds  of  the  second  order  are 
superimposed  a  multitude  of  smaller  anticlines  and  synclines  of  various 
dimensions.  They  are  more  numerous  and  more  closely  compressed  in 
the  iron-bearing  member  than  in  the  Goodrich  quartzite. 

In  the  iron-bearing  member,  which  is  a  thinly  bedded  rock,  these 
little  folds  are  especially  numerous  in  the  Morgan -Pascoe -Ely  syncline 
and  in  the  anticline  immediately  west  of  it.  The  effect  of  this  preva- 
lent crinkling  and  close  compression  is  to  give  a  general  northwesterly 
direction  to  the  individual  bands,  which  in  the  narrow  spaces  open  to 
observation  underground,  or  in  small  outcrops  on  the  surface,  may  lead 
to  erroneous  conclusions  as  to  the  real  direction  of  the  strike  and  dip 
of  the  rocks.  This  northwesterly  structure  is  really  at  right  angles  to 
the  direction  of  continuity  of  the  I'ock.  The  true  strike  is  determined 
by  the  plane  tangent  to  the  little  folds,  and  the  true  dip  by  the  angle 
of  pitch  of  their  axes.  Even  on  the  surface  in  the  larger  outcrops 
the  observer  may  sometimes  be  misled.  The  larger  subordinate  anticline 
between  the  Swamp  shaft  and  the  Morgan  is  topographically  indicated 
by  a  high  spur,  on  which  the  specular  jasper  outcrops.  The  jasper 
is  thrown  into  innumerable  little  folds,  the  axes  of  which  pitch  to  the 
northwest  at  an  angle  of  less  than  45°.  The  northern  slope  of  the  spur 
is   nearly  as    steep    as  the   angle  of  pitch,  and  so  the   surface   cuts  the 

'  Principles  of  North  American  pre- Cambrian  geology,  by  C.  K.  Van  Hise:  Sixteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  Part  1, 1896,  pp.  596-598. 


THE   EEPUBLIO  TROUGH.  541 

little  folds  nearly  parallel  to  their  axes.  In  effect,  therefore,  the  jasper 
outcrops  on  this  northern  slope  show  a  parallel  banding  striking  northwest. 
On  the  top  of  the  hill,  however,  the  true  strike  is  clearly  brought  out  on 
the  vertical  cross  joints. 

The  Goodrich  quartzite,  which  is  much  more  massive  and  heavily 
bedded  than  the  Negaunee  formation,  yielded  to  the  intense  compression 
by  differential  movements  of  one  bed  on  another,  and  doubtless  also  by 
thickening.  The  effects  of  the  movement  of  bed  on  bed  are  clearly  and 
strikingly  shown  at  numerous  points  in  the  horseshoe,  perhaps  particularly 
well  in  the  small  open  pit  east  of  the  Ely  shaft.  Here  the  individual 
quartzite  beds,  from  1  foot  upward  in  thickness,  are  separated  by  parallel 
selvages  of  ground-up  quartzitic  material,  varying  in  thickness  usually 
from  2  to  4  inches.  In  the  case  of  one  the  measured  thickness  was  11 
inches.  These  selvages,  known  locally  as  a  variety  of  "soaprock,"  show 
frequently  a  vertical  jiressure  cleavage. 


Maj.  T.  B.  Brooks  shows,  on  his  large-scale  map  of  Republic  Mountain^ 
and  vicinity,  the  course  of  a  probable  fault  cutting  diagonally  across  the 
syncline  in  a  northeasterly  direction  along  the  river  in  sec.  7,  north  of 
the  horseshoe.  Brooks  was  led  to  infer  the  existence  of  this  fault  from  the 
fact  that  he  regarded  the  diorites  as  regularly  interbedded  and  continuous 
members  of  the  stratified  series,  and  from  his  failure  to  recognize  the 
unconformity  at  the  base  of  the  Goodrich  quartzite. 

On  the  northeast  side  of  the  syncline  diorite  outcrops  a  short  distance 
north  of  the  Milwaukee  and  Northern  Railroad  water  tank,  on  the  east  side 
of  the  river,  and  lies  directly  in  the  line  of  strike  of  jas^jers  that  are  well 
exposed  a  few  hundred  feet  away  on  the  west  bank. 

On  the  southwest  side  of  the  syncline,  north  of  the  horseshoe,  the 
Lower  Marquette  rocks  which  lie  below  the  Goodi'ich  quai'tzite  on  the  east 
side  of  the  river  would,  if  prolonged  along  the  strike,  be  carried  directly 
against  granites  which  occupy  the  west  bank,  west  of  the  West  Republic 

'Geol.  Surv.  of  Michigan,  AtlaB  accompanying  reports  on  Upper  Peninsula,  PI.  VI,  by  T.  B.  BrookB, 
1869-1873. 


542  THE   MAEQUETTE    IROX-BEAEING   DISTRICT. 

mine.  Hence,  on  Brooks's  assumptions,  there  is  displacement  on  both  sides 
of  the  trougli  at  tlie  line  of  the  river,  and  a  fault  is  clearly  indicated. 

It  is  susceptible  of  demonstration,  however,  that  no  fault  exists  involv- 
ing the  Groodrich  quartzite.  On  the  northeast  side  of  the  fold  the  contact 
of  the  upper  quartzite  on  the  lower  series,  which  is  a  surface  of  economic 
interest,  has,  since  the  time  of  Brooks's  studies,  been  definitely  fixed  at 
numerous  points  on  both  sides  of  the  river  where  it  is  not  naturally  exposed 
by  diamond-drill  borings.  These  points,  when  accurately  platted,  fall  on  a 
line  which  shows  no  displacement  at  the  river. 

At  the  southwest  side  of  the  fold  the  upper  quartzite  is  abundantly 
exposed  on  both  sides  of  the  river,  and  its  base  has  been  located  at  many 
points  in  the  West  Republic  mine,  under  the  river,  and  in  several  test  pits 
and  di-ill  holes  on  the  western  side.  The  platting  of  these  data  shows 
conclusively  that  no  displacement  can  exist  which  has  heaved  the  iipper 
quartzite  to  the  extent  of  100  feet. 

The  disappearance  of  the  lower  series  on  the  west  side  of  the  river,  on 
the  southwestern  side  of  the  fold,  presents,  however,  a  real  difficulty.  That 
its  absence  is  due  to  a  sudden  bending  of  the  strike  toward  the  northeast  is 
very  improbable,  because  in  the  few  outcrops  of  the  iron-bearing  member 
nearest  the  river  there  is  almost  perfect  conformity  in  strike  with  the  upper 
quartzite.  Also,  underground  in  the  West  Republic  mine,  the  jasper  was 
followed  nearly  to  the  west  bank  of  the  river.  It  seems  necessary  to  believe, 
therefore,  that  the  formations  of  the  lower  series  continue  without  sensible 
change  in  strike  as  far  as  the  river,  and  there  terminate  squarely  against 
the  granite.  Such  relations  can  be  best  explained  by  supposing  that  the 
granite  on  the  west  bank  either  had  intruded  the  lower  series  or  had  been 
brought  to  the  level  of  the  old  surface  by  a  fault  before  Upper  Marquette 
time.  Between  these  two  explanations  there  is  no  present  means  of 
choosing. 

On  Brooks's  map,  already  referred  to,  a  tongue  of  the  upper  quartzite 
is  represented  as  forking  from  the  main  mass  of  the  same  rock  and  running 
northwest  along  the  top  of  the  Republic  bluff",  a  thin  wedge  of  the  under- 
lying specular  jasper  being  interposed  between  them.     No  explanation  of 


TOE   REPUBLIC   TEOUGH.  543 

this  singular  fact  was  given  by  Brooks  in  the  text  of  the  Michigan  report. 
Wadsworth^  has  hitely  endeavored  to  explain  these  relations  by  the  assump- 
tion that  the  wedge  of  the  specular  jasper  included  between  the  two  quartz- 
ites  does  not  belong  to  the  lower  series,  but  to  the  upper,  and  was  deposited 
later  than  the  quartzite  tongue.  It  is  believed,  however,  that  the  phenomena 
are  really  due  to  faulting.     (PI.  XXXIV,  fig.  2.) 

The  best  exposures  of  the  two  quartzites,  the  included  jasper,  and  the 
underlying  iron  formation  of  the  lower  series,  with  all  the  contacts,  may  be 
seen  on  the  natural  cross-section  afforded  by  the  breaking  down  of  Repulilic 
Mountain  north  of  the  Thompson  pit.  The  conglomerate  at  the  base  of  the 
main  mass  of  the  quarztite  is  exposed  on  the  steep  western  face  of  the  blutf. 
It  holds  pebbles  of  red  jasper,  of  jasper  banded  with  ore,  of  ore,  and  of 
quartz,  which  last,  with  ferruginous  matter,  forms  the  cement.  The  jasper 
inclusions  are  large,  many  of  them  are  angular,  and  near  the  contact  small 
quartz  grains  fill  irregular  cracks  in  the  underlying  jasper  tongue.  The 
conglomerate  is  distinctly  basal,  and  unquestionably  was  laid  down  on  an 
eroded  surface.  From  this  contact,  for  about  16  feet  to  the  east,  the  jasper 
tongue  comes  in.  This  rock  is  greatly  brecciated,  but  it  contains  no  mix- 
ture of  foreign  fragmental  material.  To  the  eye  and  under  the  microscope 
it  is  not  to  be  distinguished  from  the  ordinary  specular  jasper  of  the  under- 
lying iron  formation.  For  the  next  5  feet  occurs  a  mixture  of  large  angular 
pieces  of  jasper  (one  measured  3  feet  by  1  foot),  of  quartz,  and  probably 
quartzite,  many  somewhat  rounded  pebble-like  forms  of  all  these,  and  much 
siliceous  cement.  About  6  feet  of  westerly  dipping  quartzite,  constituting 
the  quartzite  tongue,  follow,  and  then  come  3  to  4  feet  of  conglomerate, 
entirely  similar  to  the  first  conglomerate  and  having  similar  relations  to  the 
specular  jasper,  which  continues  in  an  unbroken  body  to  the  east. 

The  significant  facts  at  this  contact,  which  seem  clearly  fatal  to  the  idea 
that  the  jasper  is  an  interbedded  member  of  the  upper  series,  are  these: 
The  conglomerate  at  the  base  of  the  main  quartzite  is  as  clearly  separated 
from  the  jasper  wedge  by  an  erosion  interval  as  the  conglomerate  below  the 
quartzite  tongue  is  from  the  main  mass  of  specular  jasper.    The  jasper 

1  Report  of  the  State  Board  of  Geological  Survey,  Lansing,  1893,  pp.  129-130. 


544  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

wedge  can  not  belong  to  the  upper  series  unless  there  are  two  upper  series. 
The  jasper  of  the  wedge,  also,  is  not  a  fragmental  rock,  and  in  it  no  con- 
temporaneous fragmental  material  has  been  recognized  except  near  the 
lower  and  upper  contacts.  The  jasper  disappears  a  short  distance  south  of 
this  section,  the  two  quartzites  coming  together.  If  this  is  a  member  of  the 
upper  series,  it  must  have  been  laid  down  at  the  same  time  that  a  rather 
coarse  fragmental  rock  was  being  deposited  a  few  hundi'ed  yards  away. 
It  is  hardly  conceivable  that  under  these  circumstances  clastic  material 
should  not  have  been  mingled  with  it. 

While  the  relations  of  the  quartzite  tongue  are  correctly  represented 
on  Brooks's  map,  the  vastly  better  surface  exposures  of  the  present  day  and 
the  large  amount  of  exploration  done  by  the  Republic  Company  enable  its 
position  now  to  be  fixed  with  much  greater  precision. 

Several  diamond-drill  holes  north  of  the  Thompson  pit  have  shown 
that  the  quartzite  tongue  extends  500  to  600  feet  north  of  the  point  where 
it  terminates  on  Brooks's  map,  and  becomes  steadily  naiTOwer.  As  it  does 
not  appear  at  the  Kingston  and  Kloman  exposures,  on  the  west  side  of  the 
river,  there  is  little  doubt  that  it  gradually  dies  out,  and  that  the  jasper 
wedge  finally  merges  into  the  main  body  of  sjjecular  jasper. 

Therefore  the  facts  to  be  explained  appear  to  be  these  (PI.  XXXIV, 
fig.  2):  A  quartzite  tongue  branches  in  the  south  from  a  large  mass  of 
similar  quartzite,  and  after  continuing  parallel  to  it  for  a  long  distance 
finally  tapers  to  a  point  in  the  north  in  a  mass  of  specular  jasper.  The 
quartzite  tongue  includes  between  itself  and  the  main  quartzite  mass  an 
exactly  similar  jasper  tongue,  which  starts  in  the  north  from  a  mass  of  spec- 
ular jasper  and  tapers  to  a  point  in  the  south  in  quartzite,  the  two  tongues 
interlocking.  The  quartzite  in  each  case,  in  the  tongue  and  in  the  main 
mass,  has  similar  and  unusual  relations  (those  marking  a  time-break)  with 
the  jasper  of  the  tongue  and  of  the  main  mass.  The  identity  of  the  two 
jaspers  and  of  the  two  quartzites  must  be  taken  as  established,  and  the 
explanation  of  the  facts  must  be  sought  in  faulting. 

In  the  horseshoe  turn  the  material  above  the  neutral  surface  yielded 
to  the  compression  in  part  by  slipping  along  bedding  planes.     If  for  any 


PLATE    XXXIT. 


MON   XXVIII- 


Plate  XXaIV.— GEOLOGICAL  MAP  OF  SOUTHEAST  END  OF  REPUBLIC  HORSESHOE. 

Fig.  1.  Southeast  corner  ol"  the  horseshoe,  showing  the  surface  relations  of  the  magnetite  anil  hematite 
to  the  .jasper,  quartzite,  and  soaprock.  The  larger  ore  deposits  of  magnetite  are  bottomed 
by  soaprock,  making  steeply  pitching  troughs.  The  specular  ore  is  for  the  greater  part  of 
the  distance  separated  by  a  belt  of  soaprock  from  the  magnetite  ore  and  quartzite. 

Fig.  2.  Map  of  the  vicinity  of  Republic  mine,  showing  the  contact  between  the  Lower  Marquette 
and  Upper  Marquette  series  and  the  quartzite  tongue.     It  will  be  seen  that  the  great  ore 
deposits  occur  in  the  two  plunging  synolines  at  the  southeastern  bend  of  the  trough. 
546 


GEOLOGICAL  MAP  OF  THE  SOUTHEAST  END  OF  THE   REPUBLIC  HORSESHOE. 


THE   EErUBLIC  TROUGH.  547 

reason  such  movement  could  take  place  more  readily  along  any  one  surface, 
the  neighboring  surfaces  would  be  relieved  and  one  of  maximum  movement 
would  result.  It  is  conceivable  that  in  the  same  way  one  local  maximum 
might  relieve  several  neighboring  maxima,  and  so  a  large  amount  of  move- 
ment might  be  accumulated  along  a  single  surface.  A  maximum  movement 
starting  in  the  specular  jasper  would,  on  account  of  the  slight  tipward  con- 
vergence of  dip,  necessarily  tend  to  cut  across  the  quartzite  at  the  contact. 
The  quartzite  might  be  traversed  until  a  surface  of  maximum  movement  in 
it  was  reached,  which  would  then  be  followed  and  a  fault  would  result, 
which  in  the  direction  of  the  strike  might  easily  pass  from  one  rock  to  the 
other  more  than  once.  It  is  evident  that  a  break  formed  under  such  condi- 
tions, accompanied  by  considerable  displacement,  would  result  in  the  surface 
relations  that  may  now  be  observed  on  Republic  Mountain. 


THE  ORE  DEPOSITS. 
POSITION    OF   ORE   DEPOSITS. 

The  iron  ores  of  the  Republic  area  all  belong  to  the  hard-ore  class, 
and  are  both  magnetite  and  specular  hematite,  the  specular  slate  ores  being- 
the  more  abundant.  They  occur  in  bodies  of  very  irregular  shape  and 
sometimes  of  great  size.  The  rule  that  has  generally  guided  exploration 
in  the  Marquette  district,  that  the  hard-ore  bodies  occur  immediately  at  or 
not  far  beneath  the  base  of  the  upper  quartzite,  holds  good  in  the  Republic 
area.  It  is  a  significant  fact  that  while  this  rule  of  occurrence  beneath  the 
upper  quartzite  has  few  or  no  exceptions,  the  position  of  the  ore  bodies 
with  reference  to  the  base  of  tlie  lower  series  is  exceedingly  variable.  At 
Republic  Mountain  and  at  the  Michigamme  mine  the  ore  bodies  lie  at  least 
1,500  feet  above  the  granites.  At  Champion  and  at  the  Riverside  mine  the 
distance  is  not  more  than  400  feet.  The  hard-ore  bodies  are  therefore  not 
confined  to  any  one  horizon  in  the  iron-bearing  member,  liut  occur  at  the 
particular  horizon  to  which  it  happened  to  be  eroded  at  the  time  the 
upper  quartzite  was  deposited.  The  contact  deposits  of  the  Republic  area 
have  relationships  with  both  the  lower  and  upper  series.  Some  are  appar- 
ently entirely  within  the  upper  series;  others  are  certainly  entirely  within 


548  THE  MAEQUETTE   IRON-BEARING  DISTRICT. 

the  lower  series;  others  again  are  partly  in  both.  In  form  the  bodies 
are  exceedingly  irregular,  but  they  may  be  described  in  general  terms  aa 
pod-like  in  shape,  the  two  horizontal  dimensions  being  usually  very  much 
smaller  than  the  third,  which  follows  down  roughly  parallel  to  the  dip  plane 
of  the  quartzite,  often  pitching  to  one  side  of  the  vertical  plane  normal  to  the 
dip  plane.  Of  the  two  horizontal  dimensions,  the  longer  is  usually  parallel 
with  the  stratification,  and  the  shorter  normal  to  it. 

Where  the  ore  deposits  are  wholly  contained  within  the  lower  series, 
the  contacts  between  them  and  the  rock  of  the  inclosing  iron  formation  are 
usually  as  follows:  The  siliceous  bands  in  the  ferruginous  rock  become 
separated  into  lenses  by  the  encroachment  of  the  adjoining  iron  bands  at 
frequent  intervals  along  their  length,  and  grow  nan-ower.  The  siliceous 
material  of  which  they  are  composed  becomes  mixed  with  a  larger  propor- 
tion of  the  iron  oxides,  and  as  the  boundaries  of  the  rich  ore  are  approached 
the  bands  separate  into  oval-shaped  units.  These  finally  disappear  partly 
or  entirely,  and  the  adjacent  iron  oxides  fill  the  whole  volume  of  the  banded 
rock.  Such  passages  from  the  banded  rock  of  the  iron  formation  to  rich 
ore  take  place  both  along  and  across  the  strike.  Sometimes  the  passage  is 
very  gradual,  leaving  a  large  zone  of  more  or  less  lean  ore  between  the 
rich  ore  and  the  rock;  but  often  it  is  very  sudden,  and  the  line  of  demar- 
cation is  sharp.  Such  sudden  passages  have  been  observed  at  the  line  of 
cross  joints,  along  which  minute  faultings  have  taken  place.  It  is  unusual 
to  find  any  ore  deposit,  however,  that  is  directly  surrounded  Ijy  the  iron- 
formation  rock  on  all  sides.  Generally  on  either  the  hanging  or  foot  wall 
soaprock  intei'veues  between,  somewhere  along  the  surface  of  contact;  and 
in  these  cases  the  iron  formation  is  usually  present  on  one  side,  while  the 
rich  ore  comes  up  to  the  soaprock  on  the  other.  These  bodies  of  soaprock 
have  already  been  referred  to  as  old  dikes  and  intrusive  sheets  of  igneous 
material. 

Of  the  deposits  entirely  within  the  upper  series  two  classes  may  be 
distinguished.  The  first  are  those  deposits  which  lie  at  the  base  of  the 
upper  series,  and  really  represent  an  enriched,  very  ferruginous  phase  of 
the  basal  conglomerate.  At  many  localities  the  Upper  Marquette  conglom- 
erate is  made  up  of  siliceous  pebbles  embedded  in  a  cement  of  iron  ore,  part 


THE   EEPUBLIC   TEOUGH.  549 

of  which  is  in  many  cases  unquestionably  detrital.  Where  the  quavtzose 
pebbles,  from  the  conditions  of  sedimentation,  happen  to  be  few  and  small, 
or  where  they  have  been  removed  by  subsequent  changes,  the  conglomerate 
may  contain  enough  iron  to  constitute  a  valuable  ore.  No  large  deposits 
of  this  character  have  been  demonstrated  to  exist  in  the  Republic  area,  but 
some  good  examples  on  a  small  scale  may  be  seen  about  Republic  Moun- 
tain. Such  ores  are  usually  magnetic.  It  is  believed  that  a  considerable 
part  of  the  magnetite  deposits  of  the  Champion  mine  belong  to  this  class. 
In  the  Republic  area,  around  the  borders  of  the  magnetite  deposits,  where 
they  become  too  lean  to  mine,  occur  certain  peculiar  rocks,  mixtures  of 
clear  quartz  and  magnetite,  which  are  usually  known  as  "black-ore  jasper." 
Higher  in  the  lower  quartzite,  thin,  regularly  bedded  bands  of  magnetite 
and  quartz  occur,  which  occasionally  rise  high  enough  in  iron  to  become 
ores.  They  are  found  usually  a  short  distance  above  the  transgression 
plane,  and  are  separated  from  it  by  a  small  thickness  of  quartzite. 

In  certain  cases  the  line  of  contact  between  the  upper  and  lower  series 
may  be  traced  directly  through  an  ore  body,  which  thus  belongs  partly  in 
one  series  and  partly  in  the  other.  Excellent  examples  may  be  seen  at  the 
Kloman  open  pits,  north  of  Republic  Mountain. 

RELATIONS    OF    THE    ORE    DEPOSITS    TO    THE    GEOLOGICAL    STRUCTURE. 

In  the  Republic  area  the  only  deposits  that  have  had  a  commercial 
value  have  been  found  in  the  immediate  vicinity  of  the  southeast  end  of 
the  fold.  The  largest  single  body  occurs  at  the  southeast  point  of  the  horse- 
shoe, in  the  easternmost  of  the  three  main  subordinate  synclines  already 
mentioned.  The  middle,  or  Morgan-Pascoe-Ely  syncline,  is  the  locus  of  a 
great  number  of  smaller  deposits.  Several  deposits  occur  also  on  the 
straight  northeastern  side  of  the  trough,  within  three-quarters  of  a  mile  from 
the  horseshoe;  but  in  going  north  along  this  stretch  the  bodies  become 
smaller  and  farther  apart,  and  north  of  the  Kloman  practically  disappear. 
A  very  close  relation  is  thus  indicated  between  the  occuiTence  of  the  iron 
ore  in  large  deposits  and  the  main  structural  features  of  the  trough. 

These  larger  bodies  are  both  magnetite  and  specular  hematite  or  slate 
ore.    (PI.  XXXIV,  fig.  1.)    The  magnetite  bodies  always  occur  immediately 


550  THE  MARQUETTE  IRON-BEAEING  DISTRICT. 

below  the  upper  quartzite,  with  which  they  are  frequently  directly  in  con- 
tact. More  commonly,  however,  the  rich  ore  is  sejiarated  from  the  quartzite 
by  a  small  thickness  of  black-ore  jasper  or  mixed  magnetite  and  quartz, 
usually  banded,  or  sometimes  by  soaprock,  while  immediately  beneath  and 
continuous  with  the  rich  magnetic  ore  specular  ore  is  sometimes  found. 
Soaprock  usually,  and  in  the  case  of  all  the  larger  bodies  invariably,  forms 
the  foot  wall.  The  magnetite  deposits  are  mostly  confined  to  the  eastern 
and  middle  subordinate  synclines,  but  are  also  found  of  small  thickness  in 
depth  along  the  straight  eastern  limb  of  the  main  fold 

The  specular  hematite  or  slate-ore  bodies  occur  both  in  the  contact 
zone  and  below  it,  entirely  within  the  specular  jasper.  (PI.  XXXIV,  fig.  1.) 
As  a  rule  the  deposits  of  the  contact  zone  contain  the  richest  ore,  which  is 
characterized  by  the  large  size  of  the  individual  crystalline  plates.  As  a 
deposit  is  followed  back  from  the  contact  zone  into  the  jasper,  these  plates 
become  progressively  smaller,  and  at  the  same  time  the  ore  grows  more 
siliceous.  The  larger  deposits  of  specular  ore  are  associated  Avith  soaprock, 
which  may  bound  a  deposit  either  on  the  foot  or  the  hanging  wall  side. 

The  deposits  of  specular  ore  that  occur  along  the  straight  eastern  limb 
of  the  fold  all  show  a  well-marked  pitch  toward  the  north,  in  the  general 
direction  of  pitch  of  the  main  fold,  but  at  a  very  much  higher  angle. 
These  bodies  all  lie  in  the  contact  zone  at  the  surface,  having  the  tipper 
quartzite  on  the  hanging  wall.  As  they  are  followed  in  depth  they  are 
found  to  recede  from  the  quartzite,  and  to  folloAv  the  banding  of  the  under- 
lying jaspers,  which  dip  at  a  higher  angle  than  the  quartzite.  They 
terminate  in  depth  entirely  within  the  specular  jasper.  As  one  body  departs 
from  the  quartzite  and  becomes  entirely  inclosed  by  the  jasper  another 
frequently  comes  in  above  it  in  the  contact  zone. 

The  deposits  in  the  subordinate  synclines  in  the  horseshoe  turn  have 
not  shared  the  intense  crumpling  to  which  the  specular  jaspers  have  been 
subjected.  They  occur  in  thin,  unwarped  sheets,  which  start  with  one  edge 
in  the  contact  zone,  having  the  upper  quartzite  on  the  hanging  wall  and 
set  back  from  it  parallel  with  the  local  strike  of  the  closely  folded  jaspers. 
The  longest  dimension  thus  follows  down  the  contact,  pitching  with  the 
dip  of  the  quartzite,  while  the  longer  of  the  two  horizontal  dimensions  is 


THE   EErUBLIG   THOUGH,  551 

usually  normal  to  the  strike  of  the  quartzite.  These  deposits  occur  in  the 
arches  or  on  the  limbs  of  the  minor  contortions,  and  never,  so  far  as 
observed,  occupy  the  troughs.  Their  attitude  Avitli  reference  to  the  general 
strike  of  the  quartzite,  and  the  fact  that  they  do  not  show  the  contortions 
of  the  inclosing  specular  jasper,  prove  that  they  have  come  into  existence 
since  the  folding. 

ORIGIN   OF   THE   ORE   DEPOSITS. 

From  the  form  and  '  general  relations  of  tlie  rich  ore  deposits  it  is 
evident  that  they  were  not  laid  down  as  bodies  of  rich  ore  contemporaneously 
with  the  inclosing  rocks.  It  is  not  conceivable  that  nearly  pure  silica  and 
nearly  pure  iron  oxides  could  be  deposited  under  water  at  the  same  time  on 
opposite  sides  of  an  imaginary  vertical  plane.  Nor  is  it  any  more  probable 
that  they  have  come  up  from  below  as  igneous  dikes  which  have  intruded 
the  sediments  of  the  iron  formation.  The  physical  objections  alone  to  this 
view  are  such  as  entirely  to  exclude  it  from  serious  consideration.  On  the 
other  hand,  the  phenomena  of  their  relations  to  the  inclosing  rocks,  which 
have  been  described,  all  lead  to  the  conclusion  that  they  are  later  concen- 
trations and  indicate  the  main  lines  along  which  the  concentration  was 
brought  about. 

In  general,  this  process  of  concentration  has  been  a  removal  by  cir- 
culating waters,  in  favorable  places,  of  the  silica  of  the  old  rock,  and  its 
contemporaneous  replacement  by  iron  oxides.  This  process  has  gone  on  in 
the  contact  zone,  in  the  detrital  conglomerates,  and  in  the  underlying 
jaspers.  The  evidence  in  both  cases  is  abundant  and  clear.  In  the  case  of 
the  iron  formation  of  the  lower  series  the  siliceous  bands  may  be  traced 
along  the  strike  in  all  stages  of  replacement,  until  finally  they  are  wholly 
represented  by  new  iron  oxides.  In  some  cases  the  new  iron  ore  is  of  coarser 
texture  than  the  old,  and  so  the  original  banded  structure  may  still  be 
traceable  into  a  body  of  nearly  pure  ore.  In  the  case  of  the  conglomei'ates, 
we  see  in  thin  sections  original  rolled  quartz  pebbles,  which  are  sometimes 
surrounded  by  new  growths  of  quartz,  studded  with  iron  oxides  about  the 
periphery.  This  process,  too,  may  be  traced  through  all  stages,  from  cases 
in  which  the  attack  on  the  old  pebble  had  just  begun  to  those  in  which 


552  THE  MARQUETTE   IKON-BEARING  DISTRICT. 

the  quartz  of  the  original  grains  is  ahnost  entirely  gone.  It  is  an  inter- 
esting inquiry,  upon  which,  however,  little  direct  evidence  can  be  brought 
to  bear,  as  to  how  far  the  concentration  in  the  conglomerates  has  depended 
upon  the  new  growth  of  iron  ore  about  rolled  nuclei  of  iron  sand.  Bearing 
possibly  upon  the  question  is  the  fact  that  the  crystalline  plates  of  specular 
hematite  are  frequently  and  perhaps  always  coarser  in  the  slate  ores  that 
occur  in  the  contact  zone  than  in  those  belonging  lower  down  within  the 
iron-bearing  member. 

The  process  of  concenti-ation  in  the  Republic  area  has  not  proceeded 
indiscriminately  throughout  the  iron-bearing  member.  The  distribution 
of  the  important  ore  deposits  shows  that  it  has  been  localized  in  accord- 
ance with  certain  physical  conditions.  The  main  facts  of  distribution  are 
(1)  that  the  ore  bodies  occur  within  or  not  far  below  the  contact  between 
the  upper  quartzite  and  the  iron-bearing  member;  (2)  that  they  occur  in 
pitching  synclines  in  the  vicinity  of  the  greater  orogenic  disturbances; 
(3)  the  larger  bodies  usually  have  a  basement  of  soaprock.  These  relations 
of  distribution  are  so  constant  that  they  must  be  regarded  as  necessary 
conditions.  It  is  evident  that  the  first  two  conditions  were  such  as  to  pro- 
mote comparatively  free  circulation.  In  the  contact  zone  the  loose  texture 
of  the  conglomerates  afforded  connecting  open  spaces  through  which  waters 
could  readily  pass.  It  is  equally  evident  that  the  general  breaking  up 
attending  sharp  folding  in  the  underlying  iron  formation  would  not  only 
open  channels  for  percolating  waters  but  would  also  reduce  the  siliceous 
bands  to  a  condition  in  which  they  could  be  readily  attacked.  The  third 
condition  was  favorable  to  the  concentration  of  the  iron-bearing  percolating 
waters. 

From  the  relations  which  the  ore  deposits  bear  to  the  structure  produced 
at  the  time  of  the  later  folding,  it  clearly  appears  that  much  of  the  concen- 
tration has  been  effected  since  Upper  Marquette  time ;  but  it  does  not  follow 
that  some  of  the  iron  oxide  of  the  deposits  was  not  already  in  existence  at 
the  time  of  the  Upper  Marquette  transgression.  If,  as  there  is  strong  reason 
for  believing,  the  specular  character  of  the  hematite  of  the  jaspers  and  of 
the  rich  ore  deposits  of  the  Eepublic  area  is  connected  with  differential 
movements  of  bed  on  bed,  produced  at  the  time  of  the  latest  folding,  it  is 


THE   REPUBLIC   TEOUGH.  553 

necessary  to  believe  that  concentratiou  either  has  preceded  the  folding  or 
went  on  contemporaneously  with  it.  In  the  case  also  of  those  deposits 
which  are  traversed  by  the  plane  of  division  between  the  upper  and  lower 
series,  and  so  lie  partly  in  both,  the  part  of  the  deposit  in  the  lower  series 
may  have  been  partly  concentrated  at  the  time  of  the  Upper  Marquette 
transgression,  while  that  in  the  upper  series  may  be  partly  fine  ddbris 
derived  from  the  underlying  body.  In  all  the  deposits  the  processes  of 
enrichment  have  doubtless  proceeded  continuously  through  all  subsequent 
time. 


CHAPTER  VII. 
GENERAL  GEOLOGY. 


By  C.  E.  Van  Hise. 


In  considering  the  general  geology  of  the  Marqnette  district  we  have 
to  deal  with  three  series :  The  Basement  Complex,  the  Lower  Marquette, 
and  the  Upper  Marquette.  These  three  series  are  separated  by  uncon- 
formities. The  Basement  Complex  includes  granites,  syenites,  gneisses, 
many  finely  crystalline  schists,  surface  volcanics,  and  various  subsequent 
intrusives.  The  complex  south  of  the  Marquette  series  is  spoken  of  as 
the  Southern  Complex;  that  to  the  north  as  the  Northern  Complex.  The 
Lower  Marquette  series,  from  the  base  upward,  comprises  the  Mesnard 
quartzite,  110  to  670  feet  thick;  the  Kona  dolomite,  425  to  1,375  feet 
thick;  the  Wewe  slate,  550  to  1,050  feet  thick;  the  Ajibik  quartzite,  700  to 
900  feet  thick;  the  Siamo  slate,  200  to  625  feet  thick;  and  the  Negaunee 
iron  formation,  1,000  to  1,500  feet  thick.  We  thus  have  a  minimum  thick- 
ness for  the  series  of  2,975  feet,  and  a  possible  maximum  of  6,120  feet. 
It  is  not  probable  that  any  single  section  will  give  so  great  a  thickness  as 
5,000  feet.  The  Upper  Marquette  series  comprises  the  Ishpeming  fonnation; 
which  includes  the  Goodrich  quartzite  and  the  Bijiki  schist,  the  Michigamme 
formation,  and  the  Clarksburg  formation.  It  is  impossible  to  give  even 
an  approximate  estimate  of  the  thickness  of  the  Upper  Marquette  series, 
but  in  the  district  considered,  excluding  the  volcanics,  it  is  probably  less 
than  5,000  feet.  Licluding  the  volcanic  Clarksburg  formation  the  series 
is  probably  over  5,000  feet  thick. 


THE   BASEMENT  COMPLEX  OF  THE   MARQUETTE   DISTRICT.       555 

Basic  igneous  rocks  intrude  in  an  intricate  manner  both  the  Upper 
Marquette  and  the  Lower  Marquette  series. 

Tlie  aim  of  the  following  paragraphs  is  to  briefly  sketch  the  history  of 
the  district. 

THE  BASEMENT   COMPLEX. 

The  oldest  rocks  of  the  Basement  Complex  are  thoroughly  crystalline, 
foliated  schists  and  gneisses.  A  close  field  and  laboratory  study  has  failed 
to  detect  in  them  any  evidence  of  sedimentary  origin.  If  any  detrital  rocks 
are  included  in  the  Basement  Complex,  they  have  been  so  profoundly  meta- 
morphosed as  to  have  lost  all  evidence  of  their  origin.  These  gneisses  and 
schists  have  been  cut  by  various  igneous  rocks  at  different  epochs.  The 
latter  occur  both  in  the  form  of  great  bosses  and  in  dikes,  sometimes  cutting, 
sometimes  parallel  to,  the  foliation  of  the  rocks.  In  some  cases  the  number 
of  intrusive  belts  of  granite  parallel  to  the  schistosity  is  so  large  and  they 
are  so  narrow  as  to  give  very  numerous  interlaminations  of  schist  and  granite 
within  a  short  distance. 

In  the  area  of  the  Northern  Complex  there  were  volcanic  outbursts, 
and  a  vast  series  of  lavas,  agglomerates,  greenstone-conglomerates,  and 
tufts  were  piled  up.  By  far  the  greater  part  of  the  volcanic  material  is  of 
an  intermediate  or  basic  character.  While  the  material  is  undoubtedly 
a  surface  deposit,  a  search  year  after  year  in  the  field  has  failed  to  reveal 
any  decisive  evidence  of  arrangement  by  water.  The  deposits  are  strictly 
volcanic.  After  the  great  lava  beds  and  the  vast  masses  of  tuft's  were  piled 
up  there  were  granitic,  syenitic,  and  diabasic  intrusions,  for  bosses  of  these 
rocks  and  dikes  from  them  cut  through  the  volcanics. 

After,  and  also  perhaps  during,  the  building  up  of  the  volcanic  series 
the  Marquette  district  was  deeply  truncated,  as  a  consequence  of  which 
many  of  the  different  varieties  of  rocks  composing  the  Basement  Complex 
appeared  at  the  surface.  The  coarse  granites  must  have  formed  as  deep- 
seated  rocks,  and  the  foliation  of  the  schists  must  have  formed  far  below 
the  surface;  such  rocks  could  have  reached  the  surface  only  by  long- 
continued  denudation,  which  removed  mountain  masses  of  materials.  The 
process  continued  until  the  Basement  Complex  had  no  great  altitude,  for 


556  THE   MARQUETTE  IRON  BEARING  DISTRICT. 

before  a  great  thickness  of  the  Lower  Marquette  series  was  deiDOsited  the 
sea  had  entirely  overridden  the  Marquette  district. 

THE   LOWER  MARQUETTE   SERIES. 
THE    TRANSGRESSION    HORIZON. 

Toward  the  close  of  the  pre-Marquette  deiuidation  the  sea  reached  the 
northeast  border  of  the  Marquette  district.  Advancing  upon  it,  perhaps  in 
par{.  as  the  result  of  depression,  but  largely  as  a  consequence  of  subaerial 
and  marine  erosion,  the  fragmental  sediments  of  the  Mesnard  formation  were 
laid  down.  This  advance  steadily  continued  from  the  northeast  toward  the 
southwest  and  west,  the  first  deposits  being  everywhere  fragmental  sedi- 
ments, at  the  base  usually  a  coarse  conglomerate,  and  liigher  up  a  sandstone 
which  subsequently  was  changed  to  the  Mesnard  quartzite.  Long  before 
the  seashore  reached  the  western  end  of  the  district  other  formations  were 
deposited  in  the  eastern  half  of  the  area,  so  that  we  have  some  measure  of 
the  time  required  for  the  transgression.  The  formations  thus  deposited 
above  the  Mesnard  quartzite  before  the  sea  advanced  to  Michigamme  Lake 
were  the  Kona  dolomite  and  the  Wewe  slate.  It  follows,  then,  that  in 
passing  from  the  east  to  the  west  end  of  the  district  there  are  in  the  Lower 
Marquette  series  fewer  and  fewer  formations.  At  the  east  end  is  the  full 
succession ;  at  the  extx'eme  Avestern  are  only  the  two  upper  members.  Lith- 
ologically  the  whole  transgression  horizon  is  one  formation,  marking  as  it 
does  a  continuous  belt  of  conglomerate  and  metamorphosed  sandstone 
immediately  above  the  Basement  Complex.  Chronologically,  however, 
different  parts  of  it  are  to  be  equated  with  several  formations,  that  part  of 
it  only  being  called  the  Mesnard  quartzite  which  was  deposited  before  the 
beginning  of  the  deposition  of  the  next  higher  member,  the  Kona  dolomite, 
and  hence  it  is  necessary  in  the  chronological  scale  to  subdivide  this  lower 
conglomerate  and  quartzite  between  the  various  formations  from  the  Mes- 
nard quartzite  to  the  Ajibik  quartzite.  It  is  not  possible  to  do  this  accu- 
rately in  the  mapping  in  all  places,  and  the  manner  in  which  on  the  maps 
one  formation  feathers  out  against  the  shore-line,  to  be  succeeded  by  the 
next  one,  is  more  or  less  arbitrary,  although  it  so  happens  that  there  is 
no  considerable  difficulty  in  this  particular  for  most  of  the  district.     This 


UNCONFORMITY   AT   BASE   OF   LOWEE  MARQUETTE   SERIES.      557 

arbitrary  subdivision  is  most  conspicuous  in  the  quartzite  which  occurs  east 
of  Teal  Lake.     (Atlas  Sheet  XXX.) 

UNCONFORMITY  AT  THE  BASE  OF  THE  LOWER  MARQUETTE  SERIES. 

As  evidence  of  the  unconformity  between  the  Lower  Marquette  series 
and  the  Basement  Complex  is  found  all  along  the  lower  part  of  the  trans- 
gression quartzite,  the  phenomena  showing  unconformity  are  mentioned  here 
rather  than  in  connection  with  the  separate  formations  among  which  this 
belt  is  divided.  However,  for  the  exact  locations  and  detailed  descriptions 
of  particular  contacts  it  will  be  necessary  to  refer  to  the  descriptions  of  the 
individual  formations. 

At  the  east  end  of  the  south  side  of  the  Marquette  district  there  are 
numerous  localities  from  Lake  Superior  to  west  of  Lake  Mary  where  a 
granite-conglomerate  is  found  bearing  numerous  bowlders  of  granite,  gneiss, 
and  schist,  identical  with  the  rocks  constituting  the  Basement  Complex 
immediately  adjacent.  At  several  of  these  localities  the  actual  contact 
between  the  Mesnard  quartzite  and  the  Basement  Complex  is  seen.  Some 
distance  farther  to  the  west  the  Marquette  formations  reach  the  Pleisto- 
cene sand  plain,  the  Basement  Complex  not  being  exposed.  Passing  this 
area,  we  next  find  in  the  Marquette  series  two  islands  of  the  Basement 
Complex  in  sees.  22  and  23,  T.  47  N.,  R.  26  W.  (Atlas  Sheet  XXXV). 
Here  are  found  most  magnificent  exposures  of  great  bowlder-conglomerate 
and  recomposed  granite,  resting  with  visible  contact  upon  the  Basement 
Complex  and  composed  of  material  mainly  derived  from  it  (fig.  11).  In 
sec.  23  the  predominant  rock  of  the  Basement  Complex  is  a  peculiar  white 
schistose  granite,  and  the  predominant  bowlders  of  the  conglomerate  are 
of  the  same  character.  South  of  the  Cascade  range,  there  are  again  a 
number  of  localities  from  sees.  34  to  32,  T.  47  N.,  R.  26  W.  (Atlas  Sheets 
XXXII  and  XXXV),  where  are  basal  conglomerates,  the  great  bowlders 
again  being  mainly  identical  with  the  adjacent  granites,  gneisses,  and  schists 
of  the  Basement  Complex.  In  this  area  in  the  Basement  Complex  ai-e 
some  peculiar  basic  eruptives,  and  these  rocks  are  found  in  the  form  of 
well-rounded  waterworn  bowlders  in  the  conglomerate.  Toward  the  west, 
the  next  exposure  of  basal  conglomerate  is  south  of  Summit  Mountain, 


558  THE   MAEQUETTE   IRON-BEAEING   DISTIUCT. 

in  the  west  half  of  sec.  25,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXIX).  The 
conglomerate  at  this  place  grades  downward  into  a  schist  which  is  scarcely 
distinguishable  from  the  Palmer  gneiss,  with  which  it  is  in  contact.  The 
next  contact  to  the  west  is  in  sec.  28,  T.  47  N.,  R.  27  W.  (Atlas  Sheet  XXV). 
Here  the  phenomena  are  similar  to  those  south  of  Summit  Mountain. 
West  of  this  place  no  actual  contacts  between  the  qviartzite  and  the  Base- 
ment Complex  are  found  until  the  end  of  the  Republic  trough  is  reached 
(Atlas  Sheet  XI),  where  again  a  conglomerate  hangs  with  visible  contact 
upon  the  flank  of  the  granite,  bearing  well-rounded  waterwom  bowlders 
from  it. 

At  the  north  side  of  the  Lower  Marquette  series,  and  near  the  east  end 
of  the  district,  there  is  exposed  a  magnificent  basal  conglomerate  about  3 
miles  west  of  Marquette,  north  of  Mud  Lake  (Atlas  Sheet  XXXVI).  Here 
the  rocks  adjacent  to  the  Mesnard  quartzite  are  the  Mona  schists,  and  these 
peculiar  rocks  are  largely  found  as  detritus  in  the  basal  conglomerate. 
Here  also  are  found  granite  bowlders  similar  to  the  granite  masses  which  a 
short  distance  to  the  north  intrude  the  volcanics  of  the  Northern  Complex- 
The  next  known  contacts  to  the  west  are  at  the  base  of  the  quartzite  east 
and  west  of  Teal  Lake  (Atlas  Sheets  XXVII  and  XXX).  Here,  at  a  half 
dozen  places,  contacts  are  found,  each  of  the  conglomerates  having,  as 
usual,  as  their  abundant  detritus,  the  immediately  subjacent  material  at  the 
particular  locality.  At  one  place  the  relations  are  such  that  the  layers  of 
the  conglomerate  cut  across  the  foliation  of  the  subjacent  schist  at  an 
acute  angle  (fig.  14).  Still  farther  west,  in  sec.  3'0,  T.  48  N.,  R.  28  W.,  the 
quartzite  is  found  in  visible  contact  with  the  granite  at  a  number  of  places 
(Atlas  Sheet  XVIII),  and  again  its  most  abundant  material  is  exactly  like 
the  subjacent  granite.  In  some  of  the  places  the  basal  rock  is  a  con- 
glomerate, in  others  a  "recomposed"  granite — i.  e.,  it  is  composed  of  the 
separate  minerals  of  the  underlying  granite.  West  of  this  point  the  only 
actual  contact  known  is  north  of  the  Michigamme  mine,  although  at  a 
number  of  places  strongly  feldspathic  quartzites  occur  near  the  granite. 

We  thus  have  more  than  a  score  of  localities,  scattered  about  the  entire 
area  covered  by  the  Lower  Marquette  rocks,  where  occur  great  basal 
conglomerates,  a  number  of  which  rest  with  visible  contact  upon  the  rocks  of 


UNCONFOKMITY   AT   BASE   OF   LOWER  MARQUETTE   SERIES.      559 
the  Basement  Complex.     In  all  of  the  cases  the  detritus  is  most  distinctly 
watevwovn,  and  while  the  major  portion  of  the  material  in  each  case  must 
h^xve  been  derived  from  the  immediately  subjacent  part  of  the  Basement 
Complex   other  material  not  occurring  in  the  immediate  neighborhood  is 
found    thus    showing    conclusively  that   these  rocks  are  not  reibungs  or 
fiult  breccias      The  evidence  is  therefore  demonstrative  that  the  Lower 
Marquette  series  was  deposited  unconformably  upon  the  Basement  Complex. 
As  explained  later,  it  will  be  seen  that  locally,  as  a  result  of  the  power- 
ful dynamic  action  to  which  the  rocks  have  been  subjected,  the  foliation  of 
the  Basement  Complex  and  that  of  the  basal  quartzite  are  in  the  same 
direction,  and  that  at  certain  localities  the  basal  conglomerate  and  quartzite 
have  been  so  mashed  as  to  pass  into  completely  crystalline  schists,  which 
appear  to  grade  down  into  the  foliated  schist  or  gneiss  of  the  Basement 
Complex      As  a  consequence,  the  granites  of  the  Basement  Complex  have 
been  described  by  certain  geologists  as  intrusive  within  the  Lower  Marquette 
series      Others  have  said  that  it  is  a  case  of  downward-progressing  meta- 
morphism.     Taking  into  account  the  above  facts  as  to  the  contacts  and 
conglomerates,  there  is  no  escape  from  the  conclusion  that  this  apparent 
conformity  and  gradation  are  illusory,  being  produced  by  the  metamor- 
phosing processes  of  profound  dynamic  action  and  metasomatic  changes. 

DEPOSITION  OF  THE  LOWER  MARQUETTE  SERIES. 

In  the  eariier  part  of  Lower  Marquette  time,  the  sea  steadily  trans- 
gressed southwestward  from  the  northeast,  depositing  a  basal  conglomerate 
as  it  advanced.  As  soon  as  the  sea  had  progressed  a  little  beyond  a  given 
place,  the  deposition  of  sandstones  there  replaced  that  of  the  conglom- 
erates (See  Atlas  Sheet  IV.)  By  the  time  the  sea  had  transgressed  as  far 
as  Teal  Lake  on  the  north  and  Goose  Lake  on  the  south,  argillaceous  and 
sihceous  hmestones  began  to  be  deposited  in  the  east  end  of  the  district  and 
hence  the  western  limit  of  the  Mesnard  quartzite  is  placed  at  these  localities. 
The  Kona  dolomite  probably  marks  deeper  and  quieter  waters,  and  therefore 
indicates  that  depression  had  been  continuing.  A  thin  layer  of _  slate  marks 
intermediate  conditions  between  those  favorable  to  the  deposition  of  sand- 
stone and  those  in  which  the  limestone  was  deposited.     However,  the  area 


560  THE   MAEQUETTE   lEON  BEARING  DISTEICT. 

of  limestone-building  was  too  near  shore  and  the  water  too  shallow  for  a 
purenon  fragmental  formation  to  be  built  up,  especially  as  vigorous  erosion 
still  continued  on  the  adjacent  land,  and  hence  it  is  that  even  the  purest 
dolomite  beds  bear  a  greater  or  less  quantity  of  fragmental  material,  while 
they  are  frequently  interstratified  with  shale,  graywacke,  and  quartzite. 
Because  the  sea  had  not  yet  overridden  the  lands  of  the  central  part  of  the 
district,  the  Kona  dolomite  is  limited  to  its  eastern  part.  On  the  south 
side  of  the  district  the  westernmost  exposures  occur  at  Goose  Lake,  and  on 
the  north  side  the  most  westerly  exposures  which  clearly  belong  to  this 
formation  are  those  at  Morgan  Furnace,  although  a  belt  of  slates  very 
similar  to  those  associated  with  the  Kona  dolomite  occurs  interstratified 
with  the  quartzites  east  of  Teal  Lake.  This  belt  feathers  out  about  1  mile 
east  of  Teal  Lake,  and  this  suggests  that  here  was  the  western  limit  of  the 
shore-line  at  the  end  of  Kona  time. 

As  a  consequence  of  the  upbuilding  of  the  Kona  formation,  combined, 
perhaps,  with  a  cessation  of  subsidence,  the  waters  again  became  shallow, 
and  there  followed  above  the  Kona  dolomite  the  Wewe  slate.  The  inter- 
mediate conditions  favorable  for  mud  deposits  continued  for  some  time.  On 
the  south  side  of  the  district  the  western  limit  of  the  shore-line  at  this  time 
was  in  the  eastern  half  of  sec.  21,  T.  47  N.,  R.  26  W.,  and  on  the  north  side 
probably  at  or  near  Teal  Lake.  By  the  upbuilding  of  the  beds  the  waters 
became  shallower  and  shallower  until  the  waves  of  the  sea  were  able  to 
transport  sand  throughout  the  area  submerged.  There  is  evidence  that  in 
some  localities  the  compacted  mud  arose  near  to  or  above  the  surface  of  the 
water,  so  as  to  be  cut  by  the  waves  and  yield  fi'agments  to  the  succeeding 
sandstone.  The  sandstone  has  been  subsequently  indurated  to  a  quartzite, 
and  hence  there  follows  above  the  Wewe  slate  the  Ajibik  quartzite. 

During  the  time  of  the  deposition  of  the  Kona  dolomite  and  Wewe  slate 
the  sea  did  not  advance  very  rapidly,  but  erosion  had  been  steadily  wearing 
down  the  highlands,  and  during  the  deposition  of  the  sandstone  following 
these  formations  there  was  a  rapid  advance  of  the  sea  toward  the  west. 
On  the  north  the  sea  of  Ajibik  time  pushed  west  to  Michigamme,  and 
on  the  south  as  far  west  at  least  as  the  Goodrich  mine.  The  sea  there- 
fore gained  farther  at  the  north  than  at  the  south,  the  shore-line  apparently 


DEPOSITION   OF   THE   LOWER   MARQUETTE    SERIES.  561 

being  diagonal,  running  in  a  northwest  -  southeast  direction,  still  fiirtlier 
suggesting  what  was  said  at  first,  that  the  advance  of  the  sea  was  from  the 
northeast.  The  subsidence  continued  faster  than  the  upbuilding  of  the 
sands,  so  that  there  followed  above  them  mud  deposits,  which  have  been 
compacted  into  the  Siamo  slate.  During  the  time  of  mud  deposits  the  shore- 
line continued  to  advance,  and  before  this  formation  was  completed  the 
sea  had  entirely  overriden  the  Marquette  district,  with  the  possible  excep- 
tion of  the  southwestern  part.  Following  naturally  from  the  conditions 
of  deposition,  the  Siamo  slate  has  a  greater  thickness  in  the  eastern  than 
in  the  western  part  of  the  district,  and  it  does  not  appear  in  the  southwestern 
part.  Perhaps  equivalent  to  some  part  of  the  Siamo  slate  in  age  is  the 
basal  quartzite  from  Humboldt  to  Republic,  but  as  it  is  impossible  to  say 
what  part  of  the  quartzite  belongs  with  the  Siamo  slate  and  what  part 
with  the  Ajibik  quartzite,  it  is  all  mapped  as  the  latter  formation  because  of 
its  lithological  likeness  to  it. 

The  steady  subsidence  during  the  deposition  of  the  Siamo  slate  so 
increased  the  depth  of  water  that  a  nonfragmental  formation  began  to 
be  deposited.  This  was  the  siderite  slate,  which  has  been  largely  trans- 
formed into  the  varieties  of  rocks  of  the  iron -bearing  formation.  The 
conditions  which  led  to  the  deposition  of  the  iron  carbonate  are  not  certainly 
known.  At  that  time  the  Marquette  transgression  had  entirely  overridden 
the  land  of  the  district,  but  it  is  not  probable  that  all  adjacent  land  areas 
had  disappeai-ed,  or  even  that  the  green  schists  of  the  Northern  Complex 
were  entirely  covered  by  the  sea,  although  it  is  possible,  or  even  probable, 
that  the  long-continued  erosion  had  reduced  the  land  areas  nearly  to  base- 
level,  and  consequently  that  chemical  solution,  rather  than  mechanical 
wear,  was  the  more  important  agent  of  erosion.  Thus  might  be  explained 
the  large  amount  of  iron  salts  which  appeared.  Doubtless  the  supply  of 
ferruginous  material  was  in  the  form  of  iron  carbonate,  taken  into  solution 
by  direct  atmospheric  agencies,  perhaps  with  the  assistance  of  organic 
acids.  The  basic  eruptives  of  the  Basement  Complex,  and  especially  the 
surface  volcanics  on  the  northern  border  of  the  district,  are  very  rich  in 
iron.  These  latter,  being  tuffs  and  lavas,  were  porous,  and  perhaps  from 
them  came  the  greater  proportion  of  iron.  In  the  water,  also,  there  was 
MON  xxviii 3G 


562  THE    MAEQUETTB    lEON-BEAEING   DISTRICT. 

doubtless  life.  As  the  iron  carbonate  came  down  into  the  open  water  it 
was  peroxidized  and  the  iron  precipitated  as  hydi-ated  oxide.  Wben  this  was 
bm-ied  with  organic  matter  the  decomposition  of  the  latter  produced  carbon 
dioxide,  and  the  iron  was  reduced  to  the  protoxide  by  the  organic  matter. 
The  two  combined  and  reproduced  iron  carbonate.  Whether  the  area  of 
deposition  of  iron  carbonate  was  an  arm  of  a  large  sea  or  an  almost  inclosed 
lagoon,  there  are  no  means  of  ascertaining,  but  the  widespread  distribution 
of  this  inferior  iron-bearing  formation  in  the  Lake  Superior  region  suggests 
that  the  areas  of  deposition  of  such  material  -were  very  large. 

ERUPTIVES    OF    LOWER    MARQUETTE    TIME. 

At  one  locality  amygdaloids  are  interstratfied  with  the  Siamo  slates. 
In  others,  closely  associated  with  the  Negaunee  iron  formation  are  volcanic 
tuffs.  It  thus  appears  that  in  later  Lower  Marquette  time  there  was  vol- 
canic action.  Just  how  extensive  the  volcanoes  were  has  not  yet  been 
determined,  as  these  rocks  have  not  in  all  cases  been  discriminated  from  the 
later  igneous  rocks. 

UNCONFORMITY   AT   THE   TOP   OF   THE   LOWER   MARQUETTE   SERIES. 

Whether  any  later  formations  followed  conformably  upon  the  Negaunee 
iron-bearing  formation  it  is  impossible  to  say,  but  if  so  they  were  subse- 
quently removed  by  erosion.  Following  the  deposition  of  the  Negaunee 
formation  and  all  possible  later  conformable  formations,  the  land  was  raised 
above  the  sea,  gently  folded,  and  eroded.  In  general  the  discordance  between 
the  Lower  Marquette  series  and  the  succeeding  series  is  not  great,  being  meas- 
ured frequently  by  5°  to  10°,  at  other  times  by  10°  to  15°,  and  it  is  only 
rarely  that  the  plications  of  the  lower  series  are  such  as  to  make  the  beds 
abut  perpendicularly  against  those  of  the  overlying  series.  In  these  cases 
the  truncated  layers  are  those  of  the  minor  plications  rather  than  the  major 
folds  (figs.  20  and  21).  Erosion  cut  deeper  in  the  Lower  Marquette  series 
in  some  places  than  in  others.  At  the  east  end  of  the  area  it  left  a  very 
considerable  thickness  of  the  iron-bearing  formation,  but  in  places  to  the 
west  this  formation  is  quite  cut  out.  Indeed,  in  places  erosion  cut  through 
the  Siamo  slate  and  the  Ajibik  quartzite,  and  in  some  places  even  into  the 


UNCONFORMITY  AT  TOP   OP  LOWER   MARQUETTE   SERIES.       563 

Basement  Complex.  This  particularly  occurs  in  the  west  and  southwest 
parts  of  the  district,  west  of  Champion  and  along  the  Republic  tongue, 
where  but  few  members  of  the  Lower  Marquette  series  were  deposited. 
Even  within  a  short  distance  the  differential  erosion  was  considerable.  For 
instance,  at  the  south  end  of  the  Republic  tongue  the  variation  was  more 
than  1,500  feet. 

To  just  what  extent  the  Lower  Marquette  series  was  altered  during 
this  period  of  folding  and  erosion  it  is  impossible  to  say.  It  is  probable 
that  the  upper  formation,  consisting  of  the  readily  altered  iron  carbonate, 
suffered  the  most,  and  there  are  indications  that  ferruginous  chert  and  jasper 
were  formed  in  the  upper  part  of  the  formation.  At  least  fragments  of  such 
materials  are  found  in  the  succeeding  formation,  and  either  these  rocks  were 
produced  from  the  iron  carbonate  during  this  folding  and  erosion  or  else 
the  iron-carbonate  bowlders  and  fragments,  in  common  with  portions  of  the 
Negaunee  formation,  were  at  a  later  time  altered  in  a  like  manner,  so  as  to 
produce  the  same  mineral  combinations  in  the  fragments  and  in  the 
Negaunee  formation  itself  It  is  probable  that  such  subsequent  modifi- 
cation has  occurred  to  some  degree,  but  many  would  doubt  whether  it 
were  possible  for  such  exactly  similar  changes  to  have  occun-ed  as  to  make 
the  bowlders  and  fragments  of  cherty  siderite  and  the  siderite  of  the  under- 
lying Negaunee  formation  into  precisely  similar  chert  and  jasper. 

THE  UPPER  MARQUETTE  SERIES. 

DEPOSITION  OF  THE  UPPER  MARQUETTE  SERIES. 

The  Upper  Marquette  history  begins  with  the  second  transgression  of 
the  sea,  as  a  result  of  which  the  Ishpeming  formation  was  deposited.  If  we 
may  judge  by  the  greater  thickness  of  the  Goodrich  quartzite  of  this  forma- 
tion at  the  eastern  part  of  the  district,  and  the  greater  erosion  of  the  Negaunee 
formation  at  the  western  part,  an  anticline  had  formed  to  the  west,  and  the 
transgression  of  the  sea  was  again  from  the  east  or  northeast.  Thus,  the 
Negaunee  formation  in  the  eastern  part  of  the  area  was  more  quickly  buried. 
In  other  words,  the  western  part  of  the  formation  was  higher  and  was  sub- 
jected to  longer  erosion.  Therefore,  in  the  eastern  part  of  the  disti-ict  the 
sediments  of  the  Goodrich  quartzite  first  began  to  form.     The  western  part 


564  THE   MARQUETTE   IRON-BEARING   DISTRICT. 

of  the  district  i-emained  for  a  time  above  the  sea,  and  therefore  at  lirst 
received  no  deposits.  We  thus  partly  explain  the  very  considerable  thick- 
ness of  the  quartzite  in  the  Ishpeming  and  Negaunee  areas,  its  dying  down 
to  an  exceedingly  narrow  stratum  in  the  western  end  of  the  district,  the 
considerable  thickness  of  the  Negaunee  formation  about  Ishpeming  and 
Negaunee,  and  its  thinning  or  disappearance  at  the  west  end  of  the  district. 

The  first  deposit  of  the  advancing  sea  was  a  conglomerate,  the  detritus 
of  which  was  derived  mainly  from  the  immediately  subjacent  Negaunee 
formation.  Hence  it  is  that  the  basal  formation  is  so  frequently  jasper- 
conglomerate,  chert-conglomerate,  and,  where  the  detritus  is  finer,  recom- 
posed  chert  and  jasper,  ferruginous  slate,  etc.  However,  the  deti-itus  was 
derived  not  wholly  from  the  Negaunee  formation,  but  in  part  from  the 
various  lower  formations.  This  shows  that  either  within  the  district  under 
discussion  or  adjacent  to  this  district  erosion  had  cut  into  the  inferior  forma- 
tions, and  even  down  into  the  Basement  Complex.  This  is  well  illustrated 
by  the  Palmer  belt  of'  the  Goodrich  quartzite,  where  the  conglomerate  con- 
tains not  only  fragments  of  the  Negaunee  formation  but  of  the  Ajibik 
quartzite  and  of  the  Basement  Complex. 

Following  the  basal  conglomerate,  which  is  from  a  few  feet  to  several 
hundred  feet  thick,  came  a  sand  deposit.  This  sand  was  largely  composed 
of  simple,  pure  grains  of  quai'tz,  which  could  not  have  been  derived  from  the 
iron-bearing  formation,  but  must  have  come  from  lower  formations  outside 
of  the  district  discussed.  This  probably  implies  that  adjacent  to  the  district 
erosion  by  this  time  had  removed  large  areas  of  the  Negaunee  formation. 
Mingled  with  the  coarse  simple  grains  of  quartz  are  also  fine  complex  frag- 
ments of  chert  and  jasper,  which  shows  that  in  places  the  Negaunee  iron 
formation  was  still  being  cut.  This  sandstone  has  been  subsequently 
changed  to  a  quartzite. 

Early  in  the  time  of  sand  deposits  along  the  southern  part  of  the  dis- 
trict, an  east- west  fissure  was  formed  near  Clarksburg,  and  a  major  and 
probably  at  least  two  minor  volcanoes  were  developed.  As  a  consequence 
there  was  piled  up  the  Clarksburg  formation,  a  mountainous  mass  of  mate- 
rial, consisting  of  lavas  and  tuffs,  some  of  which  were  rearranged  by  water, 
and  of  volcanic  materials  interstratified  with  ordinary  sedimentary  rocks. 


DEPOSITION    OF   THE    UPPER   MAKQUETTE   SERIES.  565 

The  area  over  which  the  volcanic  material  was  deiDOsited  gradually  grew, 
reaching  east  as  far  as  Stoneville  and  west  as  far  as  Champion.  These 
more  remote  deposits  are  comparatively  thin,  and  show  evidence  of  water 
arrangement.  As  the  lavas  and  tuffs  were  piled  up,  subsidence,  possibly 
due  to  the  burdening  of  the  crust,  went  on,  so  that  there  resulted  a  great 
bend  of  the  adjacent  formations  to  the  sou.tliward.  How  far  to  the  south 
and  to  the  north  these  volcanoes  were  felt  we  do  not  know,  but  the  slates 
to  the  north  indicate  that  their  ashes  reached  to  the  extreme  northern  part 
of  the  district.  This  volcanic  activity  lasted  for  some  time  ;  for,  beginning 
in  the  time  of  the  Goodrich  quartzite,  it  did  not  cease  until  a  considerable 
thickness  of  the  Michigamme  slate  had  been  deposited.  Contemporaneously 
with  the  extrusives,  it  is  probable  that  intrusives  penetrated  the  Basement 
Complex  and  the  Lower  Marquette  series. 

In  the  western  part  of  the  district  the  Goodrich  quartzite  grades  upward 
into  a  grimerite-magnetite-schist  (the  Bijiki  schist),  and  this  into  a  ferriferous 
slate,  often  sideritic.  In  the  eastern  part  of  the  district  the  Bijiki  schist 
may  exist,  but  exposures  have  not  been  found.  As  the  schist  is  regarded 
as  developing  from  a  sideritic  slate,  it  appears  that  following  the  deposition 
of  the  sandstone  there  were  waters  favorable  to  the  deposition  of  a  non- 
fragmental  sideritic  formation — that  is,  the  conditions  for  the  production 
of  the  Negaunee  formation  of  the  Lower  Marquette  were  repeated,  but 
not  with  perfection,  for  the  ferruginous  slates  in  much  of  the  district  were 
mingled  with  greater  or  less  quantities  of  mechanical  sediments.  These 
are  more  abundant  in  the  eastern  end  of  the  area  than  in  the  western, 
where  a  considerable  belt  of  griinerite-magnetite-schist  is  comparatively 
free  from  mechanical  sediments  and  might  be  mapped  as  a  narrow  separate 
formation. 

The  zone  of  ferruginous  shales  was  apparently  of  variable  thickness. 
It  was  followed  above  by  ordinary  shales,  which,  however,  are  locally 
ferruginous.  Also  with  the  shales  was  deposited  much  organic  matter, 
as  is  shown  by  the  fact  that  the  resultant  slates  and  schists  are  anthra- 
citic  or  graphitic.  These  carbonaceous  rocks  ai-e  particularly  abundant 
at  the  horizons  which  are  heavily  ferruginous,  and  thus  confirm  the  sugges- 
tion made  in  considering  the   Negaunee  formation,  that  organic  matter 


566  THE   MARQUETTE   IRON-BEARING    DISTRICT. 

was  iDstrumental  in  reproducing  iron  carbonate  from  the  precipitated 
iron  oxide.  This  ferruginous  and  carbonaceous  shale  was  very  similar 
to  some  of  the  Paleozoic  shales  of  the  Appalachians,  and  argues  sim- 
ilar conditions  of  deposition.  Subsidence  must  have  steadily  continued 
during  the  deposition  of  the  shale,  for  it  is  of  considerable  thickness. 
The  sediments  varied  in  coarseness,  as  shown  by  the  fact  that  the  rocks 
now  found  include  fine-grained  slates,  graywackes,  and  even  rocks  which 
approach  a  quartzite.  These  rocks  indicate  waves  and  currents  of  varying 
strength  or  water  of  varying  depth,  or  both.  The  shale  and  graywacke 
have  been  modified  over  extensive  areas  into  mica-slates,  mica-schists,  or 
mica-gneisses. 

FOLDING   OF  THE  BASEMENT  COMPLEX,  LOWER  INIARQUETTE  SERIES, 
AND  LTPPER  MARQUETTE  SERIES. 

The  Marquette  district  had  been  an  area  of  deposition  since  the  begin- 
ning of  Upper  Marquette  time,  and  sediments  of  great  thickness  had  accumu- 
lated. A  physical  revolution  next  occurred,  as  a  consequence  of  which  this 
district  was  raised  above  the  sea  and  was  folded  in  a  complicated  manner. 
(See  Atlas  Sheet  IV.)  Whether  there  was  an  epeirogenic  movement  which 
raised  the  plateau  above  the  sea  before  the  orogenic  movements,  and  whether 
the  main  folds  now  found  were  formed  simultaneously  or  successively,  have 
not  as  yet  been  determined.  In  general,  the  directions  of  folding  are 
approximately  east-west  and  north-south.  The  only  important  exception 
to  this  is  in  the  southwest  part  of  the  district,  where  the  Republic  arm 
swings  away  from  the  main  area  of  Algonkian  in  a  southeast  direction. 

The  largest  but  least  conspicuous  fold  of  the  district  is  an  anticline  hav- 
ing a  north-south  axis,  running  through  Marquette.  This  fold  has  a  gentle 
dip,  but  a  breadth  of  many  miles  gives  it  a  great  amplitude.  Its  effect  upon 
the  minor  but  more  conspicuous  east-west  folds  is  to  give  them  a  westward 
pitch.  It  follows  that  in  going  west  from  Lake  Superior  the  area  of  the  Mar- 
quette rocks  becomes  broader  and  broader,  and  higher  and  higher  members 
appear  in  successive  eastward-pointing  U's,  the  ends  being,  however,  often 
crenulated,  due  to  the  folds  of  the  second  and  third  orders.  This  great 
fold  is  by  no  means  simple  in  its  character,  but  has,  especially  near  its 


FOLDING   OP   BASEMENT   COMPLEX   AND   MARQUETTE    SERIES.      567 

crown — that  is,  for  the  eastern  6  or  8  miles  of  the  district — superimposed 
upon  it  folds  of  the  second  order,  making  this  part  of  the  fold  an  anti- 
clinorium.  These  secondary  folds  have  lengths  varying  from  1  to  several 
miles,  and  therefore  a  given  formation  may  be  repeated  in  an  east-west 
direction  along  the  present  plain  of  denudation.  The  other  major  anti- 
cline belonging  to  this  system  of  folds  is  one  running  north  and  south 
through  the  east  end  of  Michigamme  Lake.  From  this  line  the  Algonkian 
belt  broadens  to  the  east  and  to  the  west.  It  then  follows  that  all  of  the 
district  between  the  center  of  range  26  west  and  the  east  end  of  Lake 
Michigamme  may  be  regarded  as  a  great  north-south  syncline. 

The  major  part  of  the  district  has  been  affected,  however,  by  much 
more  powerful  pressure  in  a  north-south  direction,  so  that  the  folds  in  an 
east-west  direction  are  much  more  conspicuous  than  the  north-south  folds  of 
greater  wave  length  and  greater  amplitude.  The  conspicuous  character 
of  these  folds  has,  in  fact,  led  to  neglecting  the  effect  of  the  folding  in  the 
other  direction,  and  thus  one  of  the  most  important  clews  to  the  distribution 
of  the  formations  was  unnoticed.  As  a  result  of  the  north-south  pressure, 
the  Upper  and  Lower  Marquette  series  together  have  been  bent  into  a 
great  synclinorium.  At  the  east  end  of  the  district  the  Mesnard  quartzite 
is  overturned  .at  one  place  and  dips  under  the  Southern  Complex  at  an 
angle  of  80°.  The  strikes  of  most  exposures  are  mainly  controlled  by  the 
east-west  folding,  but  at  the  east  and  west  ends  of  the  areas  of  the  forma- 
tions the  larger  north-south  folds  already  described  control  the  strike.  In 
passing  to  the  west  from  Lake  Superior,  on  the  south  side  of  the  district, 
from  Lake  Mary  to  Goose  Lake  and  somewhat  beyond,  the  secondary  north- 
south  folds  and  the  primary  east-west  folds  are  of  about  equal  amplitude, 
although  the  east-west  folds  are  closer  and  give  higher  dips.  As  a  conse- 
quence of  these  two  sets  of  folds  some  belts  strike  north  and  south,  some 
east  and  west,  and  some  in  intermediate  directions,  thus  giving,  at  first  sight, 
an  apparently  lawless  distribution  of  the  formations;  but  when  the  char- 
acter of  the  folding  is  understood  the  distribution  is  perfectly  explained. 
From  the  north-south  line  running  through  Goose  Lake  to  the  west  line 
of  range  28  west  is  the  area  in  which  the  Marquette  series  have  the 
greatest  width.     For  this  part  of  the  district  it  appears  that  the  less  rigid 


568  THE   MAKQUETTE    IKON-BEAEING   DISTRICT. 

rocks  of  the  Marquette  series  have,  as  it  were,  been  pushed  over  the  rocks 
of  the  Basement  Complex  on  the  north  and  south  sides  of  the  area.  The 
outer  Algonkian  formations  are  closely  plicated  into  a  series  of  overturned 
and  in  some  places  isoclinal  folds,  the  dips  on  both  the  north  and  south 
sides  being  toward  the  center  of  the  trough  and  away  from  the  Basement 
Complex  (fig.  1).  These  secondary  east-west  folds  are  usually  only  dis- 
covered by  tracing  the  contact  between  two  formations.  In  passing,  on  the 
plain  of  denudation,  toward  the  center  of  the  trough,  one  first  passes  from 
a  lower  formation  to  a  higher  formation;  then  apparently  above  this  he 
may  again  find  the  lower  formation;  and  this  infolding  in  extreme  cases  is 
repeated  several  times  (Atlas  Sheet  XVIII).  However,  on  the  whole,  the 
great  syncline  controls,  so  that  finally  the  inferior  formation  is  not  again 
found.  In  passing  inward  toward  the  center  of  the  Marquette  area  the 
minor  folds  become  more  open  in  their  character,  and  in  the  center  have 
a  symmetrical  shape  (fig.  1).  We  then  have  a  structure  in  this  district 
in  some  respects  like  the  fan-shaped  folds  of  the  Alps,  with,  however,  the 
great  difference  that  the  area  as  a  whole  is  a  synclinorium  instead  of  an 
auticlinorium;  that  is,  the  oldest  rocks  are  found  on  the  outside  of  the 
fan-shaped  areas  and  the  youngest  rocks  in  the  center  of  the  area.  The 
significance  of  this  type  of  fold,  which  I  have  named  an  abjiormal  syncli- 
norium, is  fully  discussed  by  me  in  another  place.^ 

The  overfolds  on  the  outer  borders  of  the  Marquette  belt  are  best  dis- 
covered in  places  where,  as  a  consequence  of  the  pitch  given  by  north-south 
folds,  an  east  or  west  termination  of  the  formation  appears.  A  few  of  the 
best  illustrative  areas  may  perhaps  be  mentioned.  West  of  Goose  Lake,  in 
sees.  22  and  23,  by  reference  to  the  maps  (Atlas  Sheets  IV  and  XXXV), 
it  will  be  seen  that  there  are  four  Archean  areas,  separated  by  Algonkian 
rocks  both  in  an  east-west  and  north-south  direction.  Their  separation  in 
an  east-west  direction  is  due  to  the  secondary  north-south  folding,  and 
their  separation  in  a  north-south  direction  is  due  to  the  isoclinal  northwest- 
southeast  overfolds.  The  latter  folds  are  the  closer;  consequently  the 
majority  of  the  strikes  are  northwest  and  southeast,  and  the  dips  are  mostly 

I  Principles  of  Nortli  American  lue-Cambriau  geology,  by  C.  E.  Van  Hise:  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Part  1, 1896,  ])p.  612,  615-621. 


FOLDING    OF   BASEMENT   COMPLEX   AND    MAEQUETTE    SERIES.      569 

to  the  northeast.  The  fragmental  formations  thus  appear  to  plunge  under 
the  Archean  islands  on  the  south  sides  of  the  areas  and  to  lie  above  them 
on  the  north  sides.  The  infolded  character  of  the  Upper  Marquette  and 
Lower  Marquette  series  is  illustrated  by  the  isoclinal  overfolds  along  the 
north  half  of  the  north-south  quarter  line  of  sec.  21,  T.  47  N.,  R.  27  W. 
(Atlas  Sheet  XXVI).  Here  a  north-south  section  at  one  place  shows  the 
Negaunee  formation;  above  this,  in  its  proper  position,  is  the  Goodrich 
quartzite ;  and  then  there  appears  above  this  again  the  Negaunee  formation. 
At  the  west  end  of  the  Jackson  mine  also  the  isoclinal  overfolds  of  the 
Goodrich  quartzite  and  the  Negaunee  iron  formation  are  beautifully  shown 
(Atlas  Sheet  XXVIII).  However,  the  best  locality  of  all  to  illustrate  the 
isoclinal  overfolds  is  in  sec.  30,  T.  48  N.,  R.  28  W.  (Atlas  Sheet  XVIII). 
"Here  the  infolding  is  between  the  granite  of  the  Northern  Complex  and  the 
Lower  Marquette  Ajibik  quartzite  and  Siamo  slate.  A  section  at  the  most 
favorable  place  passes  from  the  Siamo  slate  to  the  Archean  granite,  then 
again  to  the  Siamo  slate,  from  this  to  the  Ajibik  quartzite,  into  the  Archean 
granite,  in  turn  into  the  Ajibik  quartzite,  granite,  Ajibik  quartzite,  granite, 
Ajibik  quartzite,  and  probably  following  this,  although  topography  rather 
than  exposures  indicates  it,  come  again  the  Siamo  slate,  the  Ajibik  quartz- 
ite, and  the  Archean.  For  the  whole  of  this  distance  the  dips  are  to  the 
south.  Two  islands  of  Archean  are  cut  off  from  the  main  area.  The  quartz- 
ites  and  slates  occupy  the  valleys,  while  the  granite  is  more  resistant  and 
occupies  the  higher  land.  Controlled  by  the  western  pitch,  the  tongues 
of  quartzite  which  project  into  the  Archean  die  out  to  the  east,  and  open 
out  to  the  west.  We  have  here,  then,  the  conjoint  effect  of  the  close 
isoclinal  overfolding  due  to  the  north-south  pressure  and  the  great  north- 
south  folding  caused  by  east-west  pressure  which  gives  ail  of  the  formations 
a  westerly  pitch.  As  for  the  major  part  of  the  district  the  north-south  folds 
are  more  open  and  the  east-west  folds  more  conspicuous,  the  latter  may  be 
designated  the  major  folding,  and  the  former  folds  may  be  considered  as 
cross  folds  which  give  the  east-west  folds  a  pitch. 

The  western  major  north-south  anticline  at  the  east  end  of  Michigamme 
Lake  causes  the  Marquette  rocks  to  here  contract;  but  to  the  west,  in  pass- 
ing toward  the  next  syncline,  these  Algonkian  rocks  open  out  into  a  broad 


570  THE   MARQUETTE   lEON-BEAEING   DISTRICT. 

area  which  extends  beyond  the  district.  It  is  rather  probable  that  the 
eastward-projecting  land  between  the  west  and  southwest  arms  of  I.ake 
Michio-amme  marks  an  intermediate  anticline,  which,  however,  does  not  rise 
high  enough  to  bring  to  the  surface  any  rocks  higher  than  the  Michigamme 
schist.  The  Republic  tongue  and  the  Western  tongue  are  closely  com- 
pressed synclines  which  branch  off  from  this  main  area  in  southeast  and 
south  directions. 

It  has  been  seen  that  the  main  east-west  syncline  has  superimposed 
Ujion  it  secondary  folds ;  upon  these  again  are  those  of  the  third  order,  and 
upon  these  those  of  a  fourth  order,  and  so  on,  until  the  plications  in  many 
places  are  microscopic.  Pumpelly's  principle,  that  these  minor  folds  are 
often  of  the  same  character  and  usually  have  the  same  pitch  as  the  folds 
of  the  next  order  of  which  they  are  a  part,  has  been  of  great  assistance  in 
working  ou.t  the  stratigraphy  of  the  district  (PI.  XXXV). 

From  the  foregoing  description  it  is  clear  that  the  Marquette  district  is 
one  of  complex  folding.  In  fact,  no  better  example  is  known  to  me  of  this 
class  of  defoi'mation.-' 

Where  the  formations  are  brittle  the  close  plications  have  resulted  in 
their  being  fractured  through  and  through,  and  in  many  places  they  pass 
into  reibungsbreccias  (Pis.  VII,  VIII,  IX,  and  XXVI,  fig.  2).  These  phe- 
nomena are  particularly  prevalent  in  the  Negaunee  iron  formation  and  in 
the  quartzites.  The  more  plastic  formations  have  yielded  without  major 
fracturing,  but  in  a  minor  way  they  show  everywhere  the  effects  of  deforma- 
tion. A  microscopical  study  shows  that  not  a  cubic  inch  of  material  has 
escaped  dynamic  action.  Almost  every  original  grain  of  fair  size  gives 
evidence  of  interior  movement.  The  rocks  have  been  kneaded  thi-oughout. 
While,  as  a  further  consequence  of  dynamic  action,  there  has  been  local 
faulting  at  various  places,  with  two  or  three  exceptions  no  important  faults 
have  been  observed  in  the  district. 

The  only  fault  in  the  district,  besides  that  in  the  Republic  tongue 
(described  on  pp.  541-547),  large  enough  to  materially  displace  the  forma- 
tions, is  in  sec.  6,  T.  47  N.,  R.  25  W.  (Atlas  Sheet  XXXVII).     Here,  in  the 

'Principles  of  pre-Cambrian  North  American  geology,  fey  C.  R.  Van  Hise:  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Part  1, 1896,  pp.  626-631. 


S  GEOLOGICAL  SUR 


Kl(.;    1       A   IMTClllXC.    |-()|,|) 
FIG.  _:.    KAX  KOLI)  l\  I-Kl;l! 


^lAMO  Sl.ATK 
(M'S    SCHIST. 


FOLDING   OF   BASEMEISfT  COMPLEX  AND   MARQUETTE   SERIES.     571 

southeast  quarter  of  the  section,  the  Carp  River  flows  along-  the  line  of  a 
fault,  the  quartzite  formation  being  displaced  laterally  some  hundreds  of 
feet.  The  horizontal  throw  is  here  perhaps  more  than  500  feet,  but  prob- 
ably less  than  1,000  feet.  How  far  this  fault  extends  to  the  northwest  and 
southeast  the  outcrops  are  insufficient  to  determine. 

It  is  inferred  from  the  phenomena  of  deformation  that,  when  folded,  the 
rocks  which  are  now  at  the  surface  were  buried  under  a  thickness  of  several 
thousand  feet  of  sediments,  not  impossibly  as  much  as  10,000  feet.  While 
the  Upper  Marquette  slate  has  at  the  present  time  in  this  district  no  such 
thickness  as  this,  in  the  Penokee  district  10,000  feet  is  exceeded,  and  it  is 
probable  that  this  great  slate  formation  once  extended  with  nearly  or  quite 
its  full  thickness  over  the  Marquette  district.  On  the  other  hand,  it  appears 
that  the  formations  were  not  so  deeply  buried  as  to  be  beyond  the  sustain- 
ing strength  of  strong  rocks  like  quartzites,  or  else  the  layers  of  these  rocks 
would  have  been  folded  upon  themselves  without  the  production  of  reibungs- 
breccias,  as  in  the  case  of  the  Doe  River  quartzite  in  Tennessee.  Had  the 
rocks  which  are  now  exposed  not  been  deeply  covered  it  is  hardly  possible 
that  the  complicated  folding  above  described  could  have  occiuTed  without 
complicated  faulting. 

As  shown  by  the  above  facts,  the  Marquette  district  furnishes  a  beau- 
tiful instance  of  deformation  in  the  lower  part  of  the  zone  of  combined 
fracture  and  flowage.^ 

INTKITSIVES. 

Abundant  altered  diabase  and  other  rocks  were  intruded  in  both  the 
Lower  Marquette  and  Upper  Marquette  series.  This  is  shown  by  bosses 
cutting  across  the  bedding  of  the  layers  or  bending  them  (PI.  XI),  by  dikes 
branching  off  from  the  bosses  and  cutting  the  formations  of  both  the  Mar- 
quette series  (PI.  XXX),  and  by  large  and  small  inchxsions  of  griinerite-mag- 
netite-schist  in  the  greenstone  at  the  Lowthian  and  Spurr  mines  (PI.  XII). 
The  most  of  the  intrusive  greenstones  are  of  Clarksburg  or  pre-Clarksburg 
age.     They  particularly  affect  the  iron-bearing  formation  of  the  Lower 

'Principles  of  North  American  pre-Cambiian  geology,  by  C.  E.  Van  Hise:  Sixteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  Part  1, 1896,  pp.  601-603. 


572  THE   MAEQUETTE   lEOX-BEAEING   DISTEICT. 

Marquette  series,  but  occur  withiu  all  the  formations  of  the  district.  A 
few  dikes  are  later  than  any  of  the  Marquette  sedimentary  rocks.  The 
fact  that  the  intrusives  are  of  far  greater  abundance  in  the  broken  and 
fractured  Negaunee  formation  than  in  the  other  formations  suggests  that 
the  cracks  and  crevices  here  produced  by  the  folding  gave  avenues  of 
access  which  were  taken  advantage  of  by  the  igneous  rocks  to  wedge  them- 
selves in  between  the  rocks  of  the  iron-bearing  members,  to  force  them 
aside,  and  thus  to  form  great  dikes  and  bosses  of  igneous  material.  Often- 
times they  break  directly  across  the  bedding  (fig.  25);  sometimes  they 
produce  a  subordinate  folding  (fig.  18);  but  even  in  this  latter  case  the 
material  usually  breaks  across  the  bedding  to  a  greater  or  less  degree. 
In  many  instances  there  is  a  quaquaversal  arrangement  of  the  formations 
about  the  intrusive  igneous  masses,  which  suggests  that  the  igneous  material 
has  been  intruded  along  the  bedding  of  the  formation,  thus  forming  essen- 
tially laccolites  or  batholites.  At  Michigamme  and  Humboldt  the  Siamo 
slate  and  the  griinerite-magnetite-schist  may  be  seen  doming  some  of  the 
smaller  of  the  laccolites.  (PI.  XI,  and  figs.  24  and  25.)  Subsequent  erosion 
has  removed  the  capping  iron  formation  from  many  of  these  larger  domes 
and  left  the  greenstone  masses  in  the  forms  of  bosses,  the  iron  formation 
dipping  away  from  them  upon  all  sides,  just  as  do  the  sedimentary  forma- 
tions from  the  Henrj^  Mountain  laccolites.  The  major  portions  of  the 
greenstones  were  once  diabases,  but  are  now  epidiabases.  The  rather  fresh 
diabase  dikes  in  the  district  may  be  contemporaneous  with  the  igneous 
rocks  of  the  Keweenawan  period. 

DENTTDATION. 

From  the  foregoing  paragraphs  it  is  evident  that  the  rocks  of  the 
Marquette  district  were  folded  into  mountain  masses.  The  highest  parts  of 
the  mountains  were  probably  near  the  great  north-south  anticline  through 
Marquette,  and  the  mass  next  in  importance  was  probably  at  the  western 
anticline  at  Lake  Michigamme.  These  major  heights  must  have  been 
connected  by  numerous  cross  ridges,  coiTesponding  to  the  close  east-west 
folds.  During  and  subsequent  to  the  folding  these  mountains  were  cut 
down  to  an  approximate  plain,  so  that  the  district  is  at  the  present  time 


DENUDATION  OF  THE   MARQUETTE  DISTRICT.  573 

merely  l)luify.  The  highest  point  iu  the  distnct,  the  so-called  Suniinit 
Mountain,  is  1,800  feet  above  the  sea.  The  level  of  Lake  Superior  is  GOO 
feet;  so  that  the  maximum  relief  of  the  district  is  about  1,200  feet.  Begin- 
ning at  the  lake,  there  is  a  rapid  rise  to  Negaunee,  perhaps  10  miles,  the 
average  level  there  being  about  1,400  feet.  This  eastern  slope  is  a  part 
of  the  great  Lake  Superior  basin.  From  Negaunee  to  the  west  end  of 
the  district,  tliat  is,  for  much  the  larger  part  of  the  area,  the  variations 
in  elevations  are  scarcely  more  than  400  feet.  The  present  differences  of 
elevation,  with  the  exception  of  the  eastward  slope  to  Lake  Superior,  are 
mainly  due  to  differential  erosion.  The  hard  rocks,  Avhether  jaspilites,  grii- 
nerite-magnetite-schists,  quartzites,  conglomerates,  or  greenstones,  occupy 
the  higher  elevations,  and  the  soft  rocks,  the  slates,  shales,  and  most  of  the 
iron-formation  rocks,  occupy  the  valleys  and  swamps.  Since  the  forma- 
tions south  of  Marquette  were  raised  high  by  the.  eastern  anticline,  and 
the  whole  district  has  been  truncated  to  an  approximate  plain,  it  follows 
that  in  the  eastern  end  of  the  district  all  but  the  lowest  formations  have 
been  removed.  Thus  south  of  Marquette  we  find  only  the  two  lowest 
formations  of  the  Marquette  series.  In  the  great  syncline  between  the 
Marquette  anticline  and  the  Michigamme  anticline  newer  and  newer  forma- 
tions come  in,  until  the  highest  member  of  the  Upper  Marquette  series 
appears.  Tlie  Michigamme  anticline  apparently  was  not  so  high  as  the 
Marquette  anticline,  and  therefore  the  higher  members  of  the  series  are 
exposed.  However,  we  can  not  be  sure  that  several  of  the  remaining 
Marquette  formations  would  not  have  been  removed  were  the  plain  of 
denudation  600  feet  lower — that  is,  at  the  elevation  of  Marquette. 

METAMORPHISM. 

The  various  formations  of  the  Marquette  series  differ  from  one  another 
in  hardness  and  coarseness  of  grain.  It  is  probable  that  metasomatic  and 
cementing  processes  had  taken  place  to  some  extent  before  the  folding 
subsequent  to  Upper  Marquette  time,  and  thus  they  probably  differed  in 
strength.  When  this  period  of  folding  occurred  the  varying  texture  and 
strength  were  important  factors  in  the  resultant  deformations,  so  that  the 
readjustments  necessary  in  the  folding  took  place  in  large  measure  between 


574  THE   MAEQUETTE   IKON-BEARING  DISTRICT. 

the  different  formations  and  between  dissimilar  beds  of  each  formation. 
As  these  layers  were  rubbed  over  one  another  schistosity  was  developed 
parallel  to  the  bedding  in  many  places.  The  unconformable  contact 
between  the  Upper  Marquette  and  Lower  Marquette  series  was  one  of 
the  greatest  planes  of  movement,  and  adjacent  to  it  the  rocks  of  both 
were  rendered  schistose.  The  contact  between  the  Archean  and  the 
Lower  Marquette  series  was  another  such  plane  of  movement,  and  at  many 
places  a  considerable  zone  at  the  base  of  the  Lower  Marquette  series 
was  transformed  into  a  schist,  as  was  also  a  zone  of  the  rocks  of  the 
Archean  immediately  below.  Where  the  lower  quartzite  was  thin,  as  in  the 
Republic  tongue,  this  change  affected  the  entire  basal  formation.  In  other 
places,  where  the  folding  was  less  severe,  the  rocks  still  plainly  show  clastic 
characters. 

These  statements  as  to  the  adjustment  between  the  layers  and  the 
development  of  schistosity  parallel  to  the  bedding  do  not  fully  apply  to 
the  nearly  homogeneous  Michigamme  and  other  slates.  There  a2Dparently 
occurred  in  these  formations  an  actual  flowage,  the  whole  acting  in  a  way 
as  plastic  material;  consequently  there  is  frequently  a  discrepancy  between 
the  cleavage  or  schistosity  and  the  bedding.  Oftentimes  it  happens  that  the 
schistosity  nearly  corresponds  with  the  bedding  on  one  side  of  a  fold  and  cuts 
across  it  upon  the  other  (fig.  16).  In  this  case  the  complicated  character  of 
the  folding  and  the  reduplications  of  the  beds  are  particularly  likely  to  be 
overlooked.  In  the  crystalline  rocks  constituting  the  Basement  Complex 
the  north-south  pressure  was  the  predominating  force,  and  a  nearly  vertical 
schistosity  has  been  extensively  developed  with  an  approximately  east-west 
strike.  This  is  particularly  conspicuous  in  the  case  of  the  volcanic  rocks 
which,  like  the  Michigamme  slate,  were  approximately  homogeneous.  The 
whole  mass  was  mashed  together,  and  flowage  resulted  in  well-developed 
schistosity. 

During  the  time  in  which  the  dynamic  forces  were  at  work — that  is, 
while  the  folds,  fractures,  cleavage,  and  schistosity  were  being  formed — 
chemical  and  molecular  forces  were  active,  and  from  the  old  minerals  new 
minerals  were  developing.  Also  other  mineral  material  was  being  deposited 
in  the  interstices.     Thus  we  have  quartzites  or  quartz-schists  in  j)lace  of  the 


METAMOKPHISM  IN  THE   MARQUETTE   DISTKICT.  575 

sandstones;  slates,  graywackes,  mica-slates,  mica-schists,  or  mica-g-neisses 
in  place  of  the  shales  and  arkoses;  and  the  peculiar  phases  of  rocks  of  the 
iron-bearing-  formation  in  place  of  the  sideritic  slates. 

In  so  far  as  the  rocks  have  a  slaty  or  schistose  structure  it  is  believed 
that  the  metamorphism  was  contemporaneous  with  the  folding,  but  during- 
the  long  period  of  quiescence  which  has  subsequently  occurred  fui-ther 
extensive  metasomatic  and  weathering  changes  have  taken  place.  These 
appear  to  have  been  particularly  potent  in  the  iron-bearing  formation,  but 
they  have  also  doubtless  produced  important  changes  in  other  rocks.  In 
this  time  of  quiescence  must  have  occurred  the  final  enrichment  (if  the 
ore  bodies  and  the  extensive  impregnation  of  the  various  rocks  with  the 
granular  hematite  and  magnetite.  Finally,  during  this  period  of  quies- 
cence it  is  believed  that  there  developed  many  of  the  crystals  of  hornblende, 
garnet,  staurolite,  chloritoid,  and  andalusite,  and  much  of  the  secondary 
feldspar  of  the  mica-schists  and  mica-gneisses. 

The  metamorphism  is  more  nearly  complete  in  the  Avestern  part  of  the 
district  than  In  the  central  and  eastern  parts.  In  the  western  part  crystal- 
line schists  are  the  rule  for  all  the  formations,  while  in  the  central  and 
eastern  parts  of  the  district,  excluding  localities  of  excejjtional  readjustments, 
the  rocks  are  semicrystalline.  The  varying  metamorphism  corresponds 
with  the  closeness  of  folding.  In  the  western  part  of  the  district  the  folds 
are  closer  upon  the  average  than  farther  to  the  east. 

CORBEliATION. 

Reasons  have  been  given  in  previous  publications  for  regarding  the 
Upper  Marquette  and  Lower  Marquette  series  together  as  the  equivalent  of 
the  Huronian  of  the  north  shore  of  Lake  Huron.  These  will  not  here  be 
repeated.  Nor  will  the  argument  be  repeated  for  placing-  the  Upper  Mar- 
quette and  Lower  Marquette  as  the  equivalent  of  the  Upper  Huronian  and 
Lower  Huronian  of  the  other  parts  of  the  Lake  Superior  region.^  Accept- 
ing these  conclusions,  this  implies  that  the  Lower  Marquette  series  is  to  be 
equated  with  the  Lower  Felch  Mountain  and  Lower  Menominee  series. 

'  Correlation  papers,  Archean  and  Algonkian,  by  C.  R.  Van  Hise :  Bnll.  U.  S.  Geol.  Survey  No.  86, 
1892,  pp.  156-199.  Piiuciples  of  North  American  pre-Cambrian  geology,  by  C.  R.  Van  Hise:  Sixteenth 
Ann.  Rept.  U.  S.  Geol.  Survey,  Part  1, 1896,  pp.  780-807. 


576  THE  MARQUETTE  lEON-BEARING  DISTRICT. 

Smyth  has  recently  mapped  in  detail  an  area  between  and  nearly 
connecting  the  Marquette  and  Menominee  districts.  He  has  made  also  a 
general  study  of  the  latter  district.  As  the  results  of  his  studies,  lie  sum- 
marizes the  Lower  Menominee  succession  as  follows:^ 

Avoidiug  minute  subdivisions,  the  Lower  Menominee  consists  of — 

(1)  A  basal  quartzite,  rarely  conglomeratic.  The  thickness  may  reach  a  maxi- 
mum of  about  1,000  feet,  and  over  large  areas  is  at  least  700  feet. 

(2)  A  crystalline  limestone  which  averages  about  700  to  1,000  feet  in  thickness. 
On  the  Fence  River,  in  Ts.  44  and  45  N.,  R.  31  W.,  where  it  largely  if  not  entirely 
replaces  the  lower  quartzite,  the  thickness  attained,  if  there  are  no  subordinate  folds, 
is  from  1,500  to  2,000  feet. 

(3)  Red,  black,  and  green  slates  that  are  not  known  to  exceed  200  to  300  feet  in 
thickness.  The  slates  here  and  there  contain  the  iron  formation  that  affords  the  rich 
ores  of  Iron  Mountain  and  Norway.  In  the  southern  part  of  T.  44  N.,  R.  31  W.,  the 
horizon  of  the  slates  is  in  part  occupied  by  altered  eruptives  that  rapidly  increase  in 
thickness  towards  the  north,  the  whole  attaining  a  maximum  of  nearly  2,000  feet  on 
the  Fence  River,  in  T.  45  N.,  R.  31  W. 

(4)  The  highest  member,  except  volcanics,  yet  recognized  in  the  Felch  Mountain 
and  Fence  River  divisions  of  the  Lower  Menominee  is  typically  developed  at  Michi- 
gamme  Mountain,  sec.  4,  T.43  N.,  R.  31 W.,  and  sec.  33,  T.  44  K,  R.  31 W.,  and  has  been 
called  the  Michigamme  jasper.  This  is  a  greatly  altei'ed  ferruginous  rock  usually 
carrying  apparently  fragmental  quartz  grains.  Various  stages  in  the  alteration  permit 
two  or  three  types  to  be  recognized.  The  least  modified  seems  to  indicate  that  the 
rock  was  originally,  in  part  at  least,  a  clastic  sediment.  The  alteration  appears  to 
have  been  effected  by  the  infiltration  of  iron  salts,  the  formation  of  cherty  silica,  and 
the  replacement  of  the  original  constituents  to  varying  degrees.  The  most  highly 
altered  type  bears  the  closest  possible  resemblance  in  the  hand  specimen  to  the 
banded  specular  jasper  seen  on  the  Republic  bluff. 

Smyth  then  makes  the  following  statement  as  to  the  Marquette  district: 
The  Lower  Marquette  series,  in  the  western  part  of  the  Marquette  area,  where  it 
most  nearly  approaches  the  Menominee  region,  consists,  when  exposed,  of — 

(1)  A  basal  conglomerate — quartzite — quartz-schist,  probably  less  than  100  feet 
thick.    North  of  the  Michigamme  mine  the  quartzite  passes  upward  into  a  slate. 

(2)  An  iron-bearing  formation  which  may  be  divided  further  into  a  lower  member, 
composed  of  actinolite  (or  griinerite),  magnetite,  and  silica,  one  or  two  of  which  may 
locally  predominate  over  the  rest,  and  an  upper  member  usually,  but  not  invariably, 

'The  Lower  Menominee  and  Lower  Marquette  series  in  Michigan,  by  H.  L.  Smyth;  Am.  Jour. 
Sci.,  3rd  aeries,  Vol.  XLVII,  1894,  pp.  216-223. 


COEKELATION  OF   THE   MAEQUETTE   SERIES.  577 

characterized  by  bands  of  red  jasper  and  specular  hematite.  The  iron-bearing  mem- 
ber has  a  maximum  thickness  of  more  than  1,000  feet,  but  usually  it  has  been  cut 
down  greatly,  or  with  the  lower  quartzite  entirely,  by  the  Animikie  transgression. 

The  Marquette  iron  ores,  except  those  on  the  Upper  Marquette  series,  occur,  as 
Van  Hise  has  shown,  either  (a)  at  the  contact  of  the  lower  irou-bearing  member  with 
the  upper  quartzite,  when  the  ore  may  be  either  a  concentration  in  the  lower  irou- 
bearmg  member  or  a  detrital  member  of  the  upper  series,  or,  {b)  more  rarely,  entirely 
within  the  iron-bearing  member  of  the  lower  series. 

These  descriptions  are  expressed  briefly  in  the  following  table,  in  which  the  mem- 
bers of  the  two  series  are  shown  in  parallel  columns  for  lithological  comparison : 

Menominee.  Marquette. 

Michigamme  jasper Jasper  banded  with  ore.         1  Iron   forma- 

Slates  (principal  iron  formation) Maguetite-actinolite  schist.    J        tion. 

Limestone 1 

Quartzite JQuartzite. 

Archean Archean. 

Smyth  traces  the  magnetic  Michigamme  jasper  to  within  1 J  or  2  miles 
of  the  iron- bearing  formation  of  the  Marquette  series,  and  he  regards  the 
two  as  equivalent.  Toward  the  north  the  Michigamme  jasper  is  found  to 
have  a  lower  quartzitic  portion,  which  he  places  as  equivalent  to  the  lower 
quartzite  of  the  Marquette  district. 

The  whole  of  the  Lower  Marquette  series  would  thus  be  represented  by  the  high- 
est member  of  the  Lower  Menominee.  What,  then,  becomes  in  the  Marquette  district 
of  the  great  thickness  of  limestone,  quartzite,  and  eruptives  which  lie  below  the 
Michigamme  jasper  in  the  Menominee,  and  how  is  its  absence  to  be  accounted  for? 

The  most  probable  explanation  is  that  the  pre-Algonkian  basement  sank  contin- 
uously in  both  districts,  but  that  the  Marquette  was  initially  the  more  elevated,  and 
as  a  whole  was  dry  laud,  while  the  lower  quartzite,  limestone,  and  slates  were  going 
down  in  the  Menominee.  The  transgressive  movement  from  the  south  reached  it  when 
the  lower  portion  of  the  Michigamme  jasper  was  being  deposited. 

In  this  discussion  Sm3'tli  includes  under  the  name  Menominee  the  area 
which  has  heretofore  been  called  the  Menominee  district,  and  the  large  con- 
necting area  to  the  north,  which  is  as  yet  largely  undeveloped,  and  which 
will  later  be  described  in  a  monograph  entitled  The  Crystal  Falls  and  Metro- 
politan Iron-Bearing  Districts  of  Michigan.  For  convenience  in  discussion 
the  term  Menominee  will  here  be  used  in  the  sense  given  it  by  Smyth. 

MON  XXVIII 37 


578  THE  MARQUETTE   IKON-BEAKING  DISTRICT. 

The  chief  poiut  upon  which  more  evidence  is  necessary  is  the  relation 
of  the  slates  bearing  the  rich  ii'on  ores  in  the  Menominee  district  proper 
to  the  slates  associated  with  the  volcanics  farther  north  in  the  connecting 
district.  If  the  Menominee  slates  are  different  from  those  to  the  north  and 
belong  above  them,  the  succession  in  the  two  districts  would  be  very  closely 
analogous.  Using  the  succession  for  the  entire  Marquette  district  which  we 
have  made  out,  and  comparing  it  with  Smyth's  succession  in  his  Menominee 
district,  we  have  the  following  parallel  descending  succession: 

Upper  Marquette.  Upper  Menominee. 

Unconformity. 
Lotcer  Marquette.  Lower  Menominee. 

Negaunee  iron  formation,  1,000  to  1,500 fMichigamme  jasper. 

feet [Slates  bearing  rich  ores. 

Siamo   slate,  in   places   including  inter 

stratified  amygdaloids,   200  to  625  feet 

thick 

Ajibik  quartzite,  700  to  900  feet ... 

Wewe  slate,  550  to  1,050  feet 

Kona  dolomite,  550  to  1,375  feet Crystalline  dolomite,  700  to  1,000  feet. 

Mesnard  quartzite,  100  to  670  feet Basal  quartzite,  700  to  1,000  feet. 

The  succession  for  the  lower  series  would  thus  be  very  closely  parallel 
in  the  two  districts,  with  the  following  exceptions: 

(1)  The  Wewe  slate,  the  Ajibik  quartzite,  and  the  Siamo  slate  are 
placed  opposite  one  member  of  the  Menominee  series.  These  thi-ee  forma- 
tions are,  however,  all  fragmental  and  are  equated  with  a  fragmental 
formation.  Together  they  mark  a  time  of  mechanical  deposition  in  each 
district  between  the  nonfragmental  limestone  and  the  nonfragmental  iron 
formation,  and  thus  include  the  physical  change  involved  in  passing  from  a 
nonfragmental  to  a  fragmental  and  then  again  to  a  nonfragmental  forma- 
tion. The  chief  difference  is  that  in  the  Marquette  district  two  layers  of 
mud  were  separated  by  a  layer  of  sand.  Another  difference  is  that  in  the 
Menominee  district  volcanics  are  much  more  important,  and  this  may 
account  for  the  absence  of  conditions  favorable  to  sand  deposits.  However, 
it  is  interesting  to  note  that  amygdaloids  are  found  in  the  Lower  Marquette 


Slates    and    altered  volcanics,   maximum 
thickness,  2,000  feet. 


COEIIELATION  OF  THE   MAKQUETTE   SEIUES.  579 

series,  in  the  Siamo  slate — that  is,  toward  the  higher  part  of  this  great  frag- 
meutal  formation.  The  Fence  River  volcanics,  in  the  Menominee  district, 
occupy  a  similar  horizon. 

(2)  The  pure,  nonfragmental  iron  formation  of  the  Marquette  district 
is  equated  with  slates,  cherts,  jaspers,  and  the  rich  ores  of  the  Menominee 
district.  The  only  substantial  difference,  however,  is  that  in  the  Menominee 
district  the  rocks  bear,  with  the  nonfragmental,  a  considerable  amount  of 
fragmental  material.  In  other  words,  the  conditions  in  that  district  were 
not  favorable  to  pure  nonclastic  sediments,  as  they  were  in  the  Marquette 
district. 

As  there  thus  seem  to  be  closely  parallel  successions  in  the  two  dis- 
tricts in  early  Marquette  time,  it  seems  highly  probable  that  the  western 
part  of  the  Marquette  district,  where  the  lower  members  of  the  series  do 
not  appear,  jnust  have  been  a  high  area  largely  or  completely  surrounded 
by  water,  since  the  lower  members  of  the  series  were  deposited  to  the  south- 
east and  northeast.  This  elevated  tract  included  the  ai-ea  west  of  Ishpeming 
and  Negaunee  to  Lake  Michigamme  and  thence  south  to  Republic.  How 
much  farther  it  extended  to  the  west,  and  whether  it  was  an  island  or  a 
peninsula,  it  is  yet  too  early  to  venture  an  opinion. 

Until  more  detailed  studies  are  made  of  the  Upper  Huronian  rocks  in 
the  Menominee  district  it  is  unsafe  to  attempt  a  detailed  correlation  of  the 
formations  of  the  Upper  Marquette  series  with  the  formations  there  found. 
In  both  districts  there  are  certain  general  likenesses.  The  basal  formation 
in  each  district  is  frequently  an  ore  and  jasper  conglomerate,  which  passes 
up  into  a  quartzite.  In  both  the  Marquette  and  Menominee  districts,  not 
far  above  this  quartzite,  near  to  or  associated  with  the  slates,  is  an  iron- 
bearing  formation.  The  upper  predominant  formation  was  a  shale,  which 
has  been  metamorphosed  to  a  mica-slate  or  mica-schist. 


INDEX, 


A.  Page. 

Acid  bosses  in  Northern  Complex 151 

Acid  (lilies  in  Basement  Complex 150 

in  Northern  Complex 151, 159, 182-183 

A  old  interstices  in  quartz-diabase 519-520 

Acid  mona  schists 159-160 

Acid  schists 153 

in  Basement  Complex  150,159-100 

in  Northern  Complex 159-160, 162, 167 

origin  of 160 

structure  of 159 

Acid  sheets 160 

Acid  tuffs 160,109 

Acid  veins  in  Basement  Complex 150 

Actinolite-magnetite-schist    in     Lower    Marquette 

series 130 

(See  Griinerite-magnetite-schist.) 

Actinolite  of  Clarksburg  greenstone 466, 472 

of  horublende-schist 476 

of  graywacke 448 

of  siderite-slate 307 

of  slate 448 

Actinoliteschist 91,  92,  500 

origin  of 102 

passing  into  quartzite 74 

(See  Amphibole-schiat,  Grunerite-schist.) 
Adams,  P.  D.,  on  origin  of  the  mica-schists  of  the 

Grenville  series 201 

referred  to 149,190,201 

Agglomerate  at  Deer  lake 125 

in  Northern  Complex 555 

(See  Greenstone-conglomerate.) 

Agnotozoic 123,134 

proposal  of  name 112 

Ajibik  Creek 283 

AjibikHills 282,283,308,310 

-Ajibik  quartzite 221,244, 

251,  252,  257,  259,  267,  271,  273,  275,  314,  315,  316,  317,  332,  334, 
366,  382,  383,  384,  387,  388,  392,  422,  529,  534, 554,  656,  561,  562,  569 

and  Archean,  apparent  gradation  between 296-297, 

298,  305 

deposition  of 560 

described 282-313,528-529 

gradation  into  Negaunee  formation 390 

relation  to  Archean 295, 

296,  297,  298,  300,  301,  302,  309,  310,  313 

relations  to  Goodrich  quartzite 310, 409, 411 

relations  to  Kitchi  schists 302-303,  305 

relations  to  Negaunee  formation 289, 

292,  293,  298,  299,  386 


Page. 

Ajibik  quartzitf  relations  to  r.almer  gneiss 311 

rehitiuns  to  Sianio  slut,    289,300,321,333,334 

relations  t"  W,«,M, It,      271, 

■■:■!  ■:".  jst;,  287,  294,  295,  307,  309,  310 
iinconfoniKibly  iiiimi  Kil.lii  .sthists,  figure  of-..  296 

Albito  of  gneiasoid  granite 211 

of  greenstone 501 

of  iMona.schists 155,157 

Algonkian 129, 134, 143, 146, 154,  240,  460,  567, 569 

definition  of  term 127, 135 

first  use  of  term 123 

Algonkian  series 3, 161, 170 

of    Mar.nfll.-    ,li-iri,l     ,,,ir.l:,l,'d    with   Lower 

•Ti'l  I'Pl"'    ■!■  -  •     '■     ■     ■    liM  ,listrirts 3 

of  M:ii,|ii,ti.    -  ;        <,  l,h-,l   with  Original 

nuiciiiini  ,il    i';,„;mI,', 3 

Of  Marquc-tti!  district,  formations  of 3 

separatiou  from  Basement  Complex. .  149, 150, 154, 161, 162 

Algonkian  rocks 444, 525 

Alteration  in  deep-seated  zone 365,367,369 

in  zone  of  weathering 365 

of  .acid  dike  rock 182 

of  amphibole 388 

of  basic  eruptive  rock 157 

of  cherty  siderito 337,401-403 

of  Clarksburg  formation 464, 471,  473, 475, 485 

of  dike  diabase 179, 180-181, 511 

of  epidiorite 496 

of  feldspar 225,226, 

264,  265,  266,  278,  289,  290, 302,  318,  319,  327-328, 381, 
422,  435,  438,  442,  448,  449,  450,  453,  458,  527, 529,  534 

of  feldspathio  biotite-schist 197-198 

of  feldspathic  debris 230 

of  ferruginous  schist 75 

of  ferruginous  slate 380 

of  garnet 62,421,503 

of  gneissoid  granite 220 

of  greenstone 98, 396, 399 

of  grunerite 423,436 

of  griinerite-slate 380 

of  magnetite 387, 426 

of  Michigamme  jasper 576 

of  mosaic  in  gneissoid  granite 214 

of  orthoclase 173,197,198,210 

of  peridotite 183,185-186 

of  ])lagioclas6 196,  210, 496,  501-502 

of  porphyrite 521 

of  qiiartzose  sandstone 230 

of  rocks  of  Marquette  district 71 

581 


582 


Alteration  of  siderite 280, 

340,  342,  354,  367,  368,  405,  419,  422, 423,  451, 455,  562-563 

of  siderite-slate 367,446,454 

of  sideritio  chert 371 

of  uralitic  diabase 496 

products  in  Palmer  gneiss 217 

{See  "Weatbering,  Metamorpbism.) 

Altered  diabase 218,490 

altered  eruptives 152,159,176 

altered  feldapathio  biolite-scbist 199, 200 

altered  feldapathic  quartzite 512 

altered  bornblende 172 

altered  ortboclase 172 

altered  plagioclase 157,159,172,204,508 

altered  tnJla 153,201,502 

Alps,  fan  fold  of 3^,568 

Amphibole,  aloration  of 388 

analysis  of,  referred  to 140 

cellular  structure  of 206,  470,  471,  504 

plate  of 470 

contact  structure  of,  in  greenstone  scbists 206 

contact   structure  of.  in  Kepublic  greenstone, 

plate  of 470 

of  Monascbist 157 

of  Clarksburg  sediments 470, 472 

of  greenstone. . .  178,  465,  466,  467,  486,  498, 501, 502,  503-504 

of  greenstone-schist 204-205 

of  griineritemagnetite-schist 391, 418 

of  hornblende-schist 476 

of  iron  formation 382 

of  jaspilite 388,389 

of  quartzite 438 

{See  Actinolite,  Griinerite,  Hornblende,  Uralite.) 

Ampbibolescbist 152,158,159 

distinguished  from  greenstone-schist 206 

in  Southern  Complex 204,206-208 

origin  of 37 

(See     Actinolite-scbists,     Antbophyllite-scbiat, 
Griinerite-magnetite-schist,     Hornblende- 

Ampbibolite 158,472,495,527 

originof 140 

Aniygualoidal  sheet  greenstone 515,  516-517 

Amygdaloid 312 

in  Ajibik  quartzite 284 

in  lower  Marquette  series 578,579 

in  Siamo  slate 562 

in  Clarksburg  rocks 467,484 

Analysis  of  amphibole,  referred  to 140 

of  Bi.jiki  schist 418 

of  carbonaceous  slate 446 

of  cherty  siderite 337 

of  chloritoid,  referred  to 140 

of  clay,  referred  to 140 

of  diorite 495 

of  feldspathio  micaceous  schist 202,203 

of  granit*,  referred  to 62 

of  granitite 202 

of  green  schist 168 

of  greenstone 495 

of  griinerite-magnetite-schist 338 

of  iron  ore,  referred  to 21,110,129 

of  jaspilite 363 

of  Kitchi  schist 168 

>  schist 202 


Page. 

Analysis  of  sericite-schist 168 

of  serpentine 184 

of  slate 202 

of  peridotite 186 

of  Palmer  gneiss 217 

Andalusite 436 

of  Marquette  formations 574 

of  mica-schist 447,449 

Angelina  Lake.    (5ee  Lake  Angeline.) 

Animikie  district 370 

series 368 

ores  and  jaspers  of 102-103 

transgression 577 

AnthophyUite-schist 50,129,416 

Aphanitic  Mona  schists 154 

{See  Mona  schists,  dense  varieties  of) 

Aplite  in  Northern  Complex 151. 153, 182 

Appalachians,  Paleozoic  shales  of 566 

Archean 109, 

112, 123, 129, 134, 143, 146, 222,  231,  237,  238, 
256.  259,  264,  277, 278,  280,  283,  284,  287,  289, 
205,  300,  360,  412,  432,  439,  525,  534,  569,  577 
and  Ajibik  quartzite,  apparent  gradation  be- 
tween   295-297,298,315 

and  Ishpeming  formation,  apparent  gradation 

between 441 

and  Lower  Marquette,  apparent  gradation  be- 
tween   559 

and  Mesnard  quartzite,  apparent  gradation  be- 
tween   231,232,237 

denudation  of 230,537,555 

described 149-220,  526-528,  555-556 

formations  in  the  northwestern  United  States  - . .  101-104 

island 150, 

220,  221,  223,  241,  257,  258.  269,  270, 
275,  276,  282,  285,  286,  557,  568,  569 

of  Republic  area,  described 526-528 

relations  to  Ajibik,  quartzite 295, 

296,  297,  208,  300,  301,  302,  309,  310,  313 

relations  to  Ishpeming  formation 413,441,442,536 

relations  to  Lower  Marquette  series 208,  532-535. 557 

relation  to  Mesnard  quartzite 297, 557 

relation  to  Republic  tongue 525-526 

relations  to  TVewe  slate 270 

series,  divisibility  of 98, 104-106, 110-112 

separation  of  Huronian  from 145-146 

(See  Basement  Complex,  Laurentian,  Northern 
Complex,  Southern  Complex.) 

Arenaceous  slate  group 91-92,148 

Argillaceous  slate 20 

Argillite 50,58,117.118 

associated  with  diorite-schist 86 

Arkose 129 

Ashes,  volcanic 58.139,169 

{See  Tuffs.) 

Angen  of  feldspar  in  granite 270 

Augen-gneiss 301 

Augite  of  diabase 148, 179, 180 

of  diorite 178 

ofepidiorite 180 

of  greenstone 491,  492, 493, 494,  495, 498 

of  quartz-diabase 519, 520 

A  ugi  te-porphyrite 498 

A  vonian  series 129 


583 


Azoic  quartzite 27 

Azoic  rocks,  origin  of 38-40 

Azoic  series 46,  47,  64 

cliaracterizatiou  of 27 

intrusives  in :il 

occurrence  of  ores  in 31 

(Sec  Azoic  system.) 

Azoic  system 35, 71, 118, 120, 127 

composition  of 26, 29 

divisibility  of 38-40,  44,  61,  99, 127, 128, 135-136 

members  of 55 

origin  of 20, 29 

(tSVe  Agnotozoic,    Algonki.an,   Arcliean,   Azoic, 
Pre-Cambrian  series.) 


Bad  River,  Wisconsin 56 

Bancroft  Lake.    ( See  Lake  Bancroft.) 

Banded  Mona  scbists 154, 156, 158, 159, 161 

composition  of 157 

described 156-157 

origin  of 156.157-158 

structure  of 157 

Banding  of  ampbibole-scbist 207 

of  Clarksburg  sediments 468-469,  472,  474, 478 

of  Clarksburg  tuffs 474 

of  feldspathic  biotite-schist 196-197 

of  iron-bearing  formation 531 

of  jaspilite 80 

of  jasper  and  ore 59 

of  Kitchi  schists 164 

of  micaceous  scbist 192-193, 194, 199, 200-201,  203 

of  Mona  schists 156 

of  muscovite-scbist 195 

of  Palmer  gneiss 212,  213 

Baunan,  Benjamin.    (See  Daddon,  S.  H.) 

Barito  at  Lucy  mine 148 

in  iron  ore 91 

Barnum  mine 126,386,396,398 

Barron  mine 125,386,387,412.432,433 

Basal  conglomerate 263, 

275,276,278,287,294,  293,299,301, 
303,  305,  307,  311,  312,  313,  3G0,  412, 
413,420,  429,  430,  431,  432,  433,  527, 
528,    534,  536,    538,    558,  559,  564 

described 223,224, 

225,226,234,  238,  239,240,259,  264,270, 
271,  289,  295,  300,  411,  442,  533,  543,  557 

of  Wewe  slate,  figure  of 259 

Basal  conglomerate-quartzite-q  uartz-scbist  of 

Menominee  district 576 

Basal  quartzite 5(j] 

of  Menominee  district 576 

Basal  rock 44  ' 

Basalt 25,  78,  155,  218,  485,  507,  508,  522,  525 

Base  level 561 

Basement  Complex 31, 127 , 

130,  136,  223,  2,56,  264,  270,  299,  311,  312,421, 
422,  428,   429,  430,  435,  437,  450,  554,  557,  564 

constitution  of 141, 150 

contact  with  Lower  Huroniau 143 

denudation  of 230, 537, 555 

described 149-220,555-556 

distribution  of 150 

isolated  areas  of.    (See  Archean  islands.) 

mashing  of 239 


t  Coraple 


!  proposed  . 
rabers  of  . . 


150 


relations  bet 

relations  to  Algonkian 1.35,149, 150 

relations  to  Mesnard  formation 230, 231 

relations  to  Goodrich  quartzite 411 

subdivisions  of 149 

{See  Archean,  Laurentian,  Northern  Complex,' 
Southern  Complex.) 

Basic  dikes  in  Northern  Complex 150, 178-181 

diabase  dikes 178-180 

epidiorite  dikes 178, 180-181 

diorite  dikes 178, 181 

Basic  lavas 154, 155,  leo 

Basic  massive  rocks,  origin  of 74 

Basic  Mona  schists 154-159 

banded  varieties  of 156-158 

dense  varieties  of 134-156 

(See  Mona  schists.) 

Basic  schists  in  Basement  Complex 150 

in  Northern  Complex 151, 152, 154-159, 162-167 

Basic  tuffs 160,163,169,189 

(See  Clarksburg  series,  Igneous  rocks  in  South- 
ern Complex,  Kitchi  schists,  Mona  schist,  and 
Tuffs.) 

Batholites  in  Marquette  series 572 

Bayfield,  H.  \T.,  on  general  geology  of  Lake  Superior 


Bayley,  W.  S.,  on  geological  explorations  and  litera- 
ture of  Marquette  district 5-148 

on  Basement  Complex  of  Marquette  district 149-220 

on  Clarksburg  formation 460-184 

on  igneous  rocks  of  the  Marquette  district 487-522 

referred  to 2 

Beaconite,  described 140 

Beaufort  mine 127 

Bigsby,  J.  J.,  on  general  geology  of  the  Lake  Su- 
perior region  34 

on  Azoic  rocks 39-40 

referred  to 6,  71 

Bijiki  river 409;  416, 423, 434 

Bijiki  schist 409, 444,  445, 452, 554 

analysis  of 4I8 

denudation  of 417 

described 416-420 

development  of 565 

relations  to  Goodrich  quartzite 411,  419 

relations  to  Michiffamme  formation 419 

Biotite-granite,  anal  \  nim  of 202 

composition  uf 171 

in  NortbiTU  Comjili/x 171-174 

mortar  structure  in 174 

mosaic  in 173 

origin  of 175-176 

structure  of 172,173-174 

Biotite  from  feldspar 265, 

289,  318-319,  327-328,  422,  438,  450,  458 

from  griinerite 423 

from  plagioclase 501-502 

of  biotite  granite 172 

of  biotite-schist 196 

of  chlorite-sohist 514 

of  Clarksburg  formation 472,  477,  478,  479.  486 

of  feldspathic  biotite-schist 196-198 

of  granite 626 


584 


Page. 

Biotite  of  gray wacke 265, 318, 319, 448, 453 

of  greenstone 465, 485, 501, 502, 509-510, 538 

of  greenstone-acliist 205 

of  gruneritemagnetitescliist 391 

of  hornblendic  biotite-scbist 198 

of  iron  ore 374,435 

of  mica-schist 457 

of  muscovite-scbist 195 

of  qnartz-scbist 289 

of  quartzite 300,  415,  421,  529 

of  recomposed  ore 441 

of  scbist-conglomerate - 442 

of  tuff 474 

structure  of 198-200 

Biotit«-sobist 289,  294, 415, 434,  436, 443,  464, 482 

(See  Biotite-slate,  Mica-scbist,  Mica-slate.) 

Biotite-slate 290,434,436,455 

(See  Biotite-scbist,  Mica-schist,  Mica-slate.) 

Birkinbine,  J.,  on  position  of  Marquette  ores 114, 141 

referredto 7 

Blue  mine 395 

Black  ore  jasper 549,650 

Black  slate 127,133 

Bosses  in  Basement  Complex 555 

in  Clarksburg  series 460, 461, 485 

in  Marquette  series 142, 

487,  488,  518,  520,  522, 523,  571-572 

described 489-506 

in  Xegauuee  formation 329 

in  Northern  Complex 151 

{See  Acid  bosses  and  Basic  bosses,) 

Boston  mine 126,377,424 

Bradish,  Alvah,  on  life  of  Douglass  Houghton 9 

Breccia 88-89,  254,  273, 274, 281, 309,310,  311,  321,  326,  327 

associated  with  diorite-scbists 86 

described 253 

in  Clarksburg  series 460, 464, 473, 470-480, 483, 484 

(See  Chert-breccia,  Eeibungsbreccia,  Tuffs.) 
Brecciated  chert  at  base  of  Kona  dolomite,  plate  of.         246 

in  Kona  dolomite,  plate  of 250 

Brecciated  jaspilite  of  Jasper  Bluff,  plate  of 358 

Brecciated  Kona  dolomite,  plate  of 250 

Brecciated  slate -- 263 

Brecciation 200 

of  Bijiki  schist 424 

of  ferruginous  chert 361, 370,  380 

of  graywacke 268 

of  jaspilite 362,  371, 376,  380, 386,  388,  428 

of  Kona  dolomite ' 251 

of  Marquette  formations 570 

of  Kegaunee  formation 378, 383, 385 

of  quartzite 288 

of  slate 271,281,282,304,317 

of  Wewe  slate,  plate  of 262 

(See  Pre.i3ure  effects.) 

Broken  blutt's 301 

Brooks,  T.  B.,  on  correlation  of  Marquette  and  Me- 

nomminee  rocks 69 

on  geology  of  Marquette  district 48-57,  69 

on  geology  of  Menominee  district 69 

on  Huroniau  granite 63 

on  Laurentian  rocks  in  Michigan 69 

on  plications  in  jasper  and  ore 59 

on  sequence  of  rocks  in  Marquette  district. .  51, 57, 64,  65 
on  youngest  Huronian  rocks  south  of  Lake  Supe- 
rior         63-64 


Page. 

Brooks,  T.  B..  proposal  of  name  Keweenawan  by 63 

referred  to 1,6,34, 

40,  47,  48,  64,  66,  67, 70,  71,  72,  74,  75,  77,  79,  81 , 

82, 83,  86,  89,  98, 100, 102, 110,  111,  118,125,130, 

139, 143,144,  500,  403,416,  538,  541,  542. 543,544 

with  A.  A.  Julien,  catalogue  of  Huronian  rocks.  58 

Brook  section 312 

Buffalo  mine 117,118,327,395 

(See  Queen  Mining  Company.) 
Burt,  "W.  A.,  general  geology  of  the  Marquette  dis- 
trict   17-18,20-21 

referred  to 6,16,17,18,48 

C. 

Calcareous  rocks  in  Huronian  series 65 

Calcite  of  greenstone 465,473,496 

of  Kitchi  schists 166 

of  Mona  schists 155,156,157 

Calhoun,  J.  C,  referred  to 35 

Cambria  mine 395 

Cambrian  series 112.123, 134,135, 136 

classification  of 112, 113 

unconformity  with  Keweenawan 135 

Cambrian  sandstone.     (See  Potsdam,  Lake  Superior 
sandstone.  Old  red  sandstone.  Sandstone.) 

Camptonite 501 

Camptonite-like  greenstone 505 

Canada 44,63,189 

Cannon  mine 63,54,439 

Carbon  of  gray  wacke 4t6 

of  ferruginous  rock _   451 

ofmica-slate "   458 

of  slate 273,  446,  447,  565-566 

Carbonaceous  shale 50,  67 

Carbonaceous  slate 108 

analyses  of 446 

Carbonate  of  iron.     (See  Siderite,  Ferruginous  car- 
bonate.) 

Carbonate-bearing  beds 108,130,133 

Carp  River 5, 1 3, 

14, 16,  19,  21,  22,  23,  24,  25,  26,  27,  29,  35, 
59,  60,  87,  222,  241,  257,  272,  282,  284,  285, 
294,  295,  296,  299,  303,  305,  307,  314,  571 

Cascade 55 

Cascade  Brook 299,312,383 

Cascade  formation 127-128, 135, 136, 137, 138-139 

relations  to  other  formations 138,  139 

Cascade  mine 86,125,128,312 

Cascade  range 55, 

73, 115, 128, 137, 138,  298,  310,  332,  382,  383,  429,  557 

Cementation 573 

(See  Veins.) 

Census  OfSce  report 109 

Champion 62, 76, 

192, 194, 331, 332,  389,  409,  416,  434, 435, 436, 444, 446, 
452, 454,  455,  456,  460,  461,  473,  481,  483,  547,  563,  565 

Champion  mine 89, 

94-95,  129,  139,  140,  142,  193,  396, 

399,  412,  434,  435,  525,  537,  538,  549 

Channing.W.  F.,  on  geology  of  Marquette  district. . .      15-16 

referred  to 6, 21 

Chert 223,241,254,324,334 

and  jasper,  composition  of 1U6-109 

and  jasper  conglomerate,  described 413-414 

enlargement  of  quartz  grains  in 100 

of  brecciated  dolomite 250 


INDEX. 


585 


Pago. 

Chert,  hematitic,  from  Negaunee,  plate  of 348, 350 

of  conglomerate 226, 230,  311, 360, 411, 420, 424, 429 

of  dolomite 244,246,248,250 

of  ferruginous  slate 

of  Lower  Marquette  series 

of  quartz-conglomerate 

of  quartzit© 


130, 131 


412,415 


of  1 


317 


of  Upper  Marquette  series 127 


origin  of.. 


429, 564 


in  Northern  Complex 

in  slate 

[See  Veins  of  chert.) 
(See  Ferruginons  chert,  Hematitic  chert,  Jasper, 
Silica,  Quartz.) 

Chert-hreccia 

described 

Cliert-conglomerate 

Cherty  dolomite --i* 

Cherty  iron  carbonate.    (See  Cherty  siderite. ) 

Cherty  siderite 336 

alterationof 337,401^03 

analyses  of 337 

plate  of 3M 

Cherty  siderite-slate,  described 366-368 

Clierty  silica  of  siderite-slato 367 

Cherty  quartz  of  gray  wacke 229,  230 


371 


228,239,291,300,306 

Qconformity  between,  fig- 


76,473,515,517 


Page. 

Chlorite-schist 137,264,289,302, 

490,  494,  500,  503,  506,  507, 508,  609, 510,  511,  513-514,  515,  517,  523 

associated  with  iron  ore 131,140 

contact  metamorphism  produced  by 513 

con  tact  with  iron  ores 72 


dike 


181,421 
103-104 


Chlorite-slate 

Chloritic  quartzite 

Chloritic  rock 

Chloritic  schist 

at  Spurr  mine 

in  Marquette  series 

Chloritoid,  analysis  of,  referred  to  — 

including  quartz  and  feldspar 

of  arkose 

of  Clarksburg  hornblende-schist. 

of  conglomerate 

of  greenstone 


504-505,  509-510 


■  Marquette  formations 


55 


of  q  uartz-achist 

Cherty  quartzite 

and  dolomite,  pseudo- 

ure  of 

of  Mesnard  formation 

Chicago  and  Korthwestern  Eailroad 

Chippewa  Exploration 537 

ChippewaLand  District 12,27 

Chippewa  mine 53,54,439 

Chlorite  from  amphibole 388 

from  feldspar 265, 

290,  302,  318,  319, 327-328,  433,  448,  449, 450,  458 

from  garnet 50,62,300 

from  griinerite 423,436 

of  biotite-granite 172 

of  biotite-schist 302 

of  chlorite-schist 509 

of  conglomerate 264 

of  graywacke 265, 304, 318-319, 453 

of  greenstone 474,494.496,499 

of  greenstone-schist 205 

of  griinerite-magnetites-chist 369,  390,  391 

of  iron  ore 374,  399,  434,  435,  440 

of  Kitchi  schist 164,165-166 

of  mica-gneiss 416 

of  mica-schist 320,324,416,444 

of  mica-slate 455 

of  Mona  schist 155,156,157,158 

ofnovaculite 304 

of  ore  and  jasper  conglomerate 426 

of  recomposed  ore 411 

of  quartz-schist , 293,  322, 416 

of  quartzite 290,  300.  302,  303,  304,  313,  415,  42),  434,  529 

of  schist-conglomerate 442 

of  slate 292,304,320,448 

of  talcoso  schist , 510 

pseudomorphous  after  garnet 62,  94,  421 


Chocolate  <iuai  ry 

Chocolate  River 9,11,12,13,14,22,24,59 

Chocolay  River 237 

Claassen,  Edo.,  on  orthoclasc  crystals  in  hematite. . .  97 

Clarksburg 4, 

142, 446, 455, 456,  460, 462, 463, 464, 484, 485,  564, 571 

Clarksburg  formation 408,  435,  454-455,  554, 564-565 

composition  of 460 

conglomerates  in 476, 480 

deposition  of 464 

described 460-484 

folding  of 463 

gradation  types  between  sediments  and  tulfs  in.  472-473 

origin  of 480-481 

relations  to  Goodrich  quartzite 461-463 

relations  to  Michigamme  slate .461-463 

sediments  in 468-472 

tuilsiu 470,473-479 

447 


Clay,  analysis  of,  referred  to 140 

Clay-slate". 10-11,50,67,367 

Cleavage  in  argillaceous  beds 255 

in  conglomerate 437 

in  graywacke 267,316,446 

in  griinerite  and  hornblende 388 

in  iron. bearing  member 532 

in  Marquette  formations 574 

in  Michigamme  formation 452 

in  novaculite 267 

in  slate., 242,260, 

266,  267,  269. 271,  272, 274,  275,  310,  316,  317,  325,  446,  453,  455 

figure  of 243 

in  soaprock 541 

in  quartz-schist 293 

passing  into  flssility 317, 325 

relations  to  bedding 315, 317.  325,  452,  455,  57-1 

(See  Fissility,  Schistosity.) 

Cleveland  mine 46, 89, 94, 97, 119, 142, 507 

open  pit  of,  plate  of 336 

Cleveland  Cliffs  mine 379 


586 


INDEX. 


Cleveland  Lake  mine 

Cleveland  hematite  mine, 

Coal  Measures 

Columbia  School  of  llines,  article 
Concentration  of  ore.     {See  Iron 
of). 


by  students  of. .      93-97 


390 


Concretionary  structure  in  quartz 

in  ferrugnious  chert 370 

in  griinerite-magnetite-schist 373 

in  iron  oxide 376-377 

in  Menominee  j asper SSI 

in  mica-gneiss 450 

Conglomerate 134, 

227,228,230,231,235,  254,  256,  258,  266,  277,  282,  284,  292,  297, 
408,  409,  410,  411,  421,  424,  425,  428,  434,  436,  444,  446,  535,  556 

above  granite 1 47 

above  iron  ores  ....  31, 81, 89, 103, 115, 117, 125-127, 131, 144 

associated  Trith  iron  ore 41,67 

associated  with  diorite-schist 86 

associated  with  quartzite 88-89, 94 

;  base  of  Holyoke  formation 137 

t  base  of  Republic  formation 136, 138 

t  Jackson  mine 37 

;  Saginaw  mine 115,142 

isal.    {See  Basal  conglomerate.) 

1  Archean  and  Lower  Marquette  series  .  127, 143 
1  Archean  and  Upper  Marquette  series . .  127 

1  dolomite  and  sandstone 60 

1  granite  and  stratified  beds 122 

1  Kitchi  schists  and  Algonkian  beds 162 

between  Lower  Marquette  and  "Cpper  Marquette 

series 144 

between  Pabner  gneiss  and  Algonkian  beds 212 

between  quartzite  and  granite 88 

described .'...         452 

Omimi,  described 235 

replaced  by  ore 360 

State  road 305 

described 232 

in  Clarksburg  formation 460, 

462,  464,  468, 473, 476-480, 481-482,  483,  484 
in   Kitchi  schists.    {See  Conglomeratic  Kitchi 
schists.) 


rPaln 
rfginof- 


volcanic.    {See  Clarksburg  formation  and  Vol- 
canic conglomerate.) 

(See   Greenstone-conglomerate,  Schist-conglom- 
erate, and  Volcanic  conglomerate.) 
Conglomeratic  Kitchi  shist 160 

figure  of 161 

at  base  of  Huronian 188 

Conglomeratic  schist 153 

in  Clarksburg  formation 477 

Contact  action,  in  Clarksburg  series 475,  479. 480 

of  greenstone 498-499,  513-514 

structure  of  amphibole  in  Eepublic  greenstone, 
plate  of 470 

{See  Unconformity.) 
Cooper  Lake.    {See  Lake  Cooper.) 
Copper 12,13 

black  oxide  of 13 

ore  of 13 

Copper-bearing  series 48. 56, 64 

and  Huronian,  comparison  of 63 

(See  Keweenawan.) 


Page. 
Corning  Lake.    {See  Lake  Coming.) 
Correlation  of  Huronian  series  in  Lake  Superior  re- 
gion   127 

of  Marquette  series 574-579 

of  Marquette  series  and  Canadian  Huronian 117 

of  Marquette  series  and  Menominee  series G9 

of  Marquette  series  with  Huronian  and  Mount 

Alban  series 70-71 

of  Penokee  series  and  Marquette  Huronian 67-68 

Credner,  H.,  on  geology  of  the  Upper  Peninsula  of 

Michigan 45-46 

on  geology  of  the  Marquette  district 44-45 

referred  to 6,71,72,532 

Crocker,  referred  to 94 

Crosby,  on  origin  of  jaspers  and  cherts 68-69 

referred  to 6 

Crystal  Falls   and   Metropolitan  iron-hearing  dis- 
tricts, monograph  on 577 

Crystalline  rocks 58 

in  Northern  complex 150 

Crystalline  schists 158,526 

of  Basement  Complex 554 

described 555 

{See  Schists.) 

Crystallization  of  Clarksburg  rocks 464,  468, 473,  474,  475 

of  greenstone 502 

Cunningham,  Walter,  on  boundaries  of  the  Chippewa 

land  district , 12 

referred  to 6 

D. 
Daddow,  S.  H.,  with  Benjamin  Bannan,  on  deposition 

of  Marquette  ores 43-44 

Daliba  (Phenix)  mine 127 

Dana,  J.  T>.,  on  origin  and  position  of  Marquette  ores.     61, 146 

referred  to 71,72,77 

Dawes,  referred  to 94 

Deer  Lake 55,  74, 115, 1 16, 117, 121,  123, 125, 151, 1 60-167 

conglomerates 160 

{See  Kitchi  schists.) 

Dead  Elver 9, 12, 13, 18, 23, 30,  85, 125, 161 

Death  Kiver : 12 

Denudation  of  Archean 230,297,537,555 

of  Bijiki  schist 417 

of  Lower  Marquette 311,  312,  387,  440,  531,  537,  664 

of  Marquette  district 561,662-663 

of  Marquette  series 572-573 

of  Negaunee  formation 331,  334-335 

of  Upper  Marquette  series 402 

of  Wewe  slate 258 

plain  of 567,568 

See  Erosion,  Weathering.) 

Dexter  mine 377,378,424 

Diabase 65,66, 

98, 131,  132,  130,  148, 153, 155,  150, 157,  485,  487,  516,  572 

alteration  of 140,155,162,538 

in  Marquette  series 490-491,  494 

in  Northern  Complex 151,178,179-180 

intrusive  in  green  schist 74 

in  iron-bearing  rocks 119 

in  Marquette  series 142,  571-572 

in  Northern  Complex 555 

in  Eepublic  area 528 

olivine 507,520-521 

porphyritic 521 

quartz 507,519-520 


587 


Pago. 

Diabase  uralite 494,496 

Diabase  porpbyrite  in  Nortbern  Complex 180 

{See  Greenstone.) 

Diabnsic  lavas 160 

tuffs '. 160 

Diallageof  peridotite : 185 

Diamond  drill  boles 387,  542, 544 

Dikes  in  Basement  Complex 555 

described 506-514 

in  conglomerate 74 

in  gneiss 138 

in  Goodricb  quartzite,  plate  of 410 

in  granite 83 

in  Huronian 93 

in  Isbpeming  formation 421 

in  Marquette  series 506-514, 515,  522,  571-572 

in  Mesuard  formation 235 

in  peridotite 184-185 

in  Republic  trougb 527 

in  Soutbern  Complex 218 

of  acid  rocks.     {See  Of  granite.  Of  aplite.) 

of  aplite 153,182 

of  basalt 507 

,  of  basic  rocks.    (See  Of  greenstone.  Of  diabase.) 

of  cblorite-scbist 181,509 

of  diabase 153,162,178,179-180,188,218 

of  diabasein  greenstone-scbist 74 

iu  Marquette  series 142 

of  diorite.     {See  Of  greenstone.) 
of  epidiorite    {See  Of  greenstone.) 

of  granite 150,170,182,193,213 

in  Basement  Complex 150 

in  greenstone-scbist 122 

in  Laurontian 69 

in  Northern  Complex 151,162,182-183 

of  granite-porphyry 182 

of  greenstone 178, 180-181, 194,  488, 489,  517 

in  Azoic  schists 34 

in  Basement  Complex 150 

in  Clarksburg  series 460, 483 

iu  granite 10,  30 

in  Lauri:^ntian  series 69 

in  Marquette  series 487,500,506-514,518-522,523 

in  iron-bearing  formation  .  32,  74, 131, 329, 379,  395,  398 

in  Siamo  slate 323 

of  jasper 73 

of  kaolin-scbist 512 

of  malchite 182-183 

of  olivine  diabase 507 

of  quartz-diabase 140 

of  quartz-porphyry 182, 183 

of  sericite-scbist 513 

of  slate 49 

of  soapstone  in  Kegaunee 394 

of  talc-schist 510-511 

of  trap  in  metamorpbic  rocks 11 

relations  to  bosses 506 

{See  Basic  dikes  and  Acid  dikes.) 

Diorite 34, 49,  56,  66,  72,  74,  75,  79,  98, 128,  148,  476,  487,  502 

analysis  of 495 

fragments  in  conglomerate 538 

in  Clarksburg  formation 460,  464 

in  Huronian  series 94 

in  Huronian  schists 60 

in  Marquette  series 93,  487,  488,  491,  494, 495,  498,  500 

analyses  of 495 


Page. 

Diorite  in  iron-bearing  formation 90,91,119, 131 

iu  Nortbern  Complex 151, 178, 181,  555 

in  Republic  trougb 538 

in  Soutbern  Complex 206 

origin  of 99, 140 

relations  to  greenstone-schist 75 

to  iron  formation 86 

to  jaspiUte 75 

to  quartzitic  group 87 

to  sedimentary  rocks 105 

{See  Epidiorite,  Greenstone.) 

Diorite-schists ■. 49 

associated  with  conglomerate 148 

composition  and  origin  of 84-85 

in  Huronian  series 94 

in  Marquette  series 487, 488, 506 

relations  to  granite 83-84 

{See  Greenstone-scbist.) 

Dioritic  group 84-86 

relation  of,  to  sedimentaries 105 

Dip  of  Ajibik  quartzite 299,306,313 

of  Bijiki  schist 420 

of  Goodrich  quartzite 415, 427,  430,  431,  539 

of  iron  formation 329, 376, 378, 385 

of  jasper 428 

of  Kona  dolomite 252,  255, 256 

of  Lower  Marquette  series  in  Republic  trougb- .  539 

of  Mesnard  quartzite 667 

of  scbistosity  of  green  schist _ 295 

of  Siamo  slate 272,  312,  315,  325 

of  Wewe  slate 2.')7,  258,  271,  278,  281 

relation  to  folds 4 

Discordance.     {See  Unconformity.) 

Doe  River  quartzite  of  Tennessee 571 

Doleriteof  Marquette  series 93 

Dolomite 241.  243 

plates  of 246.250 

associated  with  serpentine 60, 76 

cherty '. 244 

derived  from  peridotite 183,184,185 

described ,  247, 248 

ferriferous 233 

veins  in  peridotite  and  serpentine 186 

{See  Kona  dolomite  and  Limestone.) 

Drift  deposits 162, 170, 191 

Dulutb,  South  Shore  and  Atlantic  Railway 127,  460,  498 

Republic  branch  of 432 

Dynamic  meiamorpbism  in  biotite- granite 172, 174 

in  Clarksburg  tuffs 474 

in  diabase -         181 

in  greenstone 145, 156,  501,  508, 51 4,  523 

in  gneissoid  granite 210 

in  hornblende-syenite 177 

in  bornblende-scbist 205 

in  niicacous  schist 200 

in  muscovite  granite 175 

in  tuff  deposits 169 

{See  Pressure  effects,  Mashing,  Contact  action, 
Brecciation.) 

Dynamically  metamorphosed  iutrusives 206 

tuffs 158 

E. 

Eagle  Mills 326 

East  Points  in  Lake  Michigamme 452 

Eastern  area 253 


588 


Edwards  mine,  croaa  section  oforeljotlies  at,  figure  oJ". 

EisengUmmer 

Eklogite 

Ely  shaft 

Enlargement  of  I'eldiipar  grains 290, 

of  quartz  grains 

264,  290, 293,  300,  304,  308,  318,  379,  414,  422,  439,  448, 

Epidiabase 

Epidiorite «4, 

in  Northern  Complex 151, 158, 178, 

origin  of 

Epidioritic  varieties  of  Mona  schists 

Epidote  of  acid  dikes 

of  acid  schists 


540,  541 
301-303 
100,104, 
449,  453 


of  greeiistt 
of  Kitclii  s 


416,  443 
168, 159 
416,443 


ofniioa-si-hist 

of  Mona  schist 155,156,157, 

of  quartz-schist 

of  quartzite 437, 

of  schist-conglomerate 

Erie  mine 439,440, 

Erosion.    {See  Denudation.) 

Eruptives,  altered 

associated  with  Azoic  rocks 

Huronian 

identified  by  Credner 

in  conglomerate 

in  Menominee  district - 

in  Republic  formation 

metasomatic  changes  in 

(See  Diabase,  Diorite,  Granite,  Peridotite,  Green- 


Escanaba  Eiver 23,30,384 

Eureka  mine 85, 187 

Excelsior  mine 378, 425 

Explorations  in  Marquette  district 5-148 

Y. 

Fan  fold  of  Alps 3-4,568 

of  ferruginous  schist,  figure  of 570 

Falsebedding 529 

Eault-overthrust  in  Siamo  slate,  plate  of 280 

Eault-sllppiDg 319 

Faulting 293,300,307,313,322,328,344,570^571 

in  conglomerate 437 

in  ferruginous  chert 380 

in  Ishpeming  formation 437 

injaspilite 371,380 

in  Ifegaunee  formation 378, 379, 383 

in  Eepublio  trough •- 144, 541-547 

Felch  Mountain  trough 525 

series,  lower 575 

Feldspar,  alteration  of.     (.S'(?eAlteration  of  feldspar.) 

including  magnetite 443 

of  biotiteschist 196,197,198,199 

of  Clarksburg  tuffs 474 

of  conglomerate  ....  276,  301, 413, 432,  442, 443,  477.  535,  536 

of  ferruginous  rock 45 1 

of  granite 270 

of  greenstone.schist 204,  206 

of  graywacke 265,448,463 

of  hematite  ore 374 


Page. 

Feldspar  of  Kitchi  schists 163,165 

of  mica.gneiss 447,  460,  459,  574 

of  mica-schist 320,443,574 

of  novaculite 267 

of  quartzite 236,  290,  415, 435 

of  recompoaed  ore 438 

of  siderite-slate 367 

of  slate 292,318,448 

parallel  arrangement  of  grains 228 

{See  Albite,  Microcliue,  Orthoclase,  Plagioclase.) 

Feldspathic  biotite-schist 196-198 

Feldspathio  micaceous-schist,  analyses  of 202, 203 

Felsite 139 

Felsite-porphyry,  origin  of 103 

Fence  Eiver 576 

Fence  Eiver  volcanics 579 

Ferrite.     (See  Hematite,  Iron  oxide,  Limonite,  Mag. 
netite.) 

Ferruginous  carbonate  in  Kitchi  schist 166 

in  Eepublic  formation 139 

(See  Siderite.) 

Ferruginous  chert 324,  326, 

327,  336,  365,  366,  372,  378, 379,  381,  382 

brecciation  of 361 

cavitiesin 370 

concretionary  structure  in 370 

described 361-363,370-371 

from  cherty  siderite 337, 451 

from  siderite 451 

from  siderite-slate 446,  454 

in  Northern  Complex 186,187 

passing  into  ferruginous  slate 380 

passing  intojaspilite 372 

plate  of 346 

Ferruginous  quartzite 427 

Ferruginous  schist 67,  526 

alteration  of 107 

composition  of 107 

fan  fold  in,  plate  of 570 

formation  of  ores  by  decomposition  of 75 

origin  of 106-109 

Ferruginous  slate 371,381,427,564,665-566 

alteration  of 380 

described 361,369-370 

from  cherty  siderite 337 

from  ferruginous  chert 380 

from  sideriteslate 361, 446 

inNorthem  Complex 186,187 

minor  plications  in,  figure  of 332 

plateof 344 

Ferruginous  veins  in  Northern  Complex 186-188 

Fissility  in  conglomerate 442 

in  Michigamme  formation 462 

inslate 269,319,324 

in  quartz-schist 293 

relations  to  bedding 452 

Fitch  mine 125,129,384,410,430,432 

Flag  ore,  interstratifled  with  chlorite-sohist 92, 515 

Flagstone  in  arenaceous  slate  group 92 

Flowage,  zone  of.    (See  Zone  of  flowage.) 

Folded  ferruginous  chert  of  Starwest  mine,  figure  of.         334 

Folding,  effect  on  metamorphism 573-675 

isoclinal.     (See  Isoclinal  folding.) 

Marquette  type  of 3-4 

of  Ajibik  quartzite 285,286,301,308 

of  Bijiki  schist 417,424 


INDEX. 


589 


Fulding  of  Clarksburg  formation  . 

of  diovite  schists 

of  feiTuginous  chert 

"  at  Starwest  mine,  figure  of  . 


Pago. 
4(51,  463,  48 


334 


410^11,427,428,431 


,  slate  interlaminated  with  achis 
toso  greenstone,  figure 
of  Goodrich  quartzite . . . 

plate  of 

of  grunerite-magnetite-schist 

of  hematite 

of  interbedded  greenstone  and  sedimen 
of  Ishpeming  formation 
of  jaspilite, 

ofi^^l:,;;::::::::::::::::::::::::--:-.-^.! 

of  Low,rM;,ni.M  II,  .,1    11,  imblio  trough 


515-516 


..  356,358,362,371,380, 


of  Micbigammo  formatio 
of  Negaunee  formation  . 


32,  537,  562-563,  566-571 
222,  230, 233, 234,  236, 237, 238 
445,456 


Garnet  of  grunerite-magnetite-rock 387 

of  grunerite-magnetite-schist. . .  293, 369, 391, 419, 423, 444 

of  griinerite  schist 513-514 

of  iron-bearing  formation 529 

of  iron  ore ^^^'^''^ 

390 


jaspiUte 

■  Marquette  formatio 
'  mica-schist 


574 

444,  447,  449, 456,  457,  459 


slate  . 


436 


54-55,86 

....  525,538-541,549 
315,326,327,395,407 


333, 335,  338,  379,  383,  384,  385,  431 
of  Negaunee  jasper 
of  quartzite  series. 
of  Republic  trough 
of  Siamo  slate 

figireof -;; 

plateof ^™ 

,    ,  ^  306,312 

of  slate 

of  Upper  Marquette  time 402,4IJ 

of  Wewe  slate ^"' 

258,  271,  272, 273,  274, 275, 277, 278, 281,  316 
Foliation.    (See  Cleavage;  Fissility;  Schistosity.) 
Foster,  J.  "W. 

on  ceology  of  Marquette  district ^''-'* 

referred  to 6,16,17,38,39,40 

41,  43,  44,  46, 47, 48,  52,  56, 60,  64, 71,  72,  76, 77,  79,  99, 113, 125 
(See  'Whitney  and  Foster.) 

on  Azoic  system 26-27,  29-30,  3^33 

on  general  geology  of  Upper  Peninsula  of 

Michigan '- 

on  geology  of  Marquette  district 24-2 

on  occurrence  and  origin  of  iron  ores 

on  systems  of  elevation  in  North  America. . 

Foster  mine 

Fracture  and  flowage,  zone  of.    (See  Zone  of  frac- 
ture and  floivage.) 
Fragmental  aggregate  i 

Fragmental  rocks 

of  Huronian  series 

volcanic 

(SeeoisoTuflfs;  Ashes;  Volcanic-breccias.) 

Fragmental  series 

Fundamental  Complex.    {See  Arohean  Basement  Complex 


Gabbro ""•  • 

Galena 

Garnet,  alteration  of 300,  < 

chlorite  pseudomorphs  of "-, 

development  by  greenstone  intrusion '• 

of  hiotite-slate ■ 

of  chlorite-schist 

of  conglomerate 434,478,479,483, 

of  contact  phases  of  chlorite-schist 

of  feldspathic  biotite-schist 

of  greenstone 


I  biotite-granite  . 


27-34 

3,  27-34 

31-34 

26 

45,  383 


58,  133 


of  quartzschist •="" 

of  quartzite 300,313,422,529 

of  slate ^^ 

Garnetiferons  green  schist 513-514 

Geological  Society  of  America 141-142 

Geological  Society  of  London 135 

Geological  Survey  of  Michigan.     (See  Michigan  Geo- 
logical Survey.) 

Georgian  Bay ^5 

Gibbon  mine ^^ 

Gilmore  mine ^ ^09 

Glaciation  of  grunerite-magnetite-schSt 380 

of  Archean  526 

of  Ishpeming  formation *29 

Glass  fragments  in  Clarksburg  tuffs 474^75 

in  matrix  of  pebbles  of  Kitchi  schists 164 

Gneiss 3,40,230,526,527,634 

associated  with  granite ^3 

of  Basement  Complex 98, 134,  654 

described 555 

of  Cascade  formation 138 

of  granite-conglomerate 557 

of  Grenville  series,  origin  of 201 

of  Northern  Complex 169, 178, 188-189 

of  Southern  Complex 190, 192, 194, 195,  209-2U 

origin  and  age  of 101,104,147 

relations  to  greenstone 120-121 

relations  to  sedimentary  beds,  Epidote 1U3 

(See  Gneissoid  granite.   Granite-gneiss,  Horn- 
blende-gneiss, Mica-gneiss,  andPalmergneisa.) 

Gneissoid  granite ^69 

described 169-176,209-211 

in  isolated  areas ^20 

in  Northern  Complex 152, 153, 169-176, 188 

in  Southern  Complex 190, 192, 194, 195,  209-211 

intrusive  in  schist  1^3 


figure  of- . 


river  course  through,  plate  of 170 

(See  Biotite-granite  and  Muscovite-granite.) 

Goethite f 

Goetz,  G.W.,  on  analyses  of  iron  ores 129 

Gold ''^ 

Goodrich  mine 89,125,126, 

142  299,  332,  335,  383,  381,  396, 409, 430,  431,  432,  560 

Goodrich  quartzite 288, 311,  332,  333,  360,  363, 306,  394,  402, 

406,  417, 419,  429, 529,  635,  540,  541,  542,  564,  565,  569 
and  Bijiki  schist,  gradation  between,  plate  of. . .  412 

contact  with  plicated  Negaunee  jaspilite,  figures 

of ''' 

deposition  of 563-564 

described •'«!'-*l« 

^^.      ^j  639 

in  Lake  Superior  mine,  plate  of 338 

relations  to  Ajibik  quartzite 310, 409, 411 

relations  to  Basement  Complex 411,  <13, 536 

relations  to  Bijiki  schist -HI.  *'i^ 

relations  to  Clarksburg  formation 461,  463 


590 


Page 
Goodrich  quartzite.  relations  to  iron-ore  deposits  396, 399, 517 

relations  to  Lower  Marqaette  series 536 

relations  to  llichigammo  formation 411 

relations  to  Negaunee  formation 377-378, 

382,  384,  410-411, 42,5,  427,  428, 430, 431,  433 

relations  to  Negaunee  jasper 383 

section  of,  at  Michiganime  mine,  showing  rela- 
tions of  .jasper  ore  and  conglomerate,  figure  of.         420 

■with  minor  fold  cut  by  dike,  plate  of 410 

(See  Ishpeming  formation.) 

Goose  Lake 55,240,241,252, 

255,  256,  257,  258,  269,  271,  272,  273,  275.  281,  282, 
283,  285,  294,  295,  299,  308,  310,  314,  559,  560,  567,  563 

Grand  Eapids  mine 354,366,380 

Granite 3,  8,  9, 10, 1 1, 13, 15, 10,  79,  82,  98, 139, 

189, 190,  223,  225,  230,  231,  234,  239,  270, 276,  526,  541 
age  of 40,63,66,69,75-70,78,83-84,101,104,111,128 


alter 


231 


analyses  of,  referred  to 62 

and  quartzite,  gradation  between 147 

(See  Archean,  apparent  gradations.) 

composition  of 201-202 

described 18,278 

dikes  in  Basement  Complex 150,193 

in  Laurentian 69 

Huronian 03 

intrusive  in  Azoic  slate 31 

in  gneiss 138 


I  iron-bearing  formation  . 

I  Kitchi  schists 

I  greenston 


magnetite-schist . 
schist,  figure  of.  - 


193 


in  Northern  Complex 178,182,555 

in  quartzite 29,76 

metamorphic ■       66 

of  conglomerate 232,  112,  442,  477,  483,  533, 557 

of  Basement  Complex 554 

origin  of 40,62-63,69,82,101,147 

pebbles  in  conglomerate 412, 442, 477, 483, 533 

range  in  composition  of 201-202 

recomposed  (see  Kecomposed  granite). 

relations  to  fragmental  series 121-122 

relations  to  greenstone-shist 105,  111,  122-123 

relations  to  Huronian  fragmentals Ill 

relations  to  iron-ore  deposits 547 

relations  to  Mesnard  quartzite 239 

relation  to  Wewe  slate 276,277 

stucco 271 

(See  Gneiss,  Gneissoid  granite.  Granitic  group. 
Hornblende-granite,  Microgranite. ) 

Granite  Point 8, 15, 34, 35, 59, 113 

Granite-conglomerate 240,  557 

Granite-gneiss,  relations  to  Mesnard  quartzite 238 

{See  Gneissoid  granite.) 
Granite-porphyry,  intrusiTe  in  Northern  Complex ...         182 

Granitic  areas  in  Lake  Superior  region 101 

Granitic  group 83-84 

relations  to  sedimentary  rocks 147 

Granitite.    (See  Biotite  granite.) 
Granitoid  gneiss.    (See  Gneissoid  granite.) 

Granophyric  growths  in  quartz  diabase 520 

Graphite'of  slate 446,453 

Graphitic  sclust 134 

Gray,  A.  B.,  on  geology  of  the  mineral  lands  of  Lake 


Page. 

Gray,  A.  B.,  referred  to 6 

Graywacke 223,224,230, 

231, 233,  234,  235, 239,  242,  256,  258,  259,  260,  263,  267, 
269,  271,  272,  273,  275,  276,  277,  278,  282,  287,  290, 292, 
304,  306,  309,  313,  316, 318,  319, 320,  321,  324,  326,  332, 
410,  415,  424,  425,  427,  433,  434,  445,  452,  468,  560,  566 

brecciation  of 268.  281 

described 228,  232, 265,  446 

enlargi-nien t  of  quartz  grains  in 100 

in  Clarksburg  scries 460,  462,  482,  483 

iuterstratified  with  conglomerate 287 

nietamorphismof 229,230,232,233 

of  conglomerate 431 

relations  to  Clarksburg  series 462 

Gray  wacke-slate 223 

described 228 

Green  Island 521 

Green  schist 113, 236, 237, 256, 476 


age  ot 
analya: 


138 


and  granite  areas,  contrast  in  topography  of 170 

described 116 

of  conglomerate 224,  233,  234,  235,  240 

of  Eureka  mine 187 

of  Mona  schist  area 152 

(See  Greenstone-schist.) 

Greenslate,  origin  of 30 

Greenstone 10, 20, 46, 49,  76, 153, 159, 160, 190 

alteration  of 98,396,399,538 

analyses  of 495 

associated  with  iron-bearing  rock 86, 131 

bluffs  near  Lake  Angeline,  plate  of 332 

blufl's  near  Lake  Bancroft,  plate  of 334 

bosses  in  Clarksburg  series 460,461 

hosses  in  Marquette  series  -  -  487,  489-506,  518,  530,  522,  523 
contact  action  of  (see  Contact  action), 
dikes  (see  Dikes). 

dikes  and  bosses  associated  with  ores 131 

in  grunerite-magnetite-schist,  plate  of 328 

figure  of 330,386 

in  Ishpeming  formation 421,425,443 

in  Marquette  series.  487, 489-506,  506-514,  518-532,  523,  571 

in  Negaunee  formation 330, 331, 333, 

375, 379,  385-386,  389,  395,  396,  399, 402,  407,  529,  572 
effect  on  developmentof  griineritic  and  mag- 

netitic  phases  of 381,383 

in  Eepublic  trough 470, 500,  503,  526, 528, 538 

in  Upper  Marquette 408 

including  griinerite-magnetite-schist,  plate  of  - .         330 

jnt«rbcdded  with  sediments 500 

of  Clarksburg  formation 464-467, 

482,  483, 484,  485,  488-499, 499-506,  523 

of  post  Clarksburg  age 518,  522 

of  pre-Clarksburg  age 488-518 

origin  of 66-67,102,103,148 

relations  to  greenstone-schist 1, 147 

relations  to  Ishpeming  formation 425 

relations  to  sedimentary  rocks 101, 489^90, 499 

sheets  in  Marquette  series 500,507,514-517,522,523 

(See  Sheet  greenstone.) 

varieties  of 58 

veins  in  granite 8, 10 

(See  Basalt,  Diabase,  Diorite,  Epidiorite,  Porphy- 
rite.     Greenstone-conglomerate,     Greenstone- 
schist.) 
Greenstone-conglomerate,  at  Deer  Lake 74,  86, 115, 117, 125 


591 


Page. 
Greenstone-conglomerate  in  Northern  Complex.  151, 109, 555 
(See  Kitchi  schists  aufl  conglomerate  in  Clarks- 
biirj;  series.) 

Gr.riist..ii.    ^.liist 10.40,75,120-123,123-125 

:is-,M  i:,i,.l  «  nil  Iragmental  rocks 4S7,504 


in  Niirtlicrn  Comples 151, 

153, 158, 160, 161, 162-167, 171, 189, 192 

analysis  of 168 

(See  Kitchi  schists  and  Mona  schists.) 

in  Southern  Complex 194, 201, 204-206 

origin  of 41,43,58,70,126,146,204^205 

relations  to  fragmental  series 111,121 

relations  to  granite  and  gneiss 105,  111,  122-123 

relations  to  acid  greenstone 75,147 

(See  Green-schist.) 

Greenstone-tuffs  in  Marquette  series 514, 517, 522, 523 

in  Northern  Complex 151,152,154 

{See   Clarksburg   series,  Kitchi  schists,  Mona 
schists.  Tuff,  Tuffaceous  greenstone.) 

Greisen  at  Eepublio 74,139,144 

Gren  ville  series ; 149 

Gresley.W.  S.,  description  of  hemal  ite  specimen. . . .         136 

Gribbenmine 89 

Grit 444,453 

Groundmass  of  conglomerate  in  Clarksburg  series. .  477-478 

of  dike  greenstone    509 

of  Kitchi  schists 164,165-167 

of  sheet  greenstone 516 

Groth,  referred  to 177 


1,436 


342,419,423,455 


387 


of  actinolite  and  anthophyllite. schist 129 

of  biotite-achist 294 

of  cherty  siderite-slate 367 

of  chlorite-schist 513 

of  Clarksburg  sediments 472 

of  griinerite-magnetite  schist. . .  293,  373, 390, 391, 418, 423 

of  griinerite  rock,  described 387 

of  hematite  ore 374 

of  hornblendic  schist 422 

of  iron-bearing  formation 529, 530, 576 

of  jaspilite 376-3T7 

of  magnetite  o 


ulite. 


455 


of  quartz  schist 293 

of  quartzite 313 

of  siderite-slate 367 

Griinerite-magnetjte-roek 388, 422-423, 424 

Griinerite-magnetite-schist 293, 294, 323, 

324,  332,  334,  336, 344,  366,  372,  377,  378,  381, 383, 385,  388, 
389, 396,  405,  408, 417, 421,  424, 434, 436,  461, 539, 565, 572 

analyses  of 338 

caught  in  greenstone 571 

plate  of 328 

concretionary  structure  in 373 

described 337-338,368,369,422^23 

development  of 369 

dome  structure  in,  plate  of 328 

from  cherty  siderite 337 

from  siderite 45 1 


Griinerite-magnetite-schist  from  siderite-scbist 

grading  into  biotite-slate 

grading  into  ferruginous  mica-slate 

grading  into  jaspilite 

grading  into  novaculite-like  rock 

isoclinal  folds  in,  figure  of 

of  Islipeming    formation,    discriminated    from 
grUnerite-magnetite-schist  of  Negaunee. . .  417,  ■ 

pebbles  in  conglomerate 461, 477,  ■ 

plate  of ; 

relations  to  Clarksburg  formation 

relations  to  intrusive  diorite,  figures  of 

relations  to  jaspilite 

veins  in  Northern  Co7upIex 

{See  Actinolite- magnetite. schist.) 
Grunerite.magnetite-slate : 

Grunerite-schist 137,: 

Grunerite-slate,  alteration  of 

from  siderite  slate 

Gunpowder  Lake 


Hard  ores 364 

origin  of 131-132, 133 

Hard  ore  jasper 363, 392 

Harlowe  mine 85 


157 


Hawaiian  volcanoes 481 

Hematite 81,136,552 

and  magnetite,  relations  to  jasper,  quartzite,  and 

soaprock,  plate  of 546 

from  magnetite 368,387,423 

from  siderite 325,340 

in  veins 310 


of  brecciated  chert 246 

of  brecciated  dolomite 250 

of  chert  and  jasper  conglomerate 414 

of  ferruginous  chert '.  361,  370 

of  ferruginous  slate 344,351,369 

of  gray  wacke 265 

of  griinerite-magnetite-schist 342, 368, 378, 418, 424 

of  hematite-schist 432 

of  iron-bearing  formation 293, 294,  530 

of  iron  ore 364-365,547 

of  jaspilite 293,354, 

356,  358,  360,  362-363, 371, 372,  373, 376, 428 

described 388 

of  magnetite-schist 344 

of  magnetitic  chert 352 

of  ore  and  jasper  conglomerate 426 

of  recomposed  ore 438-439 

of  slate 260, 265 ,  262,  266, 281,  317,  318,  320,  432 

pseudomorphs  after  siderite 3G7 

reduction  of 405 

{See  Iron  ore  and  Iron  oxide.) 

Hematite  ore 374-375,  549-550 

Hematite-schist 434 

Hematitic  chert  from  Negaunee,  plate  of 348, 350 

Henry  Mountain  laccolites 572 

Hill,  S.  W.,  on  granite  intrusive  in  slate 40 

referred  to 71 

Hillebrand,  W.  F.,  analyses  by 185 


592 


rage. 
HobbB,  W.  H.,  on  barite,  magnatite,  and  chloritoid 

in  Marquette  rocks l**' 

referred  to ^"^ 

Holyoke  formation l-** 

unconformity  witli  Republic  formation 128, 135- 

136, 137, 138-139 

Homemine '3.89 

Hornblende  and  griinerite  intergrown  . .  388.  390, 419, 423, 436 

crystals  in  mica-schist 323, 324 

fromsiderite "9.«2 

of  biotitegranite 1"2 

of  blotite-schist 322 

of  biotite-slate 384 

of  chlorite-schist 513 

of  conglomerate 479,480,527,533 

of  diorite 178,181 

of  epidiorite 180 

of  granite 526 

of  graywacke 3*^*^ 

of  greenstone ^^5, 

466,  467,  492, 493,  496,  498,  502,  538 

of  greenstone-schist 205 

of  griinerite-magnetite-sohist 368, 369 

of  griinerite-rock 387 

of  hornblende-schist *'6 

of  hornblende-syenite 177 

of  Marquette  formations 574 

of  mica-gneiss ^50 

of  Mona  schists 155,167,158,164 

of  novaculite  304 

of  quartz-diabase 519 

of  quartzite 422,444 

of  slate 304 

(See  Amphibole.) 

Hornblende-rock,  origin  of 84-85 

dikes  in  granite 83 

Hornblende-schist 98, 152, 206, 219, 527 

associated  with  Marquette  fragmentals 503 

in  Clarksburg  series 475-476, 477, 482 

in  Republic  formation 139 

origin  of 84,102,103-104,140 

in  Mount  Alban  series 66 

relations  to  granite 83-84 

{See  Amphibole-scbist.  Actinolite-schist,  Griine- 
rile-schist.) 

Hornblende-slate 11,20 

Hornblende-syenite  described 176-178 

in  Northern  Complex 151,176-177 

relations  to  adjacent  rocks 176 

Hornblendic  biotite-schist  in  Southern  Complex 198 

granite 10 

mica-gneiss  in  Southern  Complex 195 

schist 422 

in  Southern  Complex 192,201,203-208 

amphiboh'-schist 204-206 

greenstone-schist 204-206 

micaceous  amphibole-schist 208 

origin  of 


Hon 


399 


Houghton,  Douglass,  on  geology  of  Upper  Peninsula 

of  ilichigan 9-11 

on  geology  of  the  vicinity  of  Marquette 59 

referred  to 6,12,17,19,39,72,76 

Hubbard,  Bela,  on  geology  of  Marquette  district 18-19 

referred  to 6,16,17,71,72,48 

Humboldt 283, 

284,  331,  333,  338,  385,  389,  409,  410,  412,  433,  434, 444,  561 ,  S72 


Page. 

Humboldt  mine 129,386,412,433 

Humboldt  Mountain 338,  503,  507, 513 

Hunt,  T.  S.,  analysis  by 202 

on  alteration  of  Marquette  rocks 71 

on  Azoic  system 66 

on  origin  and  age  of  iron  ore 129 

on  position  of  ore 39 

proposal  of  name  Keweenawan  by 63 

referred  to 6,7,40,47,63,64,71,72,77 

Huron  Bay 92 

Huron  Lake.    (See  Lake  Huron.) 

Huron  Mountains 40 

Huronian 48,  61,  63,  71, 75,  76,  78,  82, 105,  109, 129, 134, 146 

Huronian  areas 101 

Huronian  formations 142 

{See  Huronian  series,  Huronian  rocks.) 

Huronian  granite 63 

Huronian  group 110-112 

(See  Huronian  series.) 

Huronian  rocks 55,133 

catalogue  of 58 

classification  of 64 

origin  of 98 

relative  ages  of 54 

(See  Huronian  series.) 

Huronian  series 39, 44-45,  55-56, 65, 134, 147 

and  copper-bearing  series,  comparison  of 63 

comparison  with  copper-bearing  series 63 

correlation  between  Marquette  and  Canadian ...         117 

components  of 65-66, 135 

relations  to  granite 82 

relations  to  Mount  Alban  series 66 

structure  of 98 

succession  in 64-66,  67 

two  series  in 126 

{See  Algonkian  Animikie  series,  Huronian  formation, 

Huronian  Group,  Huronian  rocks,  Marquette  series.) 

Hydromica-schist,  Irving  on 103-104 

Igneous  rocks : 

basalt 507,522 

bosses  of,  in  Marquette  series 489-499,  499-506 

described 487-524 

dikes  of 506-514 

in  Huronian  series 65 

olivine-diabase 507,  520-521 

porphyrite 521-522 

post-Clarksburg  greenstone 518-522 

pre-Clarksburg  greenstone 488-518 

quartz-diabase 507.  519-520 

sheets  of 514,515-517 

tuflfsof 614,517 

{See  Diabase,  Diorite,  Clarksburg  series.  Green- 
stone Intrusives.) 

Ilmenite  in  Mona  schists 159 

Inter-Marquette  erosion 376,  384,  387,  392 

Interbanding  of  Clarksburg  rooks 482,  484 

of  greenstones  and  sedimentaries 515 

Intergrowtbs  of  hornblende   and    grunerite.    {See 

Hornblende  or  Griinerite.) 
Intermediate  rocks,  bosses  of,  in  Northern  Complex .         151 

dikes  of,  in  Northern  Complex 150 

Intermount  trough 3^ 

International  Congress  of  Geologists 130 

Intrusive   character   of  greenstone  in   Marquette 
series 489-490,499-600 


INDEX. 


593 


Page. 

Intrusives  in  grunerite-magnetite-sohist,  figure  of. .         330 

in  Marquette  series.  470, 487-5U,  518-522, 555, 505,  571-572 

iu  Basement  Complex 554, 555, 565 

in    Negaunee   formation.    (See   Greenstone    iu 
Negaunee.) 

in  Northern  Complex 188 

described 178-186 

in  Republic  trough 628,538 

in  Siamo  slate 323 

in  Southern  Complex 218 

{See  Diabase,  Diorite,  Clarksburg  series,  Green- 
stone, Igneous  rocks.) 
Iron  carbonate.     (See  Siderite,  Ferruginous  carbon- 
ate.) 

Iron  Cliffs  mine 115 

Iron  Mountain 576 

Iron  ore 336,  37(i,  396,  413,  426,  547 

analysis  of 21 

referred  to 110,129 

and  jasper  conglomerate 426, 429,  543, 579 

plate  of 360 

at  Cascade  range 54 

at  Champion  mine 94-95 

at  Cleveland  mine 46 

at  Eureka  mine 85 

at  Foster  mine 45 

at  Jackson  mine 37,  75 

at  Lake  Superior  mine 46,  72 

at  Republic 23 

at  Salisbury  mine 75 

at  Washington  mine 75 

concentration  of 94-96, 132, 140,  317, 495 

contact  with  chlorite-schist 73 

described 364,374,375,415 

discovery  of,  in  Lake  Superior  region 142 

from  cherty  siderite 337, 401-403 

from  ferruginous  schist 75 

from  siderite; 45I 

bard  gray 405 

horizons 391-392 

in  Kepublic  formation 145 

in  Upper  Marquette  series 141 

in  Cascade  range 54 

interbanded  with  jasper 59, 72, 81 

literature  on : 

Brooks 48,  49,  54,  59 

Credner 45,46 

Crosby 68-69 

Dana 61,146 

Foster 23,  44 

Foster  and  Whitney 27, 31, 32-33 

Goetz 128 

Gray 13 

Gresley 136 

Hunt 39, 129 

Irving 102-103,106-109,113 

J'lli™ 99 

Kimball 41^3 

Lane 140 

I-esley 3g 

Locke 15,22,23 

Munroe 95-96,97 

Newberry 61-62 

J'ayne 94,95,96 

Putnam Hq 

MON  xxviir 38 


Page. 
Iron  ore— Continued. 

literature  on— Continued. 

Eeyer 113-114 

Kivot 37 

Kominger 85,86,89,90-91,148 

Smock 99 

Smyth 145 

Van  Hise 126, 127, 130, 131, 132-133, 141 

Wadsworth 71-72, 

73-75, 78,  79-81, 119, 120, 127-128, 135-136, 139 

Winchell,  H.V 142 

Whitney 34,35-37 

Whitney  and  Wadsworth 99 

Whitney  (see  Foster). 

Whittlesey 39 

magnetic 415,420 

metasomatic  changes  in 102 

of  Animikie  series 102-103 

of  basal  conglomerate 360,  543,  548-549 

of  Penokce-Gogebic  district.". 102, 103 

origin  of Q^ 

7,  32-33,  35-37,  39,  44,  61,  62,  71-72,  73-75,  78,  79-«l, 
97,  99,  102-103,  106-109,  120,  113-114,  127-128,  129, 
131-132,   135-136,   139,  140,   146,   400-405,   551-553 

pebbles  in  conglomerate 411 

phosphoric  acid  in 80 

recomposed    399 

replaced  by  silica 346 

replacing  silica 380,  400,  403^04, 431 

specular 415 

varieties  of 49 

(Sec  Flag  ore.  Hard  ore.  Soft  ore.  Iron  ore-deposits.) 
Iron-ore  deposits 322, 329, 379 


iof.. 


406 


concentration  of 495, 510, 574 

cross-section  of,  at  Edwards  mine 96 

described 391-405,547-553 

development  of 412,  435 

discovery  of 5 

form  of 131,548 

grading  into  chlorite-schist -. .  140 

grading  into  jasper 400 

horizontal  section  of,  at  Champion  mine 95 

laws  of  occurrence  of 131-133, 

400,  405, 406,  547-549,  552,  577 

of  Republic  trough  described 547-553 

of  Upper  Marquette  series 419 

origin  of 400-405,551-553 

{See  Iron  ores,  origin  of. ) 

plate  of 394,398 

position  of  (see  Iron-ore  deposits,  laws  of  occur- 
rence of). 

prospecting  for 405-407 

relations  to  geological  structure 549-551 

relations  to  Goodrich  quartzite 396,  398, 399, 547 

relations  to  granite 547 

relations  to  greenstone 131,395,396,398,399,402 

relations  to  impervious  troughs 400-401,  403,  406 

relations  to  jasper 398 

relations  to  Siamo  slate 395,398,406 

relations  to  soaprock 394,  548,  550,  552 

pLite  of 546 

(See  Iron  ore.) 

Iron  oxide,  concretionary  structure  in 376 

from  siderite 280,562-563 

in  veins  (see  Veins  of  iron  oxide). 


594 


IXDEX. 


Page. 

Iron  oxide  of  breccia 326 

of  brecciated  slate 263 

of  conglomerate 265,413,429 

of  chert  and  j  asper  con  ejlomerate 414 

of  ferruginous  chert 370 

ofgraywacke 228,365 

of  iron-bearing  formation 381,529 

of  mica  slate 458 

of  novaculite 2(i7 

of  pseudo-conglomerate 306 

of  quartzite 227,287,292 

of  slate 266,273,292,317 

Iron  pyrites  of  slate 273 

Iron-bearing  formation 109, 

110, 113, 114, 117,  UK,  12",  127,  628,  576,  579 

handing  of 531 

conglomerate  at  top  of 89 

existence  of  two 130 

extension  of 406-407 

origin  of 531 

relations  to  diorite 86,90-91,113 

{See  Iron-bearing  series,  Negaunee  formation.) 

Iron- bearing  horizon  in  arenaceous  slate  group 148 

Irving,  K.  D.,  on  age  .ind  origin  of  gneiss  and  granite  101-104 
on  Archean   formations    of   the   Northwestern 

States 101-104,104-106 

on  classification  of  early  Cambrian  and  pre-Cam- 

hrian  formations 112-113 

on  divisibility  of  Archean  in  the  Northwest. . . .  104-106, 
112-113 
on  equivalency  of  Huronian  rocks  in  Marquette 

and  Penokeo  districts 68 

on  Huronian  group 110-112 

on  Huronian  series 97-98 

on  Keweenawan  series 97-98 

on  origin  of  Huronian  rooks 98 

on  origin  of  greenstone-schist 122-123 

on  origin  of  jaspers  and  ores 102-103, 106-109 

on  position  of  greenstone-schist 120-123 

on  serpentine  of  Presquo  Isle 103 

on  stratigraphy  of  Huronian  series 67 

on  structure  of  Lake  Superior  region 101-104 

on  structure  of  Marquette  district 101-102 

on  succession  in  Lake  Superior  reigon 110-112 

on  succession  in  Marquette  district 102 

on  term  Aguotozoic 112 

referred  to 1,7,14,115,116,119,125,130, 

139, 141, 146, 150, 102,  232,  395,  400, 449,  457 
with  C.  K.  Van  Hise,  on  enlargement  of  quartz 

grains  in  sandstone 100 

Ishpeming 3, 

4,  32,  53,  67,  74,  75,  77,  86, 89, 90,  91,  93, 103,  115, 117, 
128,139,151,  329,  330, 331,  332,  336, 338,  356,  358,  373, 
376, 378, 379, 380, 382, 383, 392, 395, 408, 409, 410, 416, 
425, 427,  432,  444, 455, 488, 489, 490,  498, 501,  504,  579 

Ishpeming  basin 396, 410 

Ishpeming  formation 312, 365,  376,  388,  399, 408,  447,  554 

and  Archean,  apparent  gradation  between 441 

and  Negaunee,  apparent  gradation  between 433 

deposition  of 503-504 

described 409^44 

metamorphism  of 441 

relations  to  Arcbcim 441-442 

relations  to  greenstone 425 

relations  to  Michigamme  formation 452 

relations  to  Negaunee  formation 334- 

335, 433,  437, 439,  420 
iSee  Goodrich  quartzite.) 


Page. 
Islands  of  Archean.    (See  Archean  islands.) 

Islands  in  Lake  Michigamme 456-457 

in  Lake  Superior 236 

Isoclinal  folding 285- 

286,  308-312,  317,  327, 410,  421,  431,  568,  569 
of  griinerite-raagnetite-schist,  figure  of 384 


Jackson,  C.  T.,  on  geology  of  mineral  lands  of  Mich- 
igan       21-22 

referred  to 6,14,15,17,22,23,24,27,48 

Jackson  Iron  Company 15 

Jackson  Location 22 

Jackson  mine 37, 

54, 74,  89,  90,  94, 125, 128, 142,  346,  300,  366, 
396,  403,  410,  412,  427,  428,  433,  521,  569 

Jasper  Bluff 356,358,380 

Jasper.    (Sec  Jaspilite.) 

Jasper-conglomerate 564 

Jasper-hematite-schist 90 

Jasperization  of  Negaunee  formation 404-405 

Jaspilite 72, 

79,  80,  81,  120, 127,  128,  223,  26?,  312,  327,  334, 
336,  314,  365,  366,  372,  379,  380,  382,  386,  390, 
392,  394,  396,  499,  515,  542,  543,  550,  552,  544 

analysis  of 363 

at  Lake  Superior  mine 109 

banded  with  ore 577 

banding  of 81 

concretionary  structure  in 373 

contact  with  ores 81,128 

dikes  at  Home  mine 73 

described 362-364, 371-372, 375-376 

enlargement  of  quartz  grains  in 100 

from  cherty  siderite 337 

from  ferruginous  chert 372 

from  Grand  Rapids  mine,  plate  of 354 

from  Jasper  Bluif,  plate  of 356,358 

from  Jackson  mine,  plate  of 360 

from  Lake  Superior  mine,  plate  of 338 

from  north  of  Lowthian  mine,  plate  of 360 

from  siderite 451 

horses  of 399 

in  Animikie  series 102-103 

in  veins 268,291 


interbanded  with  ore 59 

layersin  conglomerate 311 

Literature  on : 

Brooks 59 

Crosby 68-69 

Irving 102-103,100-109 

Irving  and  Van  Hise 100 

Julien 99 

Pumpelly 109 

Rominger 90 

Van  Hise 127,130,131 

Van  Hise  and  Irving 100 

Wads  worth. ...  73,  75,  79, 118-119, 127-128, 135-136, 138 

"Whitney 99 

Whitney  and  Wadsworth 99 

Winchell,  N.  H 115 

name  proposed '9 

of  conglomerate 226,  234,  236,  303,  360, 425,  431, 543 

of  iron-bearing  formation 298,530 

of  Lower  Marquette  series 127, 130, 131 

of  Penokee-Gogebio  district 102-103 


595 


Jaapilite  of  quartz-conglomerate 432 

ofquartzite 412,415 

of  recomposed  ore 438 

of  Eepuhlio,  compared  witU  Micbigamme  jasper         576 

of  Republic  mine,  figure  of 362 

origin  of 68-69.  99,  102-103,  104,  106-109, 

115, 118-119, 127-128, 135-136, 138 

pebbles  in  conglomerate 287, 3C5,  411, 420, 429 

recomposetl 414 

relations  to  iliorite "5 

relations  to  griinerite-magnetite-scbist 375 

relations  to  Siamo  slate 328 

Julien,  A.  A.,  on  rocka  in  Marquette  district 58 

on  genesis  of  iron  ores 99 

referred  to 8,57 

{See  Brooks.) 


Kaolin  from  feldspar ^-^ 

of  conglomerate 264 

of  gray  wacke 228 

of  novacnlite 267 

of  quartz-scbist 239 

ofquartzite 290,300 

Kaolin-scbist 512 

Keller,  H.  F.,  referred  lo 148,504 

(See  Lane,  A.  C.) 

Keewatin  formation 142 

Keweenaw  Bay 9 

Keweenaw  Point 15,37,518,524 

Keweenawan  district 1 18-120 

Keweenawan  period 134, 135, 572 

Keweenawanseries 03,97-98,112,135 

proposal  of  name 63,97-98,112 

unconformity  witb  Cambrian 135 

unconformity  witb  Upper  Huronian 135 

Keystone  mine 52,82,92 

Kimball,  J.  I".,  on  geology  of  Maniiiotto  district....      40-43 

on  origin  of  green  scbist 40-41, 43 

on  origin  of  iron  ore 42-43 

referred  to 0,44,47,71,72,77,79,111 

Kingston  mine 544 

Kitcbi  bills 160,305 

Kitcbiacbist 303,306,313 

analysis  of 168 

described 160-169 

in  Nortberu  Complex 151, 152, 160-169,  517 

origin  of 160-161,163,167,168-169 

relations  to  A  jibik  quartzito 302-303,  305 

relations  to  adjacent  rocks 162 

relations  to  Mona  scbists 153-154 

structure  of 163, 164, 165-167 

Kloman  mine 51,  396, 439, 440, 442, 529,  .544, 549 

Knotenaobiefer 260 

Kona  dolomite 221, 224, 236, 237, 238, 239, 257, 258, 

209,  273,  275, 304,  306,  554,  556 ,  560 

described 240-256 

deposition  of 559 

plate  of 246,250 

relations  to  Mesnard  quartzite 251,  254 

relations  to  "Wewe  slate 269, 273,  274 

Kona  hills 254, 258 

Kach,  von,  referred  to 177 


92,30,113,518 


Laccolites  in  Marquette  aeries  . 


Page. 
Lacolites  of  greenstone  in  Negaunee  formation 329 

of  Henry  Mountains ; 572 

Laccolitic  bosses  of  greenstone 489,  490 

Lake  Angeline 54 

basin 390 

greenstone  bluffs  near,  plats  of 332 

Lake  Angeline  mine 379, 395 

LakoBancroft 378 

greenstone  bills  near,  plate  of 334 

Lake  Cooper 88.  324 

Lake  Corning 425, 505 

Lake,  Deer.     (See  Deer  Lake.) 

Lake  Gogebic 56 

Lake,  Goose.    {See  Goose  Lake.) 

Lake,  Gunpowder.     {See  Gunpowder  Lake.) 

Lake  Huron 101,102,105,110 

Huronian,  of  north  shore  of 574 

Lake  Mary 221,222,223,239,241,518,557,567 

Lake  Micbigamme 2. 3, 4, 23, 

54,  09,  92,  98, 150,  192,  282,  332,  416,  417, 
423,  436,  444,445,  447,452,  436  457,465, 
489,499,  519,  521,  525,  526,  531,  572,579 

islands  in 450-457 

Lake  Michigan 21, 110 

Lake,  Mud.    (.See  Mud  Lake.) 

Lake  Palmer 213 

Lake,  Silver.     (See  Silver  Lake.) 

Lake  Superior 2,  4,  56,  191, 194,  230.  240, 425,  557, 566 

basin 396,  573 

island  in 216 


north  shore  of 537 

Lake  Superior  hematite  mine 395 

Lake  Superior  Iron  Company 398 

Lake  Superior  mine 46, 

53,  54,  72,  73,  89,  94, 109, 117, 125, 128.  379. 425,  489 

No.  1  pit  of,  plate  of 338 

Lake  Superior  region 1, 1 00,  111,  133, 150,  574 

succession  in 110 

Lake  Superior  sandstone 100, 104 

{See  Potsdam   sandstone,   Old    Red  sandstone, 
Sandstone). 

Lake  Superior  Specular  mine 396 

Lake,  Teal.    {See  Teal  Lake  ) 
Lake,  Tigo.    {See  Tigo  Lake.) 

Lake  Wabassin 240,241.251 

Lane,  A.  C,  with  H.  F.  Keller  and  F.  F.  Sbarpless  on 

chloritoid  in  Marquette  rocks 129 

Lapworth,  referred  to 4 

Laurentian 47, 48,  54,  61, 67,  69,  71,  76,  78.  129,  1 4 1 


sof  1 


Laurentian  rocks 94 

Laurentian  series 44,55,62,66,105,109,149 

relations  to  Marquette  series 118 

Lavaa 154, 155, 156, 158, 160, 189,  464, 467, 483,  564-56.". 

diabaaic 1 25 

fragments  on  Kitcbi  achists 109 

in  Clarksburg  series 460,461,467-468,483,485 

in  Upper  Marquette  aeries 142 

{See    Clarksburg  rocks.  Volcanic    rocks,  Sheet 


Lava-breccias 

{See  Volcanic  breccias  ) 
Lawaon,  A. C,  referred  to... 
Lawton,  C.  D.,  referred  to . . . 
Lead  ore 


481 


596 


rage. 

Lesley,  J.  P.,  on  occurrence  of  iron  ores 38 

Lenooxene  of  biotite-granite 172 

of  epidiorite 180,181 

of  greenstone  of  Eastern  knobs 493 

of  hornblende-syenite 1"7 

of  Mona  schists 155,157,159 

Lherzolite 99  100,183 

Light-House  Point 60,110,187,518 

Limestone 134,  242,  254,  559,  560,  577 

at  Lake  Superior  mine 109 

of  Menominee  district 570 

origin  of 103-104 

siliceous 87 

(See  Kona  dolomite,  Marble,  Dolomite.) 

Limonite  from  magnetite 368 

of  brecciated  chert 246 

of  brecciated  dolomite 250 

of  ferruginous  chert 301 

of  ferruginous  slate 344,361,369,380 

of  ferruginous  rock 451 

of  griinerite-magnetite-schist 342 

of  iron  ore 375,447,454 

of  mica-gneiss 450 

of  shatteredslate 262 

of  slate 266,452 

pseudomorphs  after  siderito 340,  367 

Limoni tic  hematite 375 

Little  Presque  Isle 9,30 

Locke,  J.,  on  geology  of  the  mineral  lands  in  Michi- 
gan     14-15,22,23 

referred  to 6,15,16,21,72 

Logan,  Sir  William,  referred  to 105,110,146 

Lessen,  referred  to 157 

Lower  Fekh  Mountain  series 575 

Lower  Huronian  series 66,130,143 

characterization  of 135 

contact  with  Basement  complex 143 

{See  Lower  Marquette  series.) 

Lower  iron-bearing  series 109-110 

{See  Negaunee  formation.) 

Lower  Marquette  series 136, 152,  507, 527,  541,  554 

and  Archean,  apparent  gradation  between 559 

area  of 3 

141,145 


constitution  of 

contact  with  Upper  Marquette 546 

correlated  with  Lower  Felch  Mountain 574 

correlated  with  Lower  Huronian 577 

correlated  with  Lower  Menominee 574 

denudation  of 311,  312,  387,  440,  531,  537,  564 

described 3,  221-407,  528-535,  556-557 

formations  of 221,554 

proposal  of  name 127 

relations  to  Archean 298,  532-535,  557 

relations  to  Goodrich  quartzite 536 

relations  to  Upper  Marquette 562-563 

of  Republic  trough,  described 528-535 

summarized 576-577 

{See  Lower  Huronian.) 

Lower  Menominee  series 575 

summarized 576-577 

Lower  Silurian 55 

Lowthian  mine 360,383,430,431,571 

{See  Winthrop  mine.) 

Lucy  mine 148 


141 


Page. 

Magnesian  schist 49,  487 

Magnetic  mine 53,64, 

338,  389,  390,  391, 439, 441, 444,  503,  513,  534,  537 

Magnetic  ore 109,133,415,420,434,436,440 

((See Magnetite  ore.) 

Magnetic  siliceous  schist 74 

Magnetic  survey 407 

Magnetite 81,  294 

alteration  of 288,  387, 426 

and  hematite,  relations  to  jasper,  quartzite,  and 

soaprock,  plate  of 546 

from  feldspar 381 

from  hematite 405 

from  siderite 419,  422-423,  455 

in  veins 310 

included  In  garnet 387 

included  in  quartz 388,389,426 


of  tin  M  367 

of  rl:uk-iiinu  11111-    475 

of  conglomerate 420,431 

of  diabase-porphy  rite,  figure  of 180 

of  ferruginous  chert 361 

of  ferruginous  chert-breccia 370 

of  graywacke 265 

of  hematite-schist 432 

of  grunerite-magnetite-schist 342,  387, 390, 391 

of  iron  ore 426,435,547,574 

described 364-365 

of  iron-bearing  formation 293-294,  382,  530,  576 

of  jasper 293,  356,  358,  362,  372,  373,  376,  387,  428 

described 388 

of  mica-scliist 443 

of  porphyrite 522 

of  quartzite 313,413,421,437,438,529 

of  recomposed  ore 438,  439 

of  schist-conglomerate 442 

of  siderite  slate 367 

of  sl.-ite 266,317,318,320 

of  slate  ore 432 

projecting  into  quartz   389,  414,  426, 429 

replacing  chlorite-schist 140 

titaniferous 475 

Magnetite  ore 377,549 

described 374 

{See  Magnetic  ore.) 

Magnetite-actxnolite-schist 577 

(Si'cMagnetite-grunerite-schist.) 

Magnetite-griinerite-rock 375 

Magnetite-griinerite-schist 375, 531 

relations  to  Siamo  slate 369 

(SecGurnerite-magnetito-schist.) 

Magnetite-griinerite-siderite-slate 367 

Magnetitic  chert  from  Michigamme  mine,  plate  of. .         352 

Magnetitic  schist 422 

Makwa  hills 272,304 

Malchite  in  Northern  Complex 182-183 

Manganite  at  Lucy  mine 148 

Manganese  in  iron  ore 68-69 

Manganese  ore  in  hematite  mines 365 

Maps 492,508 

by  Bayley 508 

with  Van  Hise,  accompanying  atlas. 

by  Brooks 57 

reproduction  by  Putnam  referred  to 110 


INDEX. 


597 


Page. 

Maps  by  Burt 20 

referred  to 12,18 

by  Credner,  referred  to 46 

by  Foster  and  Whitney 28 

referred  to 25,26,28 

by  Gray,  referred  to 13 

by  Irving 28 

referred  to 98, 104, 123 

by  Putnam,  referred  to 110 

by  liominger,  referred  to 81,82 

by  Smyth 540 

by  Van  Hise,  referred  to 130 

with  Bayley,  accompanying  atlas. 

Mapping  of  Marquette  district,  methods  of 2 

of  Southern  Complex 190 

of  Palmer  gneiss 213 

Marble 60,134,251,253 

in  Mesnard  series 137 

(See  Kona  dolomite.) 

Marble  series 67 

Marcasite  of  slate 453 

Mareniscan 149 

proposal  of  name 134 

Marquette 8,9,29,74, 

87, 113, 115, 116, 123,  137, 139, 151, 183, 187, 
221,  222,  234,  237,  253,  331,  558,  566, 572,  573 

Marquette  district 145, 

298,  302,  332,  340,  361,  364,  401, 457, 461,  501, 525 

average  elevation  of 573 

denudation  of 561,  562-563,  572-573 

general  geology  of ... .  31,  77, 118, 120, 127-128, 130, 146-148 

metamorphism  in 573-575 

method  of  work  in 2 

relations  to  Menominee  district 576-579 

series  of 2,554 

structure  of 3,111,114,131 

succession  in 28,51,52-53, 

56-57,  64-66, 69,  83, 102,  U2, 133-134, 141-142, 146-147 

Marquette  series 66, 150,330,373 

comparison  with  Menominee  series 578-579 

comparison  with  North  Sliore  Huronian 98 

correlation  with   Huronian   and  Mount  Albau 

series 70-71 

correlation  with  Huronian  series  of  North  Shore 

Lalie  Huron 575 

correlation  with  Menominee  series 09, 578-579 

correlation  with  Penoliee  series 67-68 

intrusives  in 142,487-514,518-522 

origin  of 56-57 

relations  to  Laureutian 118 

sequence  of 51-52,  56-57,  69,  83, 133-134 

width  of 567 

(See  Algonliian,  Lower  Huronian,  and  Lower 
Marquette  series,  Upper  Huronian,  Upper 
Marquette  series.) 

Marquette  synciinorium 154, 525 

Marquette  type  of  fold 3-1 

Marquettian,  proposal  of  name 118 

Martite 413 

from  magnetite 288, 426, 429 

from  paint-rocll 511 

in  veins 288 


Mary  Lake.    (5ee  Lake  Mary.) 

Mashing  along  contact  between  Basement  Comp 

and  Algonkian 

of  Basement  Complex 


Mashing  of  biotite-granite 172-174 

of  Clarksburg  rocks 468, 475 

of  gneissoid  granite 220 

of  greenstone 494,497,501,503,504 

of  hornblendic  schist 208 

of  Kitchi  schist 162, 163, 165, 169 

of  Mesnard  quartzite 239 

of  micaceous  schist 199-200 

of  Michigamme  form.ation 457 

of  muscovite-granite 175 

of  Palmer  gneiss 217,219 

of  quartz-porphyry igs 

of  slate 260,  266 

of  tutfs 476 

(See  Brecciation,  Dynamic  metamorphism,  Pres- 

Masonite 143 

of  arkoso 229 

Matrix.    (See  Groundmass.) 

McComber  mine 515 

Melaphyre 139 

Melville,  "W.  H.,  analyses  by 338 

Menominee  district 69, 70, 120-123, 146 

area  included  in 577 

jasper  of 531 

relations  to  Marquette  district 576-579 

Menominee  River 23 

Menominee  series  compared  with  Marquette  series . .  578-579 

lower 575 

Mesabi  range 531 

Mesnard  quartzite 136, 137, 142, 

253,  256,  287,  304,  306,  307,  554,  556,  558 
and  Archean,  apparent  gradation  between..  231, 232, 237 

horizons  composed  of 224 

contact  with  Kona  dolomite 254 

deposition  o!' 559 

described 221-240 


dike 


230-231 
231, 


folding  of.     (See  Folding.) 

relations  of 

relations  to  Archean 

232,  237, 238,  239,  297,  557, 567 
resisting  power  of 222 

Mesnard  range 87,  224 

Metamorpic  rocks 11,16,17, 

18,  19,  25,  37-38,  46,  47,  65,  66,  83, 103, 104 

Metamorphism 72 

in  Marquette  district 573-575 

of  gneiss 447 

of  green  schist 137 

of  greenstone 145 

of  Ishpeming  formation 441 

of  rocks  associated  witli  iron  ores 107 

relation  to  folding 573-575 

(See  Alteration,  Contact  action.  Dynamic  meta- 
morphism. Mashing,  Metasomatism,  Pressure 
effects.) 

Metasomatism 573 

in  Clarksburg  conglomerate 479 

in  eruptive  rocks 102 

in  greenstones 494, 514 


(See  Alteration  and  Metamorphism.) 

Metropolitan  mine 439,  537 

Mica  fromfeldspar 226,290,302,435,448,449,453,534 


598 


Mica  of  conglomerate. . . 
of  ferruginous  cliert 
of  ferruginous  slate . 

of  gray  wacke 

of  mica-slate 

of  recomposed  jaspe 
of  quartzite 


302 


318,320,448 

(See  Biotite,  Muscovite,  Serit-iti.) 

Mica.gneiss 413,  416, 443.  445,  447,  450, 452,  458, 460 

concretionary  structure  iu 4d0 

Mica-hornblende-scbist 514,  523 

Mica-schist ^"^ 

92.  93,  137,  256,  289,  302,  317,  322,  433,  435,  440, 442,  444, 
445,  446,  447,  449,  452, 456,  457,  526.  528,  534,  535,  566,  679 

ilevelopmentof ^^ 

inNortliern  Complex 1^1 

intruded  by  granite,  figure  of 193 

of  conslomerate 533 

originof 100,103-104 

relations  to  intrusire  greenstone 323 

(See  Biotitescbist,  Micaceous  schist,  Muscovite- 
schist,  Biotite  slate,  Mica  slate,  Muscovite- 
slate,  Sericite-scbist.) 

Mica-slate 10.11.256, 

266, 275,  298,  305,  310,  312,  320,  435, 
436, 445,  446,  449,  463,  456,  566,  579 

from  quartzite 323 

interstratified  with  conglomerate 287 

relations  to  intrusive  greenstone 323 

(/Sf  e  Biotite-scbist,  Mica-schist,  Micaceous  schist, 
Muscovite-schist,    Biotite-slate,  Muscovite- 
slate,  Sericite-schist.) 
Micaceous-amphiboleschist  in  Southern  Complex-  -  -         208 

Micaceous  flagstone  in  arenaceous  slate  group 92 

Micaceous  hematite 91, 363, 374, 387, 429 

pelybedral  cavities  in ^7 

Micaceous  garnetifeious  schist 92 

Micaceous  schist,  analyses  of 202 

at  Miohigamme 92 

in  Southern  Complex 192, 195-203, 219 

92,117, 

118, 284,  324, 329,  331,  378,  384, 388,  390, 
409, 412,  417,  423,  436, 452,  518,  660,  572 

Michigamme  anticline 573 

Michigamme  area 300 

Michigamme  formation 408,415,440,443,  .WS.  554 

described 444-459 

relationsto  Bi,iiki  schist 419 

relations  to  Goodrich  quartzite 411 

relations  to  Isbpeming  formation 452 

Michigamme  jasper ^'J 

alteration  of 576 

compared  with  Eepublio  jasper 576 

of  Menominee  district 531,576 

Michigamme  Lake.    (See  Lake  Michigamme.) 

Michigamme  mine 89, 92, 

94,  100, 116, 126,  300,  314,  322,  352,  376,  377,  389,  396, 
399,  410,  412,  420, 421,  424,  475,  503,  504,  547,  558,  576 
section  of,  showing  relations  of  jasper,  ore,  con- 
glomerate, and  quartzite 420 

Michigamme  Mountain 576 

Michigamme  Elver 2:!,  52, 191,  445, 489, 526, 535 

Michigamme  slate 417.  533,  565,  574,  570 

relations  to  Clarksburg  form.ation 461,  462, 463 


Michigamme . 


Page. 

Michigan 5,44,62,63,64,09,504 

Michigan  Geological  Survey 1, 6, 

7,  39, 46, 47,  48,  57,  59,  81, 127, 135, 136, 142, 146-148 
Michigan  Lake.    (See  Lake  Michigan.) 

Michigan,  State  Geologist  of 118,119 

Michigan  street,  Marquette 187 

Microcline,  cleavage  of,  developed  by  pressure 264 

from  orthoclase 172 

of  biotite-granite 172-173,174 

of  conglomerate 442,535 

of  gneissoid  granite 211 

of  granite 526 

of  niica-gneias 459 

of  luica-scbist 443 

origin  of 173 

Micropegmatite  in  quartz-diabase 519 

Migisi  bluffs 238,252 

Milwaukee  and  Northern  Railroad 473, 541 

Mineral  lands  on  Lake  Superior 12, 14-17 

Mines: 

Barnum 126,386,396,398 

Barron 125,386,387,412,432,433 

Beaufort 127 


Blu 


395 


Boston 1 26,  377,  424 

Buffalo 117,118 

(See  Queen  Mining  Company.) 

Cambria 395 

Cannon 63,54,439 

Cascade 86, 125, 128,  312 

Champion 89,  94, 95, 129,  139,  140, 

142,  193, 396,  399,  412,  434,  435.  525.  537,  538.  549 


oin'u  pit  of,  plate  of 336 

Clevebmd  Cliffs 379 

Cleveland  hematite 395 

Cleveland  Lake 395 

Daliba,  Pbenix 127 

Dexter 377,378,424 

Edwards,  cross  section  of  ore  bodies  at,  figure  at.  96 

Ely  shaft 540,541 

Erie 439,440,441,537 

Eureka 85,187 

Excelsior 378,425 

Fitch 125,129,384,410,430,432 

Foster 45,383 

Gibbon 89 

Gilmore 509 

Goodrich 89,125,126,142, 

299, 332.  335,  383,  384,  396,  409,  430,  431,  432, 560 

Grand  Rapids 354,366,380 

Gribben 89 

Harlowe 85 


Hom 


73, 


129,386,. 


,433 


Humboldt 

Iron  Cliffs 115 

J^ackson 22, 

37,  54,  74,  89,  90,  94, 125. 128, 142,  346,  360, 
366,  396,  403,  410,  412, 427,  428,  433, 521, 569 

J  ackson  Iron  Company 15 

Keystone 52,  82,  92 

Kingston 544 

Kloman 51,  396.  439,  440, 442,  529,  544,  649 

Lake  Angeline 379. 395 

Lake  Superior 46,  53, 

54,  72,  73,  89,  94, 109, 117, 125, 128.  379,  425,  489 
No.  1  pit  of,  plate  of 338 


599 


Mines— Continued. 

Lake  Superior  hematite 395 

Lalse  Superior  Iron  Company 398 

Lalie  Superior  specular  hermatite 390 

Lowthian 360,383,430,431,571 

(Se«  Winthrop.) 

Lucy 148 

Magnetic 53, 54^ 

338,  389, 390,  391, 439, 441, 444, 503,  513,  534,  537 

McComber 515 

Metropolitan 439, 537 

Michigammn _ 89,92, 

94,  Kill,  I  IC,  Ijil,  l;(|ii,  ;:i  I,  :;jj,  :;r,L',  :;T0,  377,  389,  396, 
399,  410.  11;;,  rji),  4-Jl,  4-4,  175,  .".(i:;,  .504,  547,  558,  576 
section  of,  sbowiug  relations  of  jasper,  ore, 

conglomerate,  and  quartzite 420 

National 394 

New  York 73,  94 

New  York  and  Lake  Superior  Mining  Company.  22 

Nonpareil 324,  327 

(See  St.  Lawrence.) 

North  Champion 454 

Pascoe  shaft 540 

Phenix,  Pittsburg,  and  Lake  Superior 73 

Piatt 212,213,215,298,299,311,383 

Queen  Mining  Company 327,395,398 

Republic 51,74,89,126, 

128, 142, 342,  344,  396,  399, 437, 442,  537,  540,  546 

Republic  Mining  Company 544 

Riverside 396,399,439,547 

Saginaw 89,  115,  125, 142,  299,  366,  383,  384,  416,  430, 431 

Salisbury 75,89,379,395 

Section 16,  398 

Spurr 52,62,94,126,330,352,376,378, 

389, 410, 412, 420, 421,  424,  452, 475,  500,  503,  504, 671 

St.  Lawrence 324 

(S<;e  Nonpareil.) 

Standard 439,  537 

Starwest  mine 334 

See  Wheat  mine.) 

Taylor 346 

Volunteer 138,  284,  312,  333,  360,  396, 399, 412,  429, 431 

Washington 74 

West  Republic 541, 542 

Wetmore 127 

Wheat 125 

(See  Starwest.) 

Winthrop 54,125,137 

(See  Lowthian.) 

Minnesota 5,  44 

Mississippi  River 35 

Missouri 15,26,35 

Mona  schists 224,  232,  234, 470,  558 

age  of 154 

described 152-160 

division  into  two  classes 154 

in  Northern  Complex 151, 152-160, 164,  220 

origin  of 155,156-158,159 

relations  to  adjacent  rocks 153-154 

river  course  through 153 

plate  of 152 

schistosity  of 154,156 

structure  of 154,157.158 

(See  Basic  schists,  Basic  Mona  schists.  Acid 
Mona-schists,  Acid  schists.) 
Monograph  XIX,  referred  to 366 


Page. 

Morgan  furnace 54,240,244,246,253,560 

Morgan  shaft 540 

Morgan-Pascoe-Ely  syncline 540, 549 

Mortar  structure  in  hiotite-granite 174 

Mount  Alban  series 66,70-71,129 

relations  to  Huronian 55 

Mount  Chocolay 237,238,253 

Mount  Humboldt 222  329 

332,  384,  385,  386,  387,  388,  389, 432,  433,  454" 

Mount  Mesnard 137,221,231,236,237,251,253 

Mount  Omimi.    (See  Omimi  bluffs.) 

Mud  Lake 223,  232,  234,  235,  240,  252,  253,  558 

Munroe,  H.  S.,  on  deposition  of  iron  ore 95-97 

referred  to 94 

Murray,  Alexander,  referred  to 39,71,110,146 

Muscovitefrom feldspar.  289, 319, 327-328,  422,  438,  442, 458,  527 

of  hiotite-granite 172, 173 

of  biotite-schist ige 

of  chert  and  jasper  conglomerate 414 

of  graywacke 228,318-319 

of  greenstone 509 

of  griinerite-magnetiteschist 390 

of  iron  ore 374,415 

of  mica-schist 457 

of  muscovite-granite 175 

of  musoovite-sohist 195 

of  quartz-schist 289,293,415 


itzite 


1,534 


of  recomposed  ore 433  441 

Muscovite-biotite-gneiss 443 

(See  mica-gneiss.) 

Muscovite-biotite-schist 415  443 

(See  Mica-schist.) 

Muscovite- granite 174-175 

composition  ol 175 

in  Northern  Complex 174-175 

origin  of 175-176 

relations  to  adjacent  rocks 170-171 

relations  to  Kitchi  schists 162 

to  Mona  schists 153 

structure  of 175 

Muscovite-schist 289, 443 

in  Southern  Complex 195-196 

structure  of igg 


National  mine 394 

Negaunec 3, 4, 45,  53,  74, 75, 86,  89, 90,  91, 115, 123, 125, 

138, 146,  293,  296, 305, 329,  330, 331,  332,  334,  336,  348', 
350,  360,  366,  373, 376,  378,  379,  380,  382, 383,  392,  395,' 
408,  409,  410,  429, 488,  489,  490,  501,  515,  564,  573,  579 

Negaunee  formation 136, 137, 186,  221,  225,  281,  283, 

284, 204,  303,  312,  313,  314,  316,  317.  321,  324,  325,  326, 
412,  421,  422, 423,  424,  425,  426, 427,  428,  429,  432,  436, 
441,  447,  454,  472,  528,  540,  554,  582,  563,  564,  569.  570 
and  Ishpeming  formation,  apparent  gradation 

between 433 

denniliitioii  of 331,334-335 


561 

328-407,  529-532 


600 


Page. 

Xegaimee  formation  relations  to  Ajibik  quartzite...        289, 

292-293,  298-299,  333-334,  386 

relations  to  Clarksburg  series 461 

relations  to  Gooilricli  quartzite 377- 

378, 382,  384,  410-411,  425,  427,  428,  430,  433 

relations  to  Isbpeming 334, 335, 420, 437, 439 

relations  to  Siamo  slate. .  321-322, 325,  327, 333-334, 378-379 
(See  Iron-bearing  formation.) 

Xegauncejaspilite 322,421,429,437 

contact  witb  Goodrich  quartzite,  figures  of 335 

in  Lake  Superior  mine,  plate  of 338 

relations  to  Goodricb  quartzite 335, 383 

New  York  and  Lake  Superior  Mining  Company 22 

New  Tork  mine 73,94 

Newberry,  J.  S.,  on  character  and  origin  of  ores 61-62 

referred  to 71 

Norian  series 129 

Nonpareil  mine 324,  327 

North  Champion  mine 454 

North  shore  of  Lake  Huron,  Huronian  of 574 

North  shore  of  Lake  Superior 537 

Northern  Complex. .   192,  223,  225,  230,  295,  296,  439, 554,  558,  561 

and  Southern  Complex,  comparison  of 192,  218-219 

comparison  of  rocks  in 188-190 

constitution  of 150-151 

described 150-190 

intrusiresin 178-186 

mapping  of 151 

origin  of  rocks  in 190 

relations  between  members  of 151 

{See  Archean,  BasementComplex.) 

Norway 576 

Novaculite 11,67,254, 

259, 260,  269,  271,  274,  275,  277,  281,  31)4,  306,  309,  455 

associated  with  diorite-schist 86 

described 267 

of  conglomerate 424 

Novaculite-breccia 273 

NovacuUte-slato 263 

O. 

Old  Ecd  sandstone 8,  9 

Olivine  of  diabase 179 

of  olivine-diabaae 521 

of  peridotite 185 

of  quartz-diabase 520 

Olivine-diabase 507,520-521,526 

Omimi  bluffs 234,238,253 

Omimi  conglomerate  described 235 

Oolitic  structure  in  quartz 531 

Oothout,  referred  to 94 

Original  Huronian  of  Canada,  correlated  with  Algon- 

kian  of  Marquette  di.strict 3 

Orthoclase,  crystals  in  hematite 97 

of  biotite-granite 172, 173, 174 

of  biotite-schist 196, 197-198, 199-200 

of  conglomerate 442 

of  granite 526,533 

of  gneissoid  granite 171 

of  mica-gneiss 459 

of  mica-schist 443 

of  muscovite-granite 175 

of  recomposed  ore 438 

Overfolding 313,  569 

Overthrust  fault  in  Siamo  abate,  plate  of 280 


Paint-rock 131,392,395,399,487,506,510,511 

Paleozoic 134 

Paleozoic  rocks  of  Upper  Peninsula 60 

Paleozoic  shales  of  Appalachians 566 

Palmer 3,31, 

138,  221,  282,  330,  332,  333,  336,  406,  408, 
409,  410,  411,  413,  429,  430,  431, 432,  564 

Palmer  gneiss 194,219,299,312,512,558 

analysis  of 217 

described 211-218 

dikes  in 212-213 

mappiu  g  of 213 

origin  of 210,212,213-214,216,218,219 

relations  to  adjacent  formations 211-213 

relations  to  Ajibik  quartzite 311 

ParaUel  arrangement  of  feldspar  grains 228 

of  griinerite  blades 387 

of  mica  flakes 433,  449, 458-459,  527,  529 

of  quartz  grains 228, 

290,  301,  368,  382,  435,  449, 451, 458-459 

Pasooe  shaft 540 

Patton,  H.  B.,  on  macroscopic  features  of  Marquette 

rocks 140 

referred  to 7,512 

Payne,  C.  Q.,  on  deposition  of  iron  ores 94-95 

Peck,  Samuel,  referred  to 13 

Pegmatite  of  conglomerate 264 

Pegmatization  of  gneiss,  described 447,  448 

of  mica-schist 456 

Penokee  district 67, 

100, 104, 105, 107, 108,  132,  340,  370,  373, 400,  401,  457 

iron  ores  and  jaspers  of 102-103 

Penokee  range 395 

Penokee  series 366,  368, 449 

equivalency  with  Marquette  series 67-68 

Penokee-Gogebic.     {See  Penokee.) 

Peridotite 151,183-186,313 

age  of 128,184,185,188 

analyses  of 186 

alteration  of 186 

at  Presque  Isle 76-77, 99-100, 103, 128, 183-184 

near  Isbpeming 77,  99-100, 128, 184-186 


184-186 

184 

{See  Dolomite  and  Serpentine.) 

Phenixmine.    (Sec  Daliba  mine.) 

Phyllite-schist 

148 

Picnic  Islands 

140 

Picrite  

Pitch  of  EepubUctrougli 

539 

172  174 

of  greenstone 465, 466, 492, 493,  494, 496,  505, 516,  538 

601 


race.    [ 

Plagioclase,  of  greenstone-acUist 206 

of  Kitchi  schist 163,164 

of  mica-gneiss 459 

ofmica-schist 443 

of  Mona  schist 155,157,158 

of  mHscovite-granite 175 

of  porphyrite 521 

of  quartz-diabase 519 

of  recompoaed  ore 438 

(See  Feldspar.) 

Piatt  mine 212,213,215,298,299,311,383 

Plain  of  denudation.    (See  Denudation,  plain  of.) 

Pleistocene 257,332,557 

Point  No.  2  west  of  Presque  Isle 23 

Porodite  in  Eepublic  and  Holyoke  formutiuns  139 

Porter,  referred  to 94 

Porphyry 159,200 

Porphyry  vein  in  granite 10 

Porphyry-tuff 159 

Porphyrite 139,164,520-521,525 

Porphyritio  diabase 521 

Porphyritio  granite 209 

Porphyritic  greenstone 505,  508-509 

Porphyrltic  syenite 15 

Post-Clarksburg  greenstones 524 

described •■ 518-522 

Potsdam  sandstone 26, 27, 56, 71, 76, 115, 134, 184, 241 

unconformity -with  Azoic 120 

unconformity  with  underlying  series 112-1 13 

(See  Cambrian  sandstone,  Lake  Superior  sand- 
stone. Old  red  sandstone.  Sandstone.) 

Pre- Algonkian 149 

Pre-Cambrian  formations 129, 143 

classiflcation  of 112-113 

unconformity  between 137 

Pre-Clarksburg  greenstones 522-523 

described 488-518 

Presquelsle 9,13, 

15, 16,  22,  23,  25,  28,  35,  60,  67,  76-77, 
93,  99-100,    103,    128,  139,  183-184 

Pressure  effects  in  biotite- granite 172, 173-174 

in  conglomerate 270,  264,  289,  301,  428, 431,  442, 443 

in  feldspar 413 

in  ferruginous  chert 370 

in  gneiss 527 

in  gneissoid  granite 172, 173-174, 210, 211 

in  granite 277,278 

in  gray  wacke 229,  230,  233,  242,  267,  268,  319,  448 

in  iron  ore 431, 432, 433 

in  Ishpeming  formation 441 

injaspilito 376,427 

in  Kitchi  schist 165,166-167,169 

in  Kona  dolomite 247-248,251 

in  Mesnard  formation 223,224 

in  mica-schist 449 

in  mica-slate 449 


in  ore  and  jasper  conglomerate 426 

in  Palmer  gneiss 214-215 

iu  quartz 226,227, 

228,  265,  268,  275,  289,  290,  291,  308,  318,  368, 
373,  382,  413,  414,  415,  422, 429, 439,  453, 458 

in  quartzite 226, 

228,  229,  242,  288,  289,  290,  293,  300,  301 , 
303,  309,  310,  313,  412, 433,  434,  435. 438 


Pressure  effects  in  recomposed  jasper 

in  slate 233,242,260,261,266.267, 

(.Sec  Cleavage,  Dynamic  metaDiorphiam,Fissili1y, 
Mashing,  Schistosity.) 

Primary  rocks 

Prospecting  for  iron  ore 

Proterozoic 

Pseudo-conglomerate 223,  243,  260,  269,  274,  288, 

Pseudo-pebbles  in  conglomerate 

Pseudo-unconformity  between  cherty  quartzite  and 

truncated  layer  of  dolomite,  figure  of 

Pseudomorphs  of  calcite  alter  plagioclase 

of  chlorite  after  amphibole 

of  chlorite  after  garnet 

of  chlorite  after  plagioclase 

of  chlorite  after  olivine 

of  hornblende  after  augite 158, 

of  leucoxene  after  ilmenite 

of  leucoxene  after  sphene 

of  limonite  after  olivine 

of  quartz  after  plagioclase 

Pumpelly,  E.,  on  garnet  pseudomorphs  in  chlorite- 
schist 

on  geology  of  Marquette  district 

on  iron  ores  of  the  United  States 

on  structure  of  iron-bearing  series 

referred  to 1,7,110,404,421,503, 

Putnam,  B.  T.,  on  composition  of  iron  ores 

referred  to 

Pyrite 

of  gray  wacke 

of  iron  ore 

of  slate 

Pyroclastic  beds 

rocks 

Pyrolusite 

Pyroxene  of  diabase  dikes 

of  Mona  schists 

(See  Augite.) 


Quaquaversal  dip  of  iron-bearing  formation 

of  Marquette  series 

Quarry  at  Carp  Eiver 

Chocolate  Flux 

Quartz,  concretionary  arrangement  of 

fragments  in  Clarksburg  conglomerates 

from  feldspar 

265, 290,  302,  318-319,  327-328, 
435,  438,  442, 448,  449,  450,  453, 

from  siderite 

grains,    enlargement    of    (see    Enlargement    of 

quartz  grains), 
grains,  parallel  arrangement  of  (.we  Parallel  ar- 
rangement of  quartz  grains), 
iu  nodules 


Page. 

414 

306,  320 


381,  422, 
458,  527 
419,  422 


rtz). 


of  acid  schists 

of  amphibole-schist 

of  basal  conglomeriite 

of  biotite-granite 

of  biotite-schist 

of  brecciated  chert 

of  brecciated  slate 

of  cherty  siderate 

of  Clarksburg  sediments. 


602 


INDEX. 


Page. 

Quartz  of  conglomerate 226,231 

240, 264, 276, 413, 424, 431, 432, 
433, 442, 477, 479, 533,  535,  543 

of  aolomite 248 

of  feldspar 290 

of  feUlspathic  biotite-schist 197-198, 199 

of  ferragiuous  chert 325,326,379 

of  ferruginous  mica-slate 388 

of  ferruginous  rook *51 

of  ferruginous  slate 3"9 

of  gnoissoid  granite 210 

of  granite 526 

of  graywacke 228,  265,  318-319,  328,  448 

of  greenstone 465, 501,  503 

of  grlinerite-magnetite-schist 377, 388, 389, 418,  424 

described 372-373 

of  hornblende-syenite 177 

of  iron-bearing  formation 629 

described 376,381,530 

of  iron  ore 91,374,415,434,440 

of  jaspilite 107,  293,  354,  362,  372,  376,  383 

described 372-373 

of  Kitclu  soliists 164,166 


of  1 


chist 


344 


155, 157 


of  mica-gneiss 

of  mica-schist 

of  Mona  schists 

of  muscovite-granite 

of  novaculite 267,384 

of  ore  and  jasper  conglomerate 

of  quartz-diabase 

of  quartzite 

of  Palmer  gneiss 

of  recomposed  Jasper 

of  recomposed  ore 438,442 

of  siderite-slate 367 

of  slate 292,318,321,324,448 

of  ■\Veweslate 262 

Quartz-rock 223,303,310 

described 291 

from  quartzose  sandstone 230 

Quartz-schist 134,225, 

226,  231,  239, 270,  278,  289,  297,  307, 322, 413, 415, 433, 
434,  435,  437, 440, 441, 442, 443,  535. 

alteration  of 239 

interbedded  with  diorite-schist 85-86 

of  conglomerate 434 

Quartzite 11, 

49-50,  54,  87,  137,  139, 144,  224,  227,  231,  234,  235,  240, 
241,  242,  245,  247,  253, 254,  203,  269,  271,  291,  313,  325, 
401,  408,  409, 410,  412, 425,  433,  512,  515,  526, 534,  543, 
544,  560,  570,  577,  579. 

described 264,415 

development  of 293 

discovery  of  two 137 

Doe  Elver  of  Tennessee 571 

feldspathic 259 

gradation  into  granite 147 

gradation  into  mica-slate 323 

of  Clarksburg  series 460 

of  conglomerate 412,434,482,483 

unconformity  with  Potsdam  sandstone 27 

(See  Ajibik  quartzite,  Goodrich  quartzite,  Mes- 

Quartzite  range 272 


Page 

Quartzite  tongue  at  Republic 144 

Quartzite-breccia 254,291,309,310 

described 253 

Qnartzite-conglomerate 309, 310, 312, 313, 426 

described 259 

Quartzite-schist  pebbles  in  conglomerate 413 

Quartzitic  group 86-89 

composition  of 87 

relations  to  diorite 87 

Quartzose  sandstone,  alteration  of 230 

Quartzose  schist,  alteration  of 230 

Queen  Mining  Company 327, 395, 398 

R. 

Ragged  Hills 253 

Ramy  Lake  district 519 

geology  of 189 

Railroads : 

Chicago  and  Northwestern 76,  473,  515,  517 

Duluth,  South  Shore  and  Atlantic 127, 198, 460 

Republic  branch  of 432 

Milwaukee  and  Northern 473,541 

Recomposed  chert  and  j  asper  conglomerate 564 

Recomposed  granite 278,  280,  287,  557,  558 

Recomposed  jasper 413 

described 414 

Recomposed  ore 399,439,441 

Reibungsbreccia 223, 

228,  247,  208,  281,  288,  290,  303, 308,  317,  326,361,  370,  380,  570 

Republic 23, 

4S  52,  74,  76,  91, 139, 143, 144, 145, 191, 194,  294,  329,  332, 
366,  389,  390,  403,  410,  436,  439,  441,  444,  470,  503,  604, 
538,  561,  579. 

RepublioBluir 542 

Republic  formation 128,  135.1:16,138,145 

delimitation  of 139 

eruptive  rocks  in 139 

unconformity  with  Cascade  and  Holyoke  forma- 
tions   138 

Republic  horseshoe,  geological  map  of 546 

{See  Republic  trough.) 

Republic  mine 51 , 

74,  80, 126, 128, 142,  342, 344, 396, 399,  437, 442,  537,  540,  546 

Republic  Mining  Company 544 

Republic  Mountain 51-52,110, 

437,  470,  499, 500,  504, 513,  529, 531, 
535,  536, 537,  538,  541,  543,  547,  549 

Republic  trough 3, 

191,  283,  386,  293,  313,  331,  338,  389, 
394,  409,  410,  412,  413, 415,  436,  439, 344, 
445,  499,503,  504,  558,  563,  566,  570,  574 

described 525-553 

relations  to  Archean 525-526 

folding  of 525 

Reyer,  E..  on  character  of  iron-bearing  series 113-114 

on  origin  of  iron  ores 113-114 

refem-dlu 7 


Ajibik  . 
Bad.... 

Bijiki.. 


409,416,423,434 

5, 

13, 14, 16, 19,  21,  22,  23,  24,  25,  26,  27,  29,  35,  59,  60, 
87,  222,  241, 257,  273,  282,  284,  285,  294,  295,  296, 
299,  303,  305,  307,  314,  571. 


INDEX. 


603 


Rivers — CoDtinued. 

Cascade 

Chocolate 

Chocolay 


Pago. 

299,312,383 

9, 11,  12, 13,  14,  22,  24.  59 


Dead.. 
Death 


Doe. 


9, 12, 13, 18, 23,  30,  85, 125, 161 


Escanaba 22, 30, 384 

Fence 676 

MeDominee 23 

Michigamme 63,191,445,489,526,535 

Mississippi 35 

Sturgeon 44-45 

Eiverside  mine 396, 399, 439, 547 

EiTot,  on  general  geology  of  Lake  Superiorregion. . .      37-38 
referred  to 6,38,41,71,73 

Eogers,  H.  D.,  on  age  of  Lake  Superior  sandstone  ...  14 

referrcdto  71 

Eominger,  C,  on  arenaceous  slate  group 91-92 

on  conglomerates  and  breccias 88-89 

on  dioritic  group 84-86 

on  eruptive  rocks 93 

on  general  geology  of  Marquette  district. .  81-93, 146-148 

on  granitic  group 83-84 

on  Paleozoic  rocks  of  the  "Upper  Peninsula  of 

Michigan 59-60 

on  quartzitic  group 86-89 

on  sequence  of  rocks  in  Marquette  district 83 

on  serpentine 00.  93 

on  structure  of  tbe  Marquette  district 82 

on  unconformity  between  Huronian  and  Pots- 


Eosenbuach,  H.,  referred 

Eothpletz,  referred  to 

Entile  of  chlorite 

ofq»art2 

Eutley,  Frank,  referred  1 


1,  5,  6,  7,  56,  76, 102, 104, 105,  111,  123, 
125,  189,  231,  239,  302,  403,  500,  515 


Saginawmine. .. .  89, 115, 125, 142, 299,  386,  383, 384, 416, 430, 431 

Saginaw  range 432 

Salisbury  mine 75, 89, 379, 395 

Salomon,  referred  to 206 

Sandstone 10,19,22,71,556,659 

age  of 8,9,14 

red 8,23,24 


I  to  peridotite 77 

unconformity  with  granite 9,  34,  no 

unconformity  with  quartzite  and  serpentine 59, 60 

(See  Lake  Superior  sandstone.  Old  red  sandstone, 
Potsdam  sandstone,  St.  Mary's  sandstone,  St. 
Peter's  sandstone.) 

Scandinavia 35 

Schalstein 169,473,485 

Schist 3  230 

at  base  of  Marquette  series 574 

described 192-209 

of  Basement  Complex 149,150,151 

of  granite  conglomerate 557 

ofNegaunee  formation 499 

of  Northern  Complex 152-169,189 

of  Southern  Complex 191, 192-208 

(See  Acid  schist,  Actinolite-schist,  Amphibole- 
schist,  Anthophylliteschist,  Basic  schist,  Bio- 


Schist— Continued.  Page, 

tite. schist,  Chlorite-schist,  Crystalline  schist, 
Diorito-schist,  Feldspathio  mica-schist.  Graph- 
itic schist.  Greenstone  -  schist.  Hornblende- 
schist,  Horneblendic  schist.  Kaolin  schist, 
Kitchl  schist.  Mica-schist,  Micaceous  schist. 
Micaceous  amphibole  -  schist,  Mona  schist, 
Muscovite -schist.  Quartz  -  schist,  Sericite- 
schlst.  Talc-schist,  Talcose  schist.) 

Schiatcouglomerate 294,  296,  298,  434,  437,  478-479 

origin  of 479 

Schistosity  and  bedding  in  Siamo  slate,  relations  of, 

figure  of 315 

of  biotite-granite 173 

of  biotite-schist 196 

of  Clarksburg  sediments 472,  478, 483 

of  granite 277 

of  gray  wacke  267 

of  greenstone 491, 497,  601,  502,  504,  508.  511 

of  greenstone-schist 194 

of  hornliloudic  schist 194 

of  iron  form.ation 385 

of  Kitchi  schists 163-164,167 

of  Marquette  series 574 

of  micaceous  amphibolo-schist 208 

of  micaceous  schist 194, 199-200 

of  Mima  schist 154,156,158 

of  muscovite-grauite 175 

of  niuscovito-sehist 196 

of  novaculite 267 

of  Palmer  gneiss 215 

of  quartz-schist 293 

of  slate 260,  266,  267,  274,  275,  306,  315,  323 

relations  to  bedding 323,447,674 

relations  to  intrusives 386 

Schoolcraft,   H.  R.,  on  geology  of  Marquette  dis- 
trict          8,35 

referred  to 6 

Seaman,  A.E.,  referred  to 294 

Secondary  quartz 304,320,326 

from  feldspar 289 

of  quartzite '. .         291 

(See  Veins  of  quartz.) 

Secondary  rocks 9 

Section  16  mine 398 

Sedimentary  beds  in  Clarksburg  series. .  464,  467,  468.  475,  476 

Sericite  from  feldspar 265,  290,  327-328 

of  acid  schist 159 

of  biotite-granite 172 

of  conglomerate 264.413.432.4.33 

of  gray  wacke 228,  229,  230,  265,  304,  448 

of  griinerite-magnetite-schist 368 

of  iron  ore 399,432,435 

of  Kitchi  schists 164 

of  Mona  schists 155,157,158 

of  novaculite 267,304,384 

of  ore  and  jasper  conglomerate 426 

of  quartz-schist 239 

of  quartzite  ....  227,  290,  292,  300,  304,  415,  434,  435,  528,  534 

of  slate 266 

Sericite-schist 220,  264,  275,  311.  433,  613 

from  feldspathio  debris 230 

in  Kitchi  schist  area 103-164, 167, 169 

analysis  of 168 

in  Mona  schist  area 152, 160 

in  Southern  Complex 190 

veins  iu  gneissoid  granite 209-210 


604 


INDEX. 


pebbles  in  conglomerate 

Serpentine 

analysis  of 

at  Presque  Isle 

C7,  70,  76,  77,  93,  99, 100, 103, 120, 

(ScePeridotite.) 
ShiUe 260,408,444, 

alteration  of 

carbonaceous 

Sbarpless,  F.  F.,  referred  to 

{.See  Lane,  A.  C.) 
Shear  zones  in  biotite-granite 

in  greenstone 

in  porphyrite 

Shearing  between  tTpper  and  Lower  Marquette 

of  Lower  Marquette  series 

of  Mesnard  formation 

planes  in  cherty  sideiite 

in  quartz  grains 

Sheet-greenstones  in  Marquette  series 

507,  514-517, 

composition  of 

amygdaloidal 

SiamobiUs 313,314,324, 

Siamo  slate 221, 283, 284, 293, 299, 312, 332 

367,377,  384,  388,392,  394,  407,  554,  562, 

deposition  of 

described 

intrusive  greenstone  in 

minor  overturned  fold  in,  figure  i.^i 

overtbrust  fault  in,  plate  of 

pitching  fold  in,  plate  of 

plate  of 

relations  to  Ajibik  quartzite 269, 

relations  to  griinerite-magnetite-scliist 

relations  to  jasper 

relations  to  Negaunee  formation 

322,  325,  327,  333,  334, 

relations  to  ore  deposits 

relations  of  schistosity  and  bedding  in,  figure  of  . 
Siderite 130, 133, 327, 408 

alteration  of 

{See  Alteration  of  siderite.) 

of  ferruginous  chert 

of  ferruginous  schist 

of  gray  wacko 

of  siderite-slate 

origin  of 

residual 371,381,401 

(See  Ferruginous  carbonate.) 
Siderite-slate 

alteration  of 

cherty.    {See  Cherty  siderite-slate.) 
Sideritic  slate,  alteration  of 

analyses  of 

described 

veins  in  Northern  Complex 

Silica  (cherty)  of  siderite-slate 

of  ferruginous  chert 

of  griinerite-magnetite-schist 

of  iron-bearing  formation 

replaced  by  ore 346,348,394 

{See  Chert  quartz.) 


522,  523 
51tU517 
510-517 
326,  327 
333,  336, 
569,  572 
561 
313-328 


Page. 

Silicates,  aluminous,  in  iron  ores 91 

Silicification  of  carbonate-bearing-schist 108-109 

of  Kitchi  schists 166 

of  sandstone 100, 104 

SUverLake 161 

Slate 92,  224,  231,  233,  235,  236,  237,  238,  242,  244, 

247,251, 253,  254,  269,271,  276,  277,  278, 282,  287,  292, 
293,  304,  306,  307,  309,  445, 452, 454,  559,  576,  577,  579 

analysis  of 202 

arenaceous 20 

black :...  127,133 

carbon  of 273 

carbonaceous,  analyses  of 446 

composition  of 201, 202 

described 232, 446 

in  dikes 49 

iron  pyrites  in 273 

in  Clarksburg  series 460, 483 

of  ferruginous  slate 369 

of  Menominee  district 576 

relations  to  Korthern  slates 578 

of  Upper  Marquette 571 

passes  into  mica-slate 266,  325 

pressure  effects  in.  {See  Pressure  efl'ects.  Cleav- 
age Fissility,  Schistosity.) 

sideritic 334 

veins  in  Korthern  Complex 186,187 

{See  Argillite,  ArgUlitic  slate.  Clay  slate,  green 
slate,  hornblende-slate,  talcose  slate,  Siamo 
slate,  Wewe  slate,  Micbigamme  slate.) 

Slate-breccia 263,282 

Slate-conglomerate 259,  263 

described 264,266,271 

relations  to  slate  and  gray  wacke 266 

Slate  ore 364,404,405,547,549-550 

described 375 

Slickensides  in  jaspilite 376 

in  slate 317 

Smiths  Bay 145,526,538 

(See  Eepublic.) 

Smock,  J.  C,  on  position  of  Marquette  ores 99 

Smyth,  H.  L.,  on  contact  between  Lower  Huronian 

and  Basement  Complex 143 

on  quartzite  tongue  at  Eepublic 144-145 

on  Eepublic  trough 625-552 

referred  to 2,7,437,576-578 

Soapstone 116,131, 

132,  392,  395,  396,  399,  403,  487,  490,  506,  510,  511,  512,  513 

at  Jackson  mine 94 

from  greenstone 396,399,538 

grading  into  greenstone 394 

relations  to  ore  deposits 394,  548,  550, 552 

Soft  ore 67,364 

origin  of 75,78,120,131-132,133,139 

Soft  ore  jasper 318,  362,  392 

Southern  Complex 225, 554 

described 190-220 

comparison  w  ith  Northern  Complex 192,  218-219 

intrusivcsin 217,218 

relations  to  Me.snard  quartzite 567 

veins  in 217 

549-550 


Specular  hematite  ore 

Specular  jasper 392,  539 

Specular  ore 415, 436 

Sphene  of  epidiorite 181 

of  hornblende-syenite 177 


605 


rage. 

Spheroidal  weathering  of  greenstone 498 

Spiirr 331,390,417,423,446 

.S].urr  mine 52, 62,  94, 120,  330,  352,  375,  376, 

:!78,  3S3,  410,  412,  420,  421,  424,  452,  500,  503,  504,  571 


St.  iI;ir.V3»ali.lst„no 50 

St.  Peter's  sandstone 100 

Stacy,  James,  referred  to 21 

Starwest  mine,  folded  furruginous  chert  at,  figure  of.         334 

Standard  mine 439,537 

State  Prison 222.236 

State  Eoad 235 

State  Road  conglomerate 305 

Staurolite,  including  quartz  and  feldspar 459 

of  mica-schist 447.449,456,457,459 

Steiger,  George,  analyses  by 168, 

202-203,  217,  336-337,  338,  363,  418, 495 

Stelzner,  referred  to 113 

Stockton,  J.,  referred  to 13 

report  on  mineral  lands 12 

Stokes,  H.  M.,  analyses  by 338,446 

Stoneville 460,565 

Strike  of  Ajibik  quartzite 313 

of  green  schist 295 

of  iron  formation 376,378,379,385 

of  Kona  dolomite 252, 255 

of  shate 258,272,278,281,312 

relations  to  folds 4 

Sturgeon  River 44-45 

Summit  Mountain 212-213,  330,  557.  558 

height  of 573 

Superior  Lake,  see  Lake  Superior. 

Syenite 10,83,98,152,188 

porphyritlc 15 

(See  Hornblende-syenite.) 

Syncline  at  Republic 52,142,525 

Synclinorium 3,4 

abnormal 4 

cross  section  of,  figure  of 4 

Systems  of  elevation  in  North  America 26 

T. 

Taconic  system 129 

at  Saginaw  mine 142 

Talc-schist 508,510-511,533 

associated  with  iron  ore 131 

origin  of 510, 511 

Talcose  schist  in  Kitohi  schist  area 164 

in  Moua  schist  area 152 

Talcose  slate 10, 11,  20,  27 

Taylor  mine 346 

Teal  Lake 29,  30,  54,  55,  75, 86,  87,  91, 115, 123, 

125, 137,  222,  240,  257,  272,  282,  284,  285, 287,  294, 
295,  296, 297,  299,  302,  304,  305,  307,  308,  313,  314, 
315,  321,  322,  324,  326,  330,  378,  557,  558,  559.  560 

Teal  Lake  iron  range 378,  395 

Tennessee,  Doe  River  quartzite  of 571 

Thompson  pit 543,  544 

TigoLake 241,252,253 

Titaniferous  magnetite  of  quartz-diabase 520 

Topographic  maps  of  Marquette  district 2 

Tornebohm,  referred  to 66 

Tourmaline  of  feldspathic  biotite  schist 197 

of  gray  wacke 448 

of  greenstone 499 


rmaline  of  greenstone-schist.. 

of  Palmer  gneiss 

of  schistose  conglomerate 

of  .slate 


wn43  N.,  R.  31  W.,  .section  4 576 

44  N.,R.31  W 576 

44  N.,  R.  31  W.,  section  33 576 

45  N.,R.31W 576 

46  N.,R.24'W 18 

46N.,R.25W 18 

46  N.,R.26  W 18 

46  N.,  R.  27  W.,  section  22 568 

section  23 568 

46  N.,  R.  29  W.,  northwest  sections  of 525 

section  6 529 

7  529,541 

8 470 

18 532 


18. 


47  N.,R.24W 18 

section  29 147 

47  N.,R.25W 18,518 

section  1 221,223,231,238 

2 221,222,223,238,241,251,253 

3 221,222,239,241,253 

4 253 


5 253,257,272,307,314 

6 92,  272,  282,  287,  294,  299,  307,  326,  570 

7 253,272,299,307 

8 223,239,253,272 

9 221,223,239,253 

16 239 


24 273 

29 222 

47  N.,  R.26'W 13,18,54 

west  half 330 

section  1 326 

2 315,316,327 

3 314-316,322,327,330,407 

4 314^316,322,327,407 

5 258,314-310,322,327 

6 18,22,147,258 

7 18,22,258,344 

8 258,  314-316,  322, 327,  515 

9 314,  316,  322,  327, 407 

10 330,407 

11 254,257,273,308 

12 254,257,273,308 


13  . 


254,256,257,258,26 


246, 

,  272,  273,  275,  281,  340 


15 18,330,407 

19 314,315,316,328 

20 314,315,316,321,327,407 

21 262,281,282,284,560 

22 88,147,258, 

259,  269, 275,  278;  280,  281,  282,  284,  285,  308,  557 
23 187,258,259,269,270,271, 

275, 276,  277,  282,  284,  285,  286, 308,  310, 314,  557 
24 256,257,272,275,281,310,358 


509 

282,  283,  308,  309,  330 


606 


INDEX. 


Paga 
Tow-n  47  N.,  E.  26  W.— Continued. 

section  28 282,  2S3, 284, 285,  308,  309,  310,  382,  407 

29 31,283,285,287,295,308,309,314,383-383 

30 31,  284,  286, 299,  312,  314,  315,  382-383 

31 283,284,382-383 

32   212, 215, 284,  298,  311,  382-883,  557 

33 284,311,382,557 

34 194,211,212,284,298,326,557 

35 211,212,217,283,284,287,298,311 

47N.,E.27  W 20 

eastbalf 33" 

section  1 10,346,380,521 

4 378,425,517 

5       378.425,505 

6 257,331,378,425 

7  257 


22,425,427,431 


331,383,430,431,509,570 


27 284,286,289,; 

28 283,  286,  289,  298,  312,  515, 1 

47N.,B.28  W 

section  3 


7 455,483 


17 470,483 


47N.,E.29W. 


12 283,385,489,1 

47N.,  E.30 

section  2 


409,  439,  440,  457,  503 


Page 

Town  48  N.,  R.  25  W 18,159 

section  14 140 

23 180 

29 122,223,232 

31 253,326 

32 253 

33 234 

34 234,236,253 

35 236 

48N.,  E.26-W 15,18,19,159,176 

section  20 83 

21 83 

22 83 

23 83 

26 159 

28 155 

30 16 

31 305,326 

32 257,306,321,326 

33 257,  314, 326 

35 314 

36 284 

48N.,  E.27W 20,184 

section  2 187 

27 185 

29 313 

30 313 

33.... 284,302 

34 302 

35 158,302,322 

48N.,  E.28'W 20,85 

8ectionl8 183 

23 179 

25 161,298 

26 162 

29 174 

30    285,286,297,301,558,569 

31 285,286,297,301 

32 301,377 

35 446 

36 161 

48N.,E.29  W 20 

aection25 285,297 

28 416 

29 445,446,452,454 

30 452,454 

31 424,452,454,460 

32 452,483,470 

33 424 

35 445,454 

48  N.,  E.  30  W., 

section  19 322,418 

20 322 

21 300,  500 

27 621 

28 456 

29 456 

30 466 

32 456 

33 456 

35  518,520 

36 436,518,520 

48  N.,  E.  31  W., 

section  24 509 


INDEX. 


607 


Pagu 

Town  49  N.,  R.  25  AV 18 

49N.,  K.  33  W.,  section  9 346 

Transgression  quartzito 153 

Trap 13,37 

of  Presquelsle 9,16,22,70 

(See  Serpentine,  Peridotite). 

Tuffs 155, 156, 158, 164, 105, 160, 167, 169,  302,  330,  564-565 

acid 160,109 

altered 153,158,159,162,201 

basic 160,163,169,18:) 

diabasic 124 

in  Clarksburg  series 460, 

461, 462,  464,  467,  468, 470-479, 483 

in  Northern  Complex 555 

in  Upper  Marquette  series 142 

porphyry 150 

schistose 163 

structure  of 464 

(See  Greenstone- tuir,   Tuffaceous    beds,    Tuffa- 
ceous  greenstone.) 

Tuffaceous  beds  in  Marquette  series 487,  514.  617 

Tuffaceous  greenstone.     (See  Greenstone  tuff.) 
Tuffaceous  Kitchi  schists.  (See  Conglomeratic  Kitchi 
schists.) 

Twinning  of  orthoclase 526 

of  staurolite 459 

U. 
Unconformity  between  Algonliian  beds  and  Kitchi 

Bcliists 162 

between  Archean  and  Ajibikquartzite 283, 

295,  296, 298,  300,  302,  303,  309, 311,  528 

between  Archean  and  Goodrich  quartzite 411,  536 

between  Archean  and  Huronlan 44-45, 105, 113, 116 

between  Archean  and  Kona  dolomite 256 

between  Archean  and  Lower  Huronian 135, 143 

between  Archean  and  Lower  Marquette.  127, 135,  557-559 

between  Archean  and  Marquette Ill,  190, 532-535 

between  Archean  and  Mesnard 223, 230,  231, 239 

between  Archean  and  Upper  Marquette 127 

between  Archean  and  \Vewe  slate 270,  276, 278, 282 

between  Azoic  and  Potsdam 120 

between  Cambrian  and  Keweenawan 135 

between  Cascade  ami  Ki-public  formations 138 

between  Clarkslini  4  -■  i  ;i  s  ;tn.l  „;  nywacke 462 

between  granitr  ;h    :  <        i  i)  nioa...  82,111,121-122 

between  green  hi  hi   i     ,!.;  i    i  _  ^  n  ntal  series 121-122 

between  Goodri.  I,  ,|ii:iii /n.    ;iiil  Ajibik  quart- 
zite   411 

between  Goodrich  quartzite  and   Lower  Mar- 
quette series 536 

between  Goodrich  quartzite  and  Negausee  for- 
mation   411,428 

between  granite  and  sandstone 9,  34, 113, 116 

between  Holyoke  and  Republic  formations  . .        138, 139 

between  Huronian  and  Laurentian 78 

between  Huronian  and  Potsdam 60 

between  iron  formation  and  overlying  conglom- 
erates   115,126 

between  Isbpeming  and  Negaunee  formations  . .         334, 
335,  377,  378,  384,  420 
(.SVe  Unconformity  between  Lower  and  Upper 
Marquette.) 

between  j  aspilite  and  quartzite 144-145 

between  Keweenawan  and  Upper  Huronian 135 

between  Lower  Huronian  and  Upper  Huronian.         135 


Page. 
Unconformity  between  Lower  Marquette  and  Upper 

Marquette  series 1,  3, 127,  402,  411,  562-563 

(See  Unconformity  between  Isbpeming  and 
Negaunce. ) 
between  Marquette  conglomerates  and  Palmer 

gneiss 212 

between   Potsdam    sandstone   and   underlying 

series 112-113 

between  pre-Cambrian  formations 137 

between  quartzito  and  sandstone 59-60 

between  sandstone  and  serpentine 60 

Unger,  H.,  analyses  by 202 

United  States  Geological  Survey 7, 97, 123, 135, 141 

United  States  geologists 16 

Upper  Huronian  series 66, 130 

characterized 135,143-144 

of  Black  Hills 457 

of  Menominee  district 579 

of  Penokee  district 457 

(S<'t'  Upper  Manjuette  series.) 

Upper  iron-bearing  series 109-110 

Upper  Marquette  series 3,  288, 

311,  331,  332.  360, 399,  507,  528,  531,  532,  554 

area  of 3 

constitution  of 141, 145 

described 408-486,  535-538,  563-566 

denudation  of 402 

folding  of 402,  419 

formations  of 408,  554 

name  proposed 127 

of  Republic  trough,  described 535-538 

relations  to  Lower  Marquette 562-563 

relations  to  underlying  rocks 536 

Upper  Marquette  transgression 552 

Upper  slate.    (See  Miobigamme  formation.) 

Upper  Peninsula  of  Michigan 55,  64 

geology  of 27-34,66-67 

Uralite  of  greenstone 492 

Uralite-diabase 494, 496,  505 

Y. 
V-fold 441 

Veins,  ferruginous  in  Northern  Complex 186-188 

in  Southern  Complex 217 

of  acid  rock  in  Easement  Complex 150 

of  chert 186,248,260,288,301,303,306 

of  dolomite  in  serpentine 186 

of  granite  in  Basement  Complex. . . .  150, 151, 182, 193, 311 

of  granite  in  green  schist 256 

of  granite  in  mica-gneiss 447 

of  granite  in  Northern  Complex 151 

of  granite  in  Southern  Complex 193 

of  greenstone  in  granite 8. 10 

of  liematite 310 

of  hornblende 9 

of  iron  oxide 260, 

268,  275,  281,  288,  290,  303,  304,  307,  308, 344 

of  jaspilite 291 

of  magnetite 310 

of  martite 288 

of  microcline  in  quartz 172, 173 

of  porphyry  in  granite 10 

of  quartz 223,227,228, 

260,  267,  268,  281,  290,  291,  304,  307, 308,  310,  31 1,  453 

of  qu.artzin  Kitclii  schist 166 

of  quartz  in  Southern  Complex 151 


608 


Veins  of  serioite-scbist 

of  siderite 

of  slate  in  Northern  Comple 

Velani,  referred  to 

Vermilion  rock 

Vesuvius ■ 

Volcanoes,  of  Hawaii 

submarine 463,  46- 

Volcanic  rocks. ...  3,  HI,  U3,  171,  408,  435.  460,  481,  564-565,  57. 


142,  463,  464,  467,  481 


bombs 169,479,481 

breccias  {see  Breccias  in  Clarksburj^  series), 
conglomerates  (see  Conglomerates  of  Clarksburg 
series). 

eruptives 481,485 

tuffs 562 

Volcanic  plugs 484 

Volcanic  vents 460,  461,  463, 466,  479, 481 ,  484,  485 

Volunteer  mine 138,  284,  312,  333,  360,  396,  399,  412,  429,  431 

Tan  Hise,  C.  E.,  on  Algonkian  system 127, 135 

on  correlation  of  Huronian  areas  of  Lake  Supe. 

rior  region 126 

on  general  geology  of  Marquette  district 125  - 

127,  130-133, 133-135, 141-142, 143-144,  554-B79 

on  Ishpeming  formaticm 409-439 

on  Lower  Marquette  scries 221-405 

onMicbigamme  formation 439-460 

on  origin  of  iron  ores 131-133 

on  pre.Cambrian  rocks  in  Lake  Superior  region .  129, 130 
on  significance  of  conglomerates  above  ore  bori. 

zon  125-127 

on  succession  in  Marquette  district 133- 

134,  141-142, 143-144 

on  two  series  in  Huronian 126 

referred  to 2,  4,  7,  227,  230,  237,  242,  268,  274,  317,  395, 

400,  449, 467, 531, 540,  568,  570,  571,  575,  577 
(.See  Irving,  R.  D.) 

■w. 

Wabaasin  Lake.    (See  Lake  Wabassin.) 
"Wadsworth,  M.  E.,  on  general  geology  of  Marquette 

district 71-79, 127-128, )  36-140 

on  microscopic  features  of  jaspilites 73 

on  origin  of  basic  rocks 74 

on  origin  of  jaspilites  in  iron  ores 73- 

78,  79-81, 119, 127-128,  135-136,  137-141) 
on  relations  of  granites  and  sedimentary  rocks. .      75-76 

(■n  relations  of  green  schist  and  jaspilite 72-73 

on  serpentine 76-77,  99-100, 183, 185 

on  subdivision  of  Azoic  system 118-120,135-137 

on  succession  in  Marquette  district 135-140 

referred  to 1,  6,  7,  103, 106,  111,  115, 126, 125, 127, 

140.  144, 183, 185,  298, 311,  497,  543 
(See  J.  D.  Whitney.) 

■Washington  mine 74 

"Weathering  of  basic  dike  rocks 181 

of  greenstones 498 

of  Kitchi  schists 164 

of  Mona  schists 156 

(See  Denudation,  Erosion.) 

■Wehrlite 183,185 

■Werveke,  referred  to 177 

"West  Eepublio  mine 541,543 

"Western  tongue 283,  286,  293,  313,  390,  439,  444, 445, 570 

"Wetmore  mine 127 

Wewe  hills 256,259,275,308 

"Wewe  slate 221,244,248,251,252 

284,  285,  287,  289,  290,  292,  300,  304,  306,  307, 554,  556 


Page. 

"Wewe  slate,  deposition  of 560 

described 256,282 

plate  of 262,280 

relations  to  Ajibik  quartzite 271 

272,  273,  277,  286,  287,  294-295,  307.  309,  310 

relations  to  Arcbean 270,  276,  277,  280 

relations  to  Kona  dolomite 209, 273,  274 

Wichmann,  A 67,70 

"Wheat  mine 125 

(See  Starwest  mine.) 

"White  Mountain  series 66 

Whitney,  J.  D.,  analyses  by 184 

on  geology  of  Marquette  district 24 

on  origin  and  occurrence  of  iron  ores 34-37 

on  presence  of  two  series  in  Lake  Superior  region  38 

referred  to 6,16,17,38,39,40 

42,  46,  47,  48,  56,  60,  61,  64,  71,  72,  76,  79,  99, 113, 125 
■with  M.  E.  "Wadsworth,  on  divisibility  of  Azoic 

system 99 

on  origin  of  ores,  jaspilites,  and  diorites 99 

(See  Foster.) 
"Whittlesey,  Charles,  on  Laurentian  series  in  Mich- 
igan         62-63 

on  origin  of  Azoic  rocks 38-39 

on  origin  of  iron  ores 39 

referred  to 6,  71 

Wichmann,   A.,  on  microscopic  features   of  Mar- 

quetterocks 67,70 

referred  to 6,64,66,74 

"Wilkins,  "William,  referred  to 12 

Williams,  G.  H.,  criticism  of  work  of,  byN.  H.  "^Vin- 

ohell 145-146 

on     microscopical    description    of  greenstone. 

schists 123-125 

on  origin  of  greenstone-schists 114, 123-125 

referred  to 7,122,130,145,  154,155,156,157, 

159,160,178,183,206,476,502 
"Winchell,  Alex.,  on  age  of  Marquette  iron-bearing 

rocks 117-118 

on  general  geology  of  Marquette  district 39,  48 

on  points  in  geology  of  Marquette  district 117-118 

referred  to 6,  7,  39,  71,  81, 121, 125 

Wincliell,  H.  V.,  sketch  of  discovery  of  mineral  de. 

posits  in  Lake  Superior  region 142 

Winchell,  N.  H.,  on  conglomerates  and  their  signifi- 
cance    115,142 

on  origin  of  Archeau  greenstones 145, 146 

on  points  in  the  geology  of  the  Marquette  dis- 
trict    115-116,142 

referred  to 7,117,121,125,489 

"Winthrop  mine 54, 125, 137 

(See  Lowthian  mine.) 

Wisconsin 5,  6, 10,  35,  37,  61, 64,  69 

Wright,  C.  E.,  on  geology  of  "Upper  Peninsula  of 

Michigan 66-67 

on  microscopical  features  of  Marquette  rocks. . .  59 

referred  to 6,57,64,71,72,77,119 


Zirkel,  F.,  referred  to 06,177,201 

Zoisite  of  mica.gneiss 416,  450 

of  mica-schist 4.6 

of  quartz-schist 416,443 

Zone  of  fracture  and  flowage. ...  251, 268, 280,  344, 350, 356,  571 

of  flowage 269 

of  fracture 269 


[MoiiOKnii.h  XXVIII.J 


The  sfcitiito  approved  Marcli  8,  1S7M,  psfciblisliitig  tlin  lliiitcil  Status  (ieoloKii-al  Survey,  contains 
the  following  provisions : 

"The  publications  of  the  Geoloj;ii;il  Siir\  i  y  <Ii:ill  consisi  i.t  llic  annual  rej>ort  of  operations,  geo- 
logical anil  economic  maps  illustratiii.i;  lln-  i.^.iincc-  an. I  i  lissiii,  ;ii  mii  of  the  lands,  and  reports  upon 

general  and  economic  geology  and  pal( Ini;\.      I'h.- jmiinil   ii-|init  of  operations  of  the  Geological 

Survey  shall  ac-nmiiaiiy  tin-  aiiunal  r.')..,ii  of  tli.-  S, nvtaiy  ..t  tlir  lMi,.rior.  All  sp,M-ial  niruioirs  and 
reports  of  said  Siii-vcy  sli.ill  i..-  i^siir.l  in  iiniloini  .|ii;nt..  .sriirs  ;  f  .Ir.iiM'il  n. '.■.•<>:. r\   h\-  t\ir  I  liivctor,  but 

and  for  .sale  at  the  |irire  ot  |iiil>lii'iil  iuii ;  mihI  all  litriaiy  and  raitoi^rapliic  materials  rereived  in  exchange 
shall  be  the  property  of  the  United  States  and  form  a  ]>art  of  tiie  library  of  the  organization  :  And  the 
nu)iiey  resulting  from  the  sale  of  such  publications  shall  be  covered  into  the  Treasury  of  the  United 
States." 

Excejit  in  those  cases  in  which  an  extra  number  of  any  special  memoir  or  report  has  been  sup- 
plied to  the  Survey  by  special  resolution  of  Congress  or  has  been  ordered  by  the  Secretary  of  the 
Interior,  this  office  has  no  copies  for  gratuitous  distribution. 

ANNUAL  EEPOETS. 

I.  First  Annual  l{ei)ort  of  the  United  States  Geological  Survey,  by  Clarence  King.  1S80.  8'^.  79 
l)p.     1  ma]i. — A  pn  limiuary  icport  describing  plan  of  organization  and  publications. 

II.  Sec(md  Annual  Report  of  the  United  States  Geological  Survey,  1880-'81,  by  J.  W.  Powell. 

1882.  8°.     Iv,  .58S  pp.     ti:.'  pi.     1  map. 

III.  Third  Annual  Ueport  of  the  United  States  Geological  Survey,  1881-'82,  by  J.  W.  PoweU. 

1883.  8°.     xviii,  564  pp.     liT  pi.  and  maps. 

IV.  Fourth  Annual  Report  of  the  United  States  Geological  Survey,  1882-'83,  by  J.  W.  Powell. 

1884.  8^.     xxxii,473pp.     85  pi.  and  maps. 

V.  Fifth  Annual  Report  of  the  United  States  Geological  Survey,  1883-'84,  by  J.  AV.  Powell. 

1885.  8°.     xxxvi,  409  pp.     .58  pi.  and  maps. 

VI.  Sixtli  Annual  Re].. at  ot  the  United  States  Geological  Survey,  1884:-'85,  by  J.  W.  PoweU. 
1885.     8°.     XNix,  .-Mii|'l'-     ii">  pi.  :nid  maps. 

VII.  Seve.iili  Annual  Rrpoi  i  of  the  United  States  Geological  Survey,  1885-'86,  by  .J.  W.  Powell. 

1888.  8°.     xx.(i.-.(i  |,|..      71  |.l.  ,,ih1  ina],s. 

VIII.  Eighth  Annual  i;.]...!  t  of  the  United  States  Geological  Survey,  1886-'87,  by  .T.  W.  Powell. 

1889.  8°.     2pt.     xix.  171,xii  I,),,     .-.Spl.audmaps;  Ip.l.     475-1063  pp.     54-76  pi.  and  maps. 

IX.  Niutb_A_nnual  R.port  ot  the  United  States  Geological  Survey,  1887-'88,  by  J.  \V.  PoweU. 

X.  Tentli'  Annual    h'eport  ot  tlu'  I'nited  States   Geological   Survey,  1888-'89,  bv  .1.  W.  Powell. 

1890.  8-.     2i)t.      XV. 771). p.     9N|.l.  an.lmaiis;   viii.l23pp. 

XI.  Eleventh  Annual  Rejioit  ot  the  rnited  States  Geological  Survey,  1889-'90,  by  ,1.  AV.  Powell. 

1891.  8>^.     2  jit.     \v.7.^7pii.     (iii  111.  and  maps;  ix.  ;i."il  pj).     30  pi.  and  ma])S. 

XII.  Twelfth  Annn.il   Kcpm  i  ,,l'  the  Inite.!  States  ( k'ohi.uieal  Survey.  1S90-'91,  by  J.  W.  Powell. 

Xlil.  Tliii(e,utli  Annual  l;.|Mut  ojth.''  United  Stati's  (ieoloVieal  Survey,  1891-'92,  by  J.  W. 
Powell.  1893.  s.  :;  pt.  vii,  21(1  pp.  2  maps;  x,  372  pp.  105  pi.  and  maps;  xi,  486  pp.  77 "pi- and 
maps. 

XIV.  Fourteenth  Annual  Report  of  the  United  States  Geological  Survey,  1892-'93,  by  J.  W. 
Powell.    1893.     8^3.     2  pt.     vi,  321  pp.     1  pi. ;  xx,  597  pp.     74  pi.  and  maps. 

XV.  Fifteenth  Annual  Report  of  the  United  States  Geological  Survey,  1893-'94,  bv  J.  W.  Powell. 
1895.     8°.     xiv,  755  pp.     48  pi.  and  maps. 

XVI.  Sixteenth  Annual  Report  of  the  United  States  Geological  Survey,  1894-'95,  by  Charles  I). 
Walcott.  1895.  (Part  I,  1886.)  8°.  4  pt.  xxii,  910  pp.  117  pi.  and  maps ;  xi.x,  598  pp.  15  pi.  and 
maps ;  xv,  646  pp.     23  pi. ;  xix,  735  pp.     6  pi. 

XVII.  Seventeenth  Annual  Report  of  the  United  States  Geological  Survey,  1895-'96,  bv  Charles 
D.  Walcott.  1896.  8°.  3  pts.  in  4  vols,  xxii,  1076  pp.  67  pi.  and  maps;  x.xv,  864  pp.  li:!]'!  and 
maps;  xxiii,  542  pp.    8  pi.  and  maps;  iii,  543-1058  pp.     9-13  pis. 

MON  XXVI 1 1 .'>9  1 


ADVEKTISEMENT. 


MONOGKAPHS. 


I.  Lake  Boiiiievillc,  by  Grove  Karl  Gilbert.     1890.     4'\     xx,  138  pp.     51  pi.     1  map.     Price  .$1.50. 

II.  Tertiary  History  of  theGraml  ( ':irinn  Distrirt,  -n-itli  Atliis,  bv  Clarence  E.  Duttou,  Capt.,  U.  S.  A. 
1882.     4°.     xiv,2lUpp.     42pl.  and^ilhis  ofL'l  slirrls  fnli,,.      I'rir,.  $10.00. 

III.  Geolofjv  of  the  Comstoik  L.mI,.  nn.l  tlir  W  :i>li.ir  Hisiiirt.  with  Atlas,  by  George  F.  Becker. 
1882.     4^^.     XV,  422  pp.     7  pi.  ;in(l  .-itlas  (ilL'l  .slic.-ls  luli,,.      I'l  ice  .+  11.00. 

IV.  Comstock  Miiiiii!;;ni.l  Miners,  by  Eliot  Eord.     11SS3.     4".     xiv,  451  pp.     3  pi.     Price  $1.50. 

V.  The  Copi-M  i;r;inim  I ;.icks  of  Lake  Superior,  by  Eolaud  Duer  Irving.  1883.  4'^.  xvi,  4H4 
pp.     151.     2S)pl.  iiinl  iiiai.s.      1 'li.i^  $1.85. 

VI.  Ooutriliiitiiiiis  to  tlic  Knowledge  of  the  Older  Mesozoic  Flora  of  Virginia,  by  William  Morris 
Fontaine.     1883.     4'.     xi,  144  pp.     54  1.     54  pi.     Price  $1.05. 

VII.  Silver-Lead  Ueijosits  of  Eureka,  Nevada,  by  Joseph  Story  Curtis.  1884.  4'.  xiii.  200  pp. 
16  pi.     Price  $1.20. 

VIII.  Paleontology  of  the  Eureka  District,  by  Charles  Doolittlc  Walcott.  1884.  4^.  xiii,  2;)S 
pp.     241.     24  pi.     Price  $1.10. 

IX.  Brachiojioda  ami  l.niiirllil.i  :ni.  liiata  of  the  Karitan  Clays  and  Grc.cnsand  Marls  of  New- 
Jersey,  by  Robert  P.  Whitlii  1.1.      I>s:..      I.     xx.:;38pp.     35  pi.     l  nuip.     Price  $1.15. 

X.  Dinoeerata.  A  Jloiio^iiipli  <il'  ;i'i  Extinct  Order  of  Gigantic  Mammals,  by  Othniel  Clnirh^s 
Marsh.     1886.     4°.     xviii,  243  pp.     56  1.     .Mi  pi.     I'l  ic.' $2.70. 

XI.  Geological  History  of  Lake  L:ilninl:in,  a  <iiu;itcruary  Lake  of  Northwestern  Nevaila,  by 
Israel  Cook  Russell.     1885.     4=.    xiv,  288  p|i.     I(i  pi.  ;iii.l  luai^s.     Price  $1.75. 

XII.  Geology  and  Mining  Industry  of  l.r.nh  ill.  .  i '..I.Tado,  with  Atlas,  by  Samuel  Franklin  Em- 
mons.    1886.     4°.     xxix,77(M'l'.     ir.  i.l.  im.l  :ith.s  ...    ;  .  -  v  .  i-  hH.,.     Price  $8.40. 

Xin.  Geology  of  the  (,iiii<  ksilv.  i  I  i.'posits  .,i  .:  -  ■  .  .n-  .pc,  with  Atlas,  by  George  F.  Hc.ckcr. 
1888.     i'^.    xix,486pp.     7  pi.  :.im1  ;,tl.is  ol   1 1  sli.  .  i  ^  .    ' ,         i    .fi'.OO. 

XIV.  Fossil  Fi.shcs  :rn.l  F.i^mI  I'limts  ..f  ih.  '  .  i--;  1."  ;,^  of  New  Jersey  and  the  Counecticnt 
Valley,  by  Johns.  Kcw  birrs.     I.xss.     I.     xi\ ,  151' I'l'-     -'' p'-     rvice$1.00. 

XV.  ThePotoiii.i.'  .ir  \..inc..r  M.s../,.i.-  Fl..!:i,  bv  Wijli.im  Morris  Fontaine.  1889.  4^'.  xiv, 
377pp.     180pL     'I'cxt  ;,n.li.h.|.-s  l...i.nds..par;it..lv.      I'ri.-c +L'..-.il. 

XVI.  The  Palcjzoic  Fishes  ,.f  Xi.rth  Amcri.a,  by  .lolui  Strong  Newberry.  1889.  4^.  340  pp. 
53  pi.     Price  $1.00. 

XVII.  The  Flora  of  the  Dakota  Group,  a  Posthumous  Work,  by  Leo  Lesqucreux.  Edited  by  !•'. 
H.  Knowlton.     189L     4".     400  pp.     66  pi.     Price  $1.10. 

XVIII.  Gasteropoda  and  Cephalopoda  of  the  Raritan  Clays  and  Greeusand  Marls  of  New  .Jersey, 
by  Robert  P.  Whitfield.     1891.     4 -\     402  pp.     50  pi.     Price  $1.00. 

XIX.  The  Penokee  Iron-Bearing  Series  of  Northeru  Wisconsin  and  Michi,uau,  by  Roland  ]). 
Irving  and  C.  R.  Van  Hise.     1892.     4^.    xix,  534  pp.     Price  $1.70. 

XX.  Geology  of  the  Eureka  District,  Nevada,  with  an  Atlas,  by  Arnold  Hague.  1892.  4".  xvii, 
419  pp.     8  pi.     Price  $5.25. 

XXI.  The  Tertiary  Ehynchophorous  Coleoptera  of  the  United  States,  by  Samuel  Hubbard  Scud- 
der.     1893.     4".     xi,  206  pp. '  12  pi.     Price  90  cents. 

XXII.  A  Manual  of  Topographic  Methods,  by  Henry  Gannett,  Chief  Topographer.  1893.  4^. 
xiv.  300  pp.     18  pi.    Price  $1.00. 

XXIII.  Geology  of  the  Green  Mountains  in  Massachusetts,  by  Raphael  Pumpelly,  T.Nelson  Dale, 
and  J.  E.  Wolff.     1894.    4'=.    xiv,  206  pp.     23  pi.     Price  $1.30. 

XXIV.  Mollnsca  and  Crustacea  of  the  Miocene  Formations  of  New  Jersey,  by  Robert  Parr  Whit- 
field.    1894.    4°.     193  lip.    24  pL     Price  90  cents. 

XXV.  The  Glacial  Lake  Agassiz,  by  Warren  Upham.   1895.   4=.  xxiv,658pp.   38  pi.    Price  $1.70. 

XXVI.  Flora  of  the  Amboy  Clays,  by  John  Strong  Newberry;  a  P  sthumous  Work,  edited  by 
Arthur  Hollick.     1895.     4°.    260  pp.     i58  pi.     Price  $1.00. 

XXVII.  Geology  of  the  Deu\er  Basin  in  < '..Idi-id..,  by  .Samuel  Franklin  Emmons,  Whitman  Cross, 
and  George  Romans  Eldridge.     1896.    4^.     5.'.ii  |ip.     ::i  pi.     Price  $1.. 50. 

XXVIII.  The  Marquette  Iron-Bearing  1  listrict  ot  Michigan,  with  Atlas,  by  C.  R.  Van  Hise  and 
W.  S.  Bayley,  including  a  Chapter  on  the  Republic  Trough,  by  C.  L.  Smyth.  1895.  4'^'.  608  pp.  35 
pi.     Price  $5.75. 

In  pre2)araUon: 

XXIX.  The  Geology  of  (lid  llamp.shire  County,  Massachusetts,  comprising  Franklin,  Hampshire, 
and  Hampden  Counties,  b\'  l;i'nj.nuin  Kendall  I'.nicrson. 

XXX.  Fossil  Mcdns;,.,  l.yCliiiiles  1  >„..  I  it  tl..  Walcott. 

XXXI.  Geologv  id  tli.   As].,  n  Mining  iM^tri.t,  Colorado,  with  Atlas,  by  Josiah  Edward  Spurr. 

XXXII.  Ge(d^g\  of  tlie  Y.ll..«  stone  Xiiti.mal  Park,  Part  II,  Descriptive  Geology,  Petrography, 
and  Paleontology,  by  Arnold  Hagne,  J.  P.  Iddings,  W.  Harvey  Weed,  Charles  D.  Walcott,  G.  H.  Girty, 
T.  W.  Stanton,  and  F.  H.  Knowlton. 

XXXIII.  Geology  of  the  Narragausett  Basin,  by  N.  S.  Shaler,  .1.  B.  Woodworth,  and  August  F. 
Foerste. 

XXXIV.  The  Glacial  Gravels  of  Maine  and  their  Associated  Deposits,  by  George  H.  Stone. 
— Sauropoda,  by  0.  C.  Marsh. 

— Stegosauria,  by  O.  C.  Marsh. 

— Broutotheridaj,  by  O.  C.  Marsh. 

— Report  on  Silver  Clitf  and  Ten-Mile  Mining  Districts,  Colorado,  by  S.  F.  Emmons. 

— Flora  of  the  Laramie  and  Allied  Formations,  by  Frank  Hall  Knowlton. 


ADVEKTISEWENT. 


nuLLpyriNS. 


1.  Ou  Hypersthene-Audesite  and  on  Tricliuic  Pyroseuo  iu  Augitio  Rocks,  by  Whitui.in  Cidss, 
with  a,  Geological  Sketch  of  Buffalo  Peaks,  Colorado,  by  S.  V.  Emmons.  1883.  Vi  .  42  pp.  2  pi. 
Price  10  cents. 

2.  Gold  and  Silver  Conversion  Tables,  giving  the  Coining  Values  of  Troy  Ounces  of  Fine  Metal, 
etc.,  computed  by  Albert  Williams,  jr.     1883.    8'^.     8  pp.     Price  5  cents. 

3.  On  the  Fossil  Faunas  of  the  Upper  Devonian,  along  the  Meridian  of  76°  30',  from  Tompkins 
County,  N.  Y.,  to  Bradford  County,  Pa.,  by  Henry  S.  Williams.     1884.     8'^.     36  pp.     Price  5  cents. 

4.  On  Mesozoic  Fossils,  by  Charles  A.  White.     1884.     8^.     36  pp.     9  pi.     Price  5  cents. 

5.  A  Dictionary  of  Altitudes  iu  the  United  States,  compiled  by  Henry  Gannett.  1884.  8°.  325 
pp.     Price  20  cents. 

6.  Elevations  in  the  Dominion  of  Canada,  by  J.  W.  Spencer.     1884.     8°.     43  pp.     Price  5  cents. 

7.  Mapoteca  Geologica  Americana.  A  Catalogue  of  Geological  Maps  of  America  (North  and 
South),  1752-1881,  in  Geographic  and  Chronologic  Order,  by  Jules  Marcou  an<l  John  Belknap  Marcou. 

1884.  8°.     184  lip      Price  10  cents. 

8.  On  Seconilaiy  F.nLa  l;(  iiiiiifs  nt'  Mineral  Fragments  in  Certain  Rocks,  by  R.  D.  Irving  and  C. 
R.  Van  Hise.     issl.     s  .     r.d  pji.      iljil.      I'riee  10  cents. 

9.  A  Report  iirWCik, lone  in  tlie  W.-isliington  Laboratory  during  the  Fiscal  Year  1883-'84.  F.W. 
Clarke,  Chief  Chemist;  T.JI.Cli.itaid,  ;isM,-i:nit  chemist.      1884.      8°.      40  pp.      Price  5  cents. 

10.  On  the  Cambrian  Faunae  nl  Niai  h  America.  Preliminary  Studies,  bv  Charles  Doolittle  Wal- 
cott.     1884.     8°.     74  pp.     10}il.      i'l  i. ,  :,  , mts. 

11.  On  the  Quaternary  ami  l.'er.m  Mnlliisca  of  the  Great  Basin;  with  Description  of  New 
Forms,  by  R.  Ellsworth  Call.  Introdiu  ed  l.y  a  Sketch  of  the  Quaternary  Lakes  of  the  Great  Basin, 
by  G.  K.  Gilbert.     1884.     8°.     66  pp.     6  pi.  '  Price  5  cents. 

12.  ACrystallographicStudy  of  the  Thinolite  of  Lake  Lahontan,  by  Edwards.  Dana.  1884.  8'-\ 
34  pp.     3  pi.     Price  5  cents. 

13.  Boundaries  of  the  United  States  and  of  the  Several  States  and  Territories,  with  a  Historical 
Sketch  of  the  Territorial  Changes,  by  Henry  Gannett.     1885.     8°.     135  pp.     Price  10  cents. 

14.  The  Electrical  and  Magnetic  Properties  of  the  Iron-Carburets,  by  Carl  Barus  and  Vincent 
Strouhal.     1885.     8°.    238  pp.     Price  15  cents. 

15.  On  the  Mesozoic  and  Cenozoic  Paleontology  of  California,  by  Charles  A.  White.  1885.  8^1. 
33  pp.     Price  5  cents. 

16.  Ou  the  Higher  Devonian  Faunas  of  Ontario  County,  New  York,  by  John  M.Clarke.  1885.  8'^. 
86  pp.     3  pi.     Price  5  cents. 

17.  On  the  Development  of  Crystallization  in  the  Igneous  Rocks  of  Washoe,  Nevada,  with  Notes 
on  the  Geology  of  the  District,  by  Arnold  Hague  and  Joseph  P.  Iddings.  1885.  8°.  44  pp.  Price  5 
cents. 

18.  On  Marine  Eocene,  Fresh-water  Miocene,  and  other  Fossil  Mollusca  of  Western  North  America, 
by  Charles  A.  White.     1885.    8"^.     26  pp.     3  pi.     Price  5  cents. 

19.  Notes  on  the  Stratigraphy  of  California,  by  George  F.Becker.   1885.   8°.   28  pp.   Price  5  cents. 

20.  Contributions  to  the  Mineralogy  of  the  Rocky  Mountains,  by  Whitman  Cross  and  W.  F.  Hille- 
hrand.     1885.     8'^.     114  pp.     1  pi.     Price"  10  cents. 

21.  The  Lignites  of  the  Great  Sioux  Reservation;  a  Report  on  the  Region  between  tlie  Grand 
and  Moreau  Rivers,  Dakota,  by  Bailey  Willis.     1885.     8"^.     16  pp.     5  pi.     Price  5  cents. 

22.  On  New  Cretaceous  Fossils  from  California,  by  Charles  A.  White.  1885.  8°.  25  pp.  5  pi. 
Price  5  cents. 

23.  Observations  on  the  Junction  between  the  Eastern  Sandstone  and  the  Keweenaw  Series  on 
Keweenaw  Point,  Lake  Superior,  by  R.  D.  Irving  and  T.  C.  Chamberliu.  1885.  S'-'.  124  pp.  17  pi. 
Price  15  cents. 

24.  List  of  Marine  Mollusca,  comprising  the  Quaternary  fossils  and  Recent  Forms  from  American 
Localities  between  Cape  Hatteras  and  Cape  Roque,  including  the  Bermudas,  by  William  Healey  Dall. 

1885.  8°.     836  pp.     Price  25  cents. 

25.  The  Present  Technical  Condition  of  the  Steel  Industry  of  the  United  States,  by  Phineas 
3S.     1885.     8°.     85  pp.     Price  10  cents. 

26.  Copper  Smelting,  by  Henry  M.  Howe.     1885.     8°.     107  pp.     Price  10  cents. 

27.  Report  of  Work  done  in  the  Division  of  Chemistry  andPhysics,mainlv  during  the  Fiscal  Year 
1884-'85.     1886.     8°.     80  pp.     Price  10  cents.  -  ^  .  . 

28.  The  Gabbros  and  Associated  Hornblende  Rocks  occurring  in  the  Neighborhood  of  Baltimore, 
Md.,  by  George  Huutingtrm  Williams.     1886.     S<^.     78  pp.     4  pi.    Price  10  cents. 

29.  On  the  Fresli-water  Invertebrates  of  the  North  American  Jurassic,  by  Charles  A.  White.  1886. 
8°.     41  pp.      4  pi.     Price  r,  cents. 

30.  Second  Coiitriliution  to  the  Studies  ou  the  Cambrian  Faunas  of  North  America,  by  Charles 
Doolittle  Walcott.     1886.     »  .     369  pp.     33  pi.     Price  25  cents. 

31.  Systematic  Review  of  our  Present  Knowledge  of  Fossil  Insects,  including  Myriapods  and 
Arachnids,  by  Samuel  Hubbard  Scudder.     1886.     8°.     128  ])p.     Price  15  cents. 

32.  Lists  and  Analyses  of  the  Mineral  Springs  of  the  United  States;  a  Preliminary  Study,  by 
Albert  C.  Peale.     1886.     8°.     235  pp.     Price  20  cents. 

33.  Notes  on  the  Geology  of  Northern  California,  by  J.  S.Diller.     1886.    8^.    23  pp.    Price  5  cents. 

34.  On  the  Relation  of  the  Laramie  MoUuscan  Fauna  to  that  of  the  Succeeding  Fresh-water  Eocene 
and  Other  Groups,  by  Charles  A.  White.     1886.     8°.     54  pp.     5  pi.     Price  10  cents, 

HON   XXVIII 40 


IV  ADVERTISEMENT. 

35.  Physical  Properties  of  the  Irou-Carbiirets,  by  Carl  Barus  and  Vincent  Strouhal.  1886.  8=^. 
62  pp.     Price  10  cents. 

36.  SubsitlenceofFineSolidParticlesiuLitiuidSjbyCarlBarus.    1886.    8°.    58pp.    PricelOcents. 

37.  Types  of  the  Laramie  Flora,  by  Lester  F.Ward.     1887.     8°.     3.54  pp.     57  pi.     Price  25  cents. 

38.  PeridotiteofElliottCounty,  Kentucky,  by  J.  S.Diller.     1887.     8-^.    31pp.    Ipl.    Price5eents. 

39.  The  Upper  Beaches  and  Deltas  of  the  Glacial  Lake  Agassiz,  by  Warren  Upham.  1887.  8°. 
84  pp.     1  pi.     Price  10  cents. 

40.  Changes  in  River  Courses  in  Washington  Territory  due  to  Glaciation,  by  Bailey  Willis.  1887. 
8°.     10  pp.     4  pi.     Price  5  cents. 

41.  On  the  Fossil  Faunas  of  the  Upper  Devonian — the  Genesee  Section,  New  York,  by  Henry  S. 
Williams.     1887.     8°.     121pp.     4  pi.     Price  15  cents. 

42.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal  Year 
1885-'86.     F.  W.  C'l:iil.c-,  (  hiiiCliemist.     1887.     8^^.     152  pp.     Ipl.     Price  15  cents. 

43.  Tertiary  :ni(l  (  ict.n iis. Strata  of  the  Tuscaloosa,  Tombigbee,  and  Alabama  Rivers,  by  Eugene 

A.  SmithandLin'vivn.,- ('..lohiison.     1887.     8°.     189  pp.     21  pi.     Price  15  cents. 

44.  Bibliogrupliy  cf  North  American  Geology  for  1886,  by  Nelson  H.  Darton.  1887.  S-^.  35  pp. 
Price  5  cents. 

45.  The  Present  Condition  of  Knowledge  of  the  Geology  of  Texas,  by  Robert  T.  Hill.  1887.  8°. 
94  pp.     Price  10  cents. 

46.  Nature  and  Origin  of  Deposits  of  Phosphate  of  Lime,  by  R.  A.  F.  Penrose,  jr.,  with  an  Intro- 
duction by  N.  S.  Shaler.     1888.     8°.     143  pp.     Price  15  cents. 

47.  Analyses  of  AVaters  of  the  Yellowstone  National  Park,  with  an  Account  of  the  Methods  of 
Analysis  employed,  by  Frank  Austin  Gooch  and  James  Edward  Whitfield.  1888.  8^.  84  pp.  Price 
10  cents. 

48.  On  the  Form  .and  Position  of  the  Sea  Level,  by  Robert  Simpson  Woodward.  1888.  8°.  88 
pp.     Price  10  cents. 

49.  Latitudes  and  Longitudes  of  Certain  Points  in  Missouri,  Kansas,  and  New  Mexico,  by  Robert 
Simpson  Woodward.     1889.     8*^.     133  pp.     Price  15  cents. 

50.  Formulas  and  Tables  to  Facilitate  the  Construction  and  Use  of  Maps,  by  Robert  Simpson 
Woodward.     1889.     8".     124  pp.     Price  15  cents. 

51.  On  Invertebrate  Fossils  from  the  Pacific  Coast,  by  Charles  Abiathar  White.  1889.  8°.  102 
pp.     14  pi.     Price  15  cents. 

52.  Subaerial  Decay  of  Rocks  and  Origin  of  the  Red  Color  of  Certain  Formations,  by  Israel 
Cook  Russell.     1889.     8°.     65  pp.     5  pi.     PricelOcents. 

53.  The  Geology  of  Nantucket,  by  Nathaniel  Southgate  Shaler.  1889.  8°.  55  pp.  10  pi.  Price 
10  cents. 

54.  On  the  Thermo-Electric  Measurement  of  High  Temperatures,  by  Carl  Barus.  1889.  8°. 
313  pp.,  incl.  1  pi.     11  pi.     Price  25  cents. 

55.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1886-'87.     Frank  Wigglesworth  Clarke,  Chief  Chemist.     1889.     8°.     96  pp.     PricelOcents. 

56.  Fossil  Wood  and  Lignite  of  the  Potomac  Formation,  by  Frank  Hall  Knowlton.  1889.  8^^. 
72  pp.     7  pi.     Price  10  cents. 

57.  A  Geological  Reconnoissance  in  Southwestern  Kansas,  by  Robert  Hay.  1890.  8°.  49  pp. 
2  pi.     Price  5  cents. 

58.  The  (;i;iri:il  I'MHinilai  y  in  Western  Pennsylvania,  Ohio,  Kentucky,  Indiana,  and  Illinois,  by 
George  Frederick  Wiiulit,  wiili  an  Introduction  by  Thomas  Chrowder  Chamberlin.  1890.  8'^.  112 
pp.,  incl.  1  pi.     8pl.     I'liri'  i:.  cents. 

59.  The  Galihros  and  Associated  Rocks  in  Delaware,  by  Frederick  D.  Chester.  1890.  8°.  45 
pp.     1  pi.     Price  10  cents. 

60.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1887-'88.     F.  W.  Clarke,  Chief  Chemist.     1890.     8-^.     174  pp.     Price  15  cents. 

61.  Contributions  to  the  Mineralogy  of  the  Pacific  Coast,  by  William  Harlow  Melville  and  Wal- 
demar  Lindgren.     1890.    8'^.     40  pp.     3  pi.     Price  5  cents. 

62.  The  Greenstone  Schist  Areas  uf  the  Menominee  and  Marquette  Regions  of  Michigan,  a  Con- 
tribution to  the  Subject  of  Dynamic  Metamorphism  in  Eruptive  Rocks,  by  George  Huntington  Williams, 
with  an  Introduction  by  Roland  Duer  Irving.     1890.     8^.     241pp.     16  pi.     Price  30  cents. 

63.  A  Bibliography  of  Paleozoic  Crustacea  from  1698  to  1889,  including  a  List  of  North  Amer- 
ican Species  and  a  Systematic  Arrangement  of  Genera,  by  Anthony  W.  Vogdes.  1890.  8^.  177  pp. 
Price  15  cents. 

64.  A  Rfiiiirt  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  18x.s-s!i.     I',  \V.  I'larke,  Chief Cbemist.     1890.     8-.     60pp.     PricelOcents. 

65.  SI  r,il  inr.ijiliy  of  the  Bituminous  Coal  Field  of  Pennsylvania,  Ohio,  and  West  Virginia,  by 
Israel  c' Wliit.r    isiii  _  "      ' 

66.  On  a  (iroup  of  Volcanic 
rence  of  Primary  Quartz  in  Certain  Basalts,  by  Joseph  Paxson  Iddings.     1890.     8°.     34  pp.     Price  5 
cents. 

67.  The  Relations  of  the  Traps  of  the  Newark  System  in  the  New  Jersey  Region,  by  Nelson 
Horatio  Darton.     1890.     S'^.    82  pp.     Price  10  cents. 

68.  Earthquakes  in  California  in  1889,  by  James  Edward  Keeler.  1890.  8°.  25  pp.  Price  5 
cents. 

69.  A  Classed  and  Annotated  Biography  of  Fossil  Insects,  by  Samuel  Howard  Scudder.  1890. 
8°.     101pp.     Price  15  cents. 


ADVERTISEMENT.  V 

70.  A  Report  on  Astronomical  Work  of  1889  and  1890,  by  Robert  Simpson  Woodwiira.    1890.    8°. 
79  pp.     Price  10  cents. 

71.  Index  to  the  Known  Fossil  Insects  of  the  World,  including  Myriiqicids  and  Aracliiiids,  by 
Samuel  Hubbard  Scudder.     1891.     8°.     744  pp.     Price  50  cents. 

72.  Altitudes  between  Lake  Superior  and  the  Rocky  Mountains,  by  Warren  Uphani.     1891.    8^. 
229  pp.     Price  20  cents. 

73.  The  Viscosity  of  Solids,  by  Carl  Barus.     1891.     8'^.     xii,  139  pp.     Gpl.     Price  1.5  cents. 

74.  The  Minerals  of  North  Carolina,  by  Frederick  Augustus  Genth.     1891.     8°.     119  pp.     Price 
15  cents. 

75.  Record  of  North  American  Geology  for  1887  to  1889,  inclusive,  by  Nelson  Horatio  Darton. 
1891.     8".     173  pp.     Price  15  cents. 

76.  A  Dictionary  of  Altitudes  in  the  United  States  (Second  Edition),  compiled  by  Henry  Gannett, 
Chief  Topographer.     1891.     8".     393  pp.     Price  25  cents. 

77.  The  Tex.an  Permian  and  its  Mesozoic  Types  of  Fossils,  by  Charles  A.  White.     1891.     8°.     51 
pp.     4  pi.     Price  10  cents. 

78.  A  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1889-'90.     F.  W.  Clarke,  Chief  Chemist.     1891.     8".     131  pp.     Price  15  cents.' 

79.  A  Late  Volcanic  Eruption  in  Northern  California  and  its  Peculiar  Lava,  by  J.  S.  Diller. 

80.  Correlation  Papers — Devonian  and  Carboniferous,  by  Henry  Shaler  Williams.     1891.     8°. 
279  pp.     Price  20  cents. 

81.  Correlation  Papers— Cambrian,  by  Charles  Doolittle  Walcott.      1891.     8°.     .547  pp.     3  pi. 
Price  25  cents. 

82.  Correlation  Papers— Cretaceous,  by  Charles  A.  White.     1891.     8-.     273  pp.     3  pi.     Price  20 
cents. 

83.  Correlation  Papers— Eocene,  by  William  Bullock  Clark.     1891.     8-.     173  pp.     2  ]il.     Price 
15  cents. 

84.  Correlation  Papers— Neocene,  by  W.  H.  Dall  and  G.  I).  Harris.     1892.     8>^.     349  jip.     3  pi. 
Price  25  cents. 

85.  Correlation  Papers— The  Newark  System,  by  Israel  Cook  Russell.    1892.     8°.     344  pp.    13  pi. 
Price  25  cents. 

86.  Correlation  Papers— Archean  and  Algonkian,  by  C.  R.  Van  Hise.     1892.     8^.     549  pp.     12  pi. 
Price  25  cents. 

90.  A  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1890-91.     F.  W.  Clarke,  Chief  Chemist.     1892.     8°.    77  jip.     Price  10  cents. 

91.  Record  of  North  American  Geology  for  1890,  by  Nelson  Horatio  Darton.     1891.     S'^.    88  pp. 
Price  10  cents. 

92.  The  Coiiiiiifssiliility  of  Liquids,  by  Carl  Barns.     1802.     S\     fir,  pp.     29  pi.     Price  10  cents. 

93.  Some  Inserts  of  S),e.i:i]  Interest  trnm  l1nriss:iHt,('i.I..i,Ml..,  .•nMl()lIirrP..intsintlieTertiaries 
of  Colorado  and  I'tah,  1)V  S.-ii I  Ilulilianl  Sindd.  r.     isiii'.     s   .     ;;:,  pp.     :',  pi.     Triic- r.  cents. 

94.  The  Mechanism  of  Solid  \iM'ositv,  iiy  Carl  Barns,      ISHL'.     .x.     Ills  i,|,.     Priee  15  cents. 

95.  Earthquakes  in  California  iu  189U  and  1891,  by  Edward  Singleton  llolden.     1892.     8'-'.     31  pp. 
Price  5  cents. 

96.  The  VolumeThermodynamicsofLiquids,  by  Carl  Barus.     1892.     8'^.     100pp.     Price  10  cents. 

97.  The  Mesozoic  Echiuodermata  of  the  United  States,  by  W.B.Clark.    1893.    8".    207  pp.    50pl. 
Price  20  cents. 

98.  Flora  of  the  Outlying  Carboniferous  Basins  of  Southwestern  Missouri,  by  David  White. 
1893.     8^.     139  pp.     5  pi.     Price  15  cents. 

99.  Record  of  North  American  Geology  for  1891,  by  Nelson  Horatio  Darton.     1892.     8\     73  pp. 
Price  10  cents. 

100.  Bibliography  and  Index  of  the  Publications  of  the  U.  S.  Geological  Survey,  1879-1892,  by 
Philip  Creveling  Warmau.     1893.     8°.     495  pp.     Price  25  cents. 

101.  Insect  Fauna  of  the  Rhode  Island  Coal  Field,  by  Samuel  Hubbard  Scudder.     1893.     8°. 
27  pp.     2  pi.     Price  5  cents. 

102.  A  Catalogue  and  Bibliography  of  North  American  Mesozoic  Invertebrata,  by  Cornelius 
Breckinridge  Boyle.     1892.     8°.    315  pp.     Price  25  cents. 

103.  High  Temperature  Work  in  Igneous  Fusion  and  Ebullition,  chiefly  in  Relation  to  Pressure, 
by  Carl  Barus.     1893.     8°.    57  pp.     9  pi.     Price  10  cents. 

104.  Glaciation  of  the  Yellowstone  Valley  north  of  the  Park,  by  Walter  Harvey  Weed.    1893.    8°. 
41  pp.     4  pi.     Price  5  cents. 


105.  The  Laramie  and  the  Overlying  Liviugstone  Formation  in  Montana,  by  Walter  Harvey 
,  with  Report  on  Flora,  by  Frank  Hall  Knowlton.     1893.    8°    68  pp.    6  pi.     Price  10  cents. 

106.  The  Colorado  Formation  and  its  Invertebrate  Fauna,  by  T.  W.  Stanton.     1893.     8°.    288 


pp.     45  pi.     Price  20  cents. 

107.  The  Trap  Dikes  of  Lake  Champlain  Valley  and  the  Eastern  Adirondacks,  by  James  Furman 
Kemp. 

108.  A  Geological  Reconnoissance  in  Central  Washington,  by  Israel  Cook  Russell.     1893.     8^^. 
108  pp.     12  pi.     Price  15  cents. 

109.  The  Erujitivo  and  Sedimentary  Rocks  on  Pigeon  Point,  Minnesota,  and  their  Contact  Phe- 
nomena, by  William  Shirley  Bayley.     1893.     8^^.     121  pp.     16  pi.     Price  15  cents. 

110.  The  Paleozoic  Section  iu  the  Vicinity  of  Three  Forks,  Montana,  by  Albert  Charles  Peale. 
1893.     8^'.    56  pp.     6  pi.     Price  10  cents. 


VI  ADVERTISEMENT. 

111.  Geologv  of  the  Big  Stono  Gap  Coal  Fields  of  Virgiuia  and  Kentucky,  by  Marius  R.  Camp- 
bell.    1893.     8-.   "106  pp.     6  pi.     Price  15  cents. 

112.  Earthquakes  in  California  in  1892,  by  Charles  D.  Perrine.    1893.    8°.    57  pp.    Price  10  cents. 

113.  A  Report  of  Work  done  in  the  Division  of  Chemistry  during  the  Fiscal  Years  1891-'92  and 
1892-'93.     F.  W.  Clarke,  Chief  Chemist.     1893.     8°.     115  pp.     Price  15  cents. 

114.  Earthquakes  in  California  in  1893,  by  Charles  D.  Perrine.    1894.    8°.    23  pp.    Price  5  cents. 

115.  A  Geographic  Dictionary  of  Rhode  Island,  by  Henry  Gannett.     1894.     8°.     31  pp.     Price 
5  cents. 

116.  A  Geographic  Dictionary  of  Massachusetts,  by  Henry  Gannett.     1894.     8°.     126  pp.     Price 
15  cents. 

117.  A  Geographic  Dictionary  of  Connecticut,  by  Henry  Gannett.     1894.     8°.     67  pp.     Price  10 
cents. 

118.  A  Geographic  Dictionary  of  New  Jersey,  by  Henry  Gannett.     1894.     8°.     131  pp.     Price  15 
cents. 

119.  A  Geological  Reconnoissance  in  Northwest  Wyoming,  by  George  Romans  Eldridge.     1894. 
8°.     72  pp.     Price  10  cents. 

120.  The  Devonian  System  of  Eastern  Pennyslvania  and  New  York,  by  Charles  S.  Prosser.     1894. 
8*^.    81  pp.    2  pi.     Price  10  cents. 

121.  A  Bibliography  of  North  American  Paleontology,  by  Charles  Rolliu  Keyes.     1894.    S°.     251 
pp.     Price  20  cents. 

122.  Results  of  Primary  Triangulation,  by  Henry  Gannett.     1894.     8^.     412  pp.     17  pi.     Price 
25  cents. 

123.  A  Dictionary  of  Geographic  Positions,  by  Henry  Gannett.     1895.     8°.     183  pp.     1  pi.    Price 
15  cents. 

124.  Revision  of  North  American  Fossil  Cockroaches,  by  Samuel  Hubbard  Scudder.     1895.     8°. 
176  pp.     12  pi.     Price  15  cents. 

125.  The  Constitution  of  the   Silicates,  by  Frank  Wigglesworth  Clarke.     1895.     8°.     109   pp. 
Price- 15  cents. 

126.  A  Mineralogical  Lexicon,  of  Franklin,  Hampshire,  and  Hampden  counties,  Massachusetts, 
by  Benjamin  Kendall  Emerson.     1895.     8^.     180  pp.     1  pi.     Price  15  cents. 

127.  Catalogue  and  ludex  of  Contributions  to  North  American  Geology,  1732-1891,  by  Nelson 
Horatio  Dartou.     1896.     8°.     1045  pp.     Price  60  cents. 

128.  The  Bear  River  Formation  and  its  Characteristic  Fauna,  by  Charles  A.  White.     1895.     8°. 
108  pp.     11  pi.     Price  15  cents. 

129.  Earthquakes  in  California  in  1894,  by  Charles  D.  Perrine.    1895.     8*^.     25  pp.     Price  5  cents. 

130.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  1892  and  1893,  by  Fred  Houghton  Weeks.     1896.     8^.     210  pp.     Price  20  cents. ' 

131.  Report  of  Progress  of  the  Division  of  Hydrography  for  the  Calendar  Years  1893  and  1894, 
by  Frederick  Hayues  Newell,  Topographer  in  Charge.     1895.     8^.     126  pp.     Price  15  cents. 

132.  The  Disseminated  Lead  Ores  of  Southeastern  Missouri,  by  Arthur  Winslow.     1896.     8°. 
31  pp.     Price  5  cents. 

133.  Contributions  to  the  Cretaceous  Paleontology  of  the  Pacific  Coast:    The  Fauna  of  the 
Knoxville  Beds,  by  T.  W.  Stanton.     1895.    8°.    132  pp.     20  pi.     Price  15  cents. 

134.  The  Cumbrian  Rocks  of  Pennsylvania,  by  Charles  Doolittle  Walcott.     1896.     8°.     43  pp. 
15  pi.     Price  5  cents. 

135.  Bibliography  and  Index  of  North  American  Geologv,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1894,  by  F.  B.  Weeks.     1896.     8^.     141  pp.     Price  15  cents. 

136.  Volcanic  Rocks  of  South  Mountain,  Pennsylvania,  by  Florence  Bascom.    1896.    8-.    124  pp. 
28  pi.     Price  15  cents. 

137.  The  Geologv  of  the  Fort  Riley  Military  Reservation  and  Vicinity,  Kansas,  by  Robert  Hay. 
1896.     8°.    35  pp.     8  pi.     Price  5  cents. 

138.  Artesian-well  Prospects  in  the  Atlantic  Coastal  Plain  Region,  by  N.  H.  Darton.     1896.     8^. 
228  pp.     19  pi.     Price  20  ceuts. 

139.  Geology  of  the  Castle  Mountain  Mining  District,  Montana,  by  W.  H.  Weed  and  L.  V.  Pirs- 
Bon.     1896.     8°.     164  pp.     17  pi.     Price  15  cents. 

140.  Report  of  Progress  of  the  Division  of  Hydrography  for  the  Calendar  Year  1895,  by  Frederick 
Haynes  Newell,  Hydrographer  in  Charge.     1896.    8°.     356  pp.     Price  25  cents. 

141.  The  Eocene  Deposits  of  the  Middle  Atlantic  Slope  in  Delaware,  Maryland,  and  Virginia, 
by  William  Bullock  Clark.     1896.    8^.     167  pp.     40  pi.     Price  15  cents. 

142.  A  Brief  Contribution  to  the  Geology  and  Paleontology  of  Northwestern  Louisiana,  by  T. 
Wayland  Vaughan.     1896.     8°.    65  pp.     4  pi.     Price  10  cents. 

143.  A  Bibliography  of  Clays  and  the  Ceramic  Arts,  by  John  C.  Branuer.     1896.     8-.     114  pp. 
Price  15  cents. 

144.  The  Moraines  of  the  Missouri  Coteau  and  their  Attendant  Deposits,  by  James  Edward  Todd. 
1896.     8°.     71  pp.     21  pi.     Price  10  cents. 

145.  The  Potomac  Formation  in  Virginia,  by  W.  M.  Fontaine.     1896.     8°.     149  pp.     2  pi.     Price 
15  cents. 

146.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1895,  by  F.  B.  Weeks.     1896.     8'\     130  pp.     Price  15  cents. 

147.  Earthquakes  in  California  in  1895,  by  Charles  D.  Perrine,  Assistant  Astronomer  in  Charge 
of  Earthquake  Observations  at  the  Lick  Observatory.     1896.     8^'.    23  pp.    Price  5  cents. 


ADVERTISEMENT. 


WATER  SUPPLY  AND  IRRIGATION  PAPERS. 

By  act  of  Congress  .approved  ,Iuno  11,  1896,  the  foUowins;  provision  was  made: 

"Froviilcil,  That  hereafter  the  reports  of  the  Geological  Survey  in  rel.ation  to  the  gauging  of 
streams  and  to  the  methods  of  utilizing  the  water  resources  m.iy  be  printed  in  octavo  form,  not  to 
exceed  one  hundred  pages  in  length  and  iive  thousand  copies  iu  number;  one  thousand  copies  of  which 
shall  be  for  the  official  use  of  the  Geological  Survey,  one  thousand  five  hundred  copies  shall  be  deliv- 
ered to  the  Senate,  and  two  thousand  five  hundred  copies  shall  be  delivered  to  the  House  of  Kepre- 
seutiitives,  for  distribution." 

Uuder  this  law  the  following  paper  has  been  issued: 

1.  Pumping  Water  for  Irrigation,  by  Herbert  M.  Wilson. 

GEOLOGIC  ATLAS  OF  THE  UNITED  STATES. 

The  Geologic  Atlas  of  Che  United  States  is  the  final  form  of  publication  of  the  topographic  and 
geologic  m.aps.  The  atlas  is  issued  iu  parts,  progressively  as  the  surveys  are  extended,  and  is  designed 
ultimately  to  cover  the  entire  country. 

Undisr  the  plan  adopted  the  entire  area  of  the  country  is  divided  into  small  rectangular  districts, 
bounded  by  certain  meridians  and  parallels.  The  unit  of  survey  is  also  the  unit  of  publication,  and 
the  maps  and  descriptions  of  each  rectangular  district  are  issued  as  a  tolio  of  the  Geologic  Atlas. 

Each  folio  contains  topographic,  geologic,  economic,  and  structural  maps,  together  with  textual 
descriptions  and  explanations,  and  is  designated  by  the  name  of  a  principal  town  or  of  a  prominent 
natural  feature  withiu  the  district. 

Two  forms  of  issue  have  been  adopted :  A  library  edition,  bound  between  heavy  paper  covers 
and  stitched;  and  afield  edition,  similarly  bound,  but  unstitched. 

Under  the  law  a  copy  of  each  folio  is  sent  to  certain  public  libraries  and  educational  institu- 
tions. A  limited  number  of  copies  are  reserved  for  distribution  to  persons  specially  interested  in  the 
region  represented.  This  distribution  is  at  first  gratuitous,  but  when  the  remaining  number  of  copies 
of  any  folio  reaches  a  certain  minimum  a  charge  equivalent  to  cost  of  publication  will  be  made.  In 
such  cases  prepayment  is  obligatory.     The  folios  ready  for  distribution  are  listed  below. 


Name  of  sheet. 


Area, : 


Price, 


Livingston* 
Ringgold*  . . 
Placerville* . 
Kingston*  . . 


Pikes  Peak* 

Sewanee* 

Antliracite-Crested  Butte* 


Harpers  Ferry*. 
Jackson* 


Estill  ville* 

Fredericksburg* 

Staimton* 

Lassen  Peak*... 
Knoxville* 


Cleveland* . . . . 
Pikeville*  .... 
McMinnville* 


Pocahontas . 
Morristown. 
Piedmont... 


fNevadaCity. 

Nevada  City . .  .^Grass  Vallev. 

(Banner  Hill  . 

{Gallatin  . . 
Canyon . . . 
Shoshone. 
Lake 


Colorado 

Tennessee 

Colorado 

Virginia....... 

Maryland'!!....; 

California 

•Virginia 

Kentucky 

.Tennessee 

'Maryland 

[Virginia 

(Virginia 

["West  Virginia. 
California 


Tennessee  . 
Tennessee  . 
Tennessee  . 
Maryland.. 

Montana. . . 
Tennessee  . 

West  v'irgi: 


Maryland 

.West  Virginia..!) 

1(12 
California <12 


iio°-ni° 

450-460 

85°-85°  30' 

340  30'-35° 

120°  30'-121° 

81°  30'-85° 

1210-121°  30' 

1050-105°  30' 

85°  30'-86° 

°  45'-107o  15' 

38°  30'-39o 
350  30'-36° 
38°  30'-39° 
35°-350  30' 
38°  30'-39o 
350-35°  30' 
380  45'-39o 

77°  30'-78o 

39°-39°  30' 

120°  30'-121° 

38°-38°  30' 

82°  30'-83o 

36°  30'-37° 

770-770  30' 

38°-38°  30' 

790-790  30' 

38°-380  30' 

1210-1220 

40°^1° 

830  30'-84o 

35°  30'-36° 

121°  30'-122o 
1210-1210  30' 

390-39°  30' 
390-390  30' 

850  30'-86o 

34°  30'-35° 

84°  30'-85° 
850-85°  30' 
85°  30'-86o 

35°~30'-36° 
35°  30'-36° 

7C°  30'-77° 

38°-38o  30' 

111°-112° 
840-81°  30' 

45°-46° 
35°  30'-36° 

81°-81o  30' 

37°-37o  30' 

830-83°  30' 

360-36°  30' 

79°-79°  30' 

390-39°  30' 

'-1210  03'  45" 
-121°  05'  04" 

39°  13'  50"-39°  17'  16" 
39°  10'  22"-39°  13'  50" 

39°  13'  50"-39°  17'  16" 


*  These  folios  < 


I  prepjiyment  of  price  stated  iu  the  last  column. 


ADVERTISEMENT. 


STATISTICAL  PAPERS. 


Mineral  Resources  of  the  United  States  [1882],  by  Albert  Willi.ams,  jr.  1883.  8^.  xvii,813pp. 
Price  50  cents. 

Mineral  Resources  of  the  United  States,  1883  and  1884,  by  Albert  Williams,  jr.  1885.  8°.  xiv, 
1016  pp.     Price  60  cents. 

Mineral  Resources  of  the  United  States,  1885.  Division  of  Mining  Statistics  and  Technology. 
1886.     8°.     vii,  576  pp.     Price  40  cents. 

Mineral  Resources  of  the  United  States,  1886, by  David  T.Day.  1887.  S°.  vui,813pp.  Price 
60  cents. 

Mineral  Resources  of  the  United  States,  1887,  by  David  T.  Day.  1888.  8°.  vii,  832  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1888,  by  David  T.  Day.  1890.  8°.  vii,  652  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1889  and  1890,  by  David  T.  Day.  1892.  8°.  viii,  671  pp. 
Price  50  cents. 

Mineral  Resources  of  the  United  States,  1891,  by  David  T.  Day.  1893.  8'=.  vii,  630  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1892,  by  David  T.  Day.  1893.  8°.  ■  vii,  850  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1893,  by  David  T.  Day.  1894.  8^.  viii,  810  pp.  Price 
50  cents. 

On  March  2, 1895,  the  following  provision  was  included  in  an  act  of  Congress : 

"Profided,  That  hereafter  the  report  of  the  mineral  resources  of  the  United  States  .shall  be 
issued  as  a  part  of  the  report  of  the  Director  of  the  Geological  Survey." 

In  compliance  with  this  legislation,  the  report  Mineral  Resources  of  the  United  States  for  the 
Calendar  Year  1894  forms  Parts  III  and  IV  of  the  Sixteenth  Annual  Report  of  the  Survey,  and  Mineral 
Resources  of  the  United  States  for  the  Calendar  Year  1895  forms  Part  III  of  the  Seventeenth  Annual 
Report  of  the  Survey. 

The  money  received  from  the  sale  of  these  publications  is  deposited  in  the  Treasury,  and  the 
Secretary  of  that  Department  declines  to  receive  bank  checks,  drafts,  or  postage  stamps ;  all  remit- 
tances, therefore,  must  be  by  MONEY  ORDER,  made  payable  to  the  Director  of  the  United  States 
Geological  Survey,  or  in  currency  for  the  exact  amount.  Correspondence  relating  to  the  publica- 
tions of  the  Survey  should  be  addressed 

To  THE  Director  of  the 

United  States  Geological  Survey, 
Washington,  D.  C,  February,  1S97.  Washington,  D.  C. 


LIBRARY  CATALOGUE  SLIPS. 

United  States.     Dejxiriment  of  the  interior.     (  T.  S.  tjeological  survey.) 

Department  of  the  interior  |  —  |  Monographs  |  of  the  |  United 
States  geoh)gioal  survey  |  Volume  XXVIII  |  [Seal  of  the  depart- 
ment] I  Washington  |  government  i)rinting  office  |  1897 

Second  title:  United  States  geologiciil  survey  |  Charles  D. 
Waleott,  director  |  —  |  The  |  Marquette  iron-bearing  district  of 
Michigan  |  with  |  atlas  |  by  |  Charles  Richard  Van  Hise  and  Will- 
iam Shirley  Bayley  |  including  |  a  chapter  on  the  Republic  trough 
I  by  I  Henry  Lloyd  Smyth  |  [Vignette]  | 

Washington  |  government  printing  office  |  1897 

4°.    608  1)]).    ys  pi. 


Van  Hise  (Charles  Richard),  Bayley  (William  Shirley),  and  Smyth 
(Henry  Lloyd). 

United"  States  geological  survey  |  Charles  D.  Waleott,  di- 
rector I  —  I  The  I  Marquette  iron-bearing  district  of  Michigan  | 
with  I  atlas  |  by  |  Charles  Richard  Van  Hise  and  William  Shirley 
Bayley  |  including  |  a  chapter  on  the  Republic  trough  |  by  |  Henry 
Lloyd  Smyth  |  [Vignette]  | 

Washington  |  government  printing  office  |  1897 

i\    60811]..   :w,ii. 

[United  States.  Department  of  ttie  interior.  {U.  S.  geological  surrey.) 
Monogr.aph  XXVIII.] 


United  States  geological  survey  |  Charles  D.  Waleott,  di- 
rector I  —  I  The  I  Marquette  iron-bearing  district  of  Michigan  | 
with  I  atlas  |  by  |  Charles  Richard  Van  Hise  and  William  Shirley 
Bayley  |  including  |  a  chapter  on  the  Republic  trough  |  by  |  Henry 
Lloyd  Smyth  |  [Vignette]  | 

Washington  |  government  printing  office  |  1897 

i°.    608  pp.    35  pi. 

[United  States.  Department  of  the  interior.  ( U.  S.  geological  survey.) 
Monograph  XXVIII.] 


\ 


